On the 23rd December 2019 I received a particularly nice early Christmas present : confirmation that my paper had, at long last, been accepted. We submitted it on 18th October 2018 and it went through three(!) reviewers before it was finally accepted, so this was much, much more of a saga than I was expecting. Whether it's more like Goldilocks And The Three Reviewers, or of a visitation of the three ghosts of Christmas, I'm not sure. It's probably best told as a mash-up of the two.
Challenge accepted. Here goes.
Once upon a time, there was a nice little blonde astronomer named Goldilocks who noticed some interesting things in his data that he hadn't noticed before. He decided to write a paper about it.
"Ho hum !", he said to himself. "This should be an easy little write-up, and then I can do something else."
But then he found that there was a weird pattern in his data that didn't make much sense. He decided to run a public poll to make sure he hadn't gone mad. He got about a hundred responses on the so-called "ghost town" that was Google Plus, and everyone agreed that the findings were correct. So he thought about it very carefully and realised that it was probably just an interesting but not terribly unlikely coincidence. He wrote up his paper and sent it to his co-authors for comments. As no-one had much to add, he submitted it to a journal and slept soundly, thinking it should not be a terribly controversial discovery.
That night he awoke with a start. A chill air filled the room and there was a most terrible wailing. Suddenly a ghastly phantom rose from the floorboards and cried,
"Rhyyyyy.... I mean, Gooolldddiiiiiloooooocks ! You shall be visited by three ghostly reviewers to inspect your paaaapppeeerrrr !"
"But why ?", cried Goldilocks. "It's nothing special. I mean, it's nice enough, but it's just some galaxies with stripped tails of gas that are pretty much exactly what we expected to find. Come on, galaxies in the Virgo cluster, losing gas exactly as predicted ? Fun, but hardly a revelation. The only real oddity is the way the streams are pointing, but we've explained that well enough. Why should it need three reviewers ?"
"I doooon't knoooooooow !" wailed the phantom. "They just woooooon't belieeeeeeeve yoooooou ! Expect the first ghost quite soooooon !".
A few weeks later Goldilocks again awoke to the clanking of chains and a mysterious wailing. Creeping downstairs, he found a middle-aged man lounging on his sofa and throwing popcorn at his TV in a carefree fashion.
"Yo," said the man, "Someone's been sleeping in MY bed... ! I'm the Daddy Ghost of Utter Pointlessness. Here's yer report." And with that he vanished.
Goldilocks picked up the report and read it carefully. It was quite long, but it didn't seem too bad at first. It was pretty darn clear that the Ghost really just wanted extra citations to their own papers and didn't understand some very basic concepts from radio astronomy. That was a bit worrying, but easy enough to address. It also came with the dreaded task of "shorten the text", which was, as usual, quite meaningless as it came with no further instructions. "Too many notes", tutted Goldilocks to himself. "But everyone says that, so it's not much to worry about".
More reassuringly, the Ghost didn't ask for anything drastic or express any major scientific skepticism, and the requests to make things "more convincing" seemed quite reasonable : clarifying the improved sensitivity from the new analysis, more labels on figures, that kind of thing. It made sense that one might be a bit skeptical about detecting this many new gas streams, even knowing that the cluster was exactly the environment where one should expect such features, since they were somewhat on the faint side.
Goldilocks did as the seemingly sensible (if rather ignorant and uninformed), Ghost suggested and duly returned the paper. Before long, the Ghost came back with a new report and vanished once more, feeling even less inclined to discourse than the last time.
Goldilocks eagerly read the report and immediately fell into despondency. This wasn't so much pointless as it was downright rude. He'd carefully addressed all the points from the first report and explained things at length in the accompanying letter. Yet the Ghost's report was barely a single paragraph and, worst of all, insisted that Goldilocks hadn't done what was asked. Even on those points that were really simple, like asking for a number which was now very clearly highlighted in bold. And the Ghost had asked for more explanation on the improved sensitivity, while Goldilocks had explained several times that sensitivity wasn't the issue, it was about visualisation. Of course he'd explained the procedure in more detail as well, just to make sure, but the Ghost either just didn't get it or was being deliberately obtuse.
"What on Earth am I to do," said Goldilocks to the co-authors, "when someone asks me what the number is, I tell them 'it's six, six is the number, and the number shall be six' and they insist that I haven't told them what the number is ?"
Goldilocks was both cross and confused. The Ghost's response was hopelessly inconsistent. Whereas before the Ghost seemed a bit concerned if the admittedly quite faint tails were real, now they were wondering if they could have been produced by something other than ram pressure stripping. That was something they easily could have pointed out at the first stage, and adding it now really felt like being strung along. And the Ghost made a bizarre claim that one source, already firmly established in the literature through several other independent observations, was only "probably" real. This was a bit like saying it bricks would only "probably" hurt if you dropped them on your toe. It was pointless.
Goldilocks couldn't see the point of answering a referee report knowing that they might just ignore everything and shift the goalposts again. Especially since they insisted the paper was now longer when it was objectively shorter.
"FFS", said Goldilocks to himself.
After consulting the co-authors, Goldilocks decided to ask the editor what to do. He was a bit disappointed that the editor hadn't already intervened, because the problems with the Ghost's response weren't subtle. They were, in fact, glaringly obvious, and he'd seen editors intervene by themselves in the past with things less blatant than this. Goldilocks complained that addressing this new response wouldn't work, since the referee was so inconsistent and asked for things which were already done and stated very clearly indeed in the main text. Trying to address things raised by someone who would simply ignore you no matter how clearly you stated things was indeed Utterly Pointless.
The editor thought for a while and declared, "Hum ! So, this porridge is a bit hot, is it ? We'll see if we can find some that's a bit cooler". And with that Goldilocks waited for a brand new Ghost.
Some time later Goldilocks again awoke to hear a low moaning. This time there was a slightly older matronly figure sitting in a more dignified position and wearing a monocle. "WoooOOooo !", said she. "Behold, I am the Mummy Ghost of Undue Skepticism. Read my report, mortal, if you dare !". She shook her fist in a dramatic fashion and disappeared.
Goldilocks read the report with some trepidation, but was soon confident he knew what to do. The report wasn't without problems. This Ghost was asking for a figure to be both improved and removed, which was very confusing. However, they were very explicit about their main concerns, which made them a lot easier to address. First, they were worried that some of the streams might not be due to ram pressure stripping. Goldilocks was fine with that, he'd never thought that the situation would be otherwise. Making this clearer was no problem. Second, the Ghost wasn't sure all the streams were even real. That wasn't too big of a deal either, as it was quite straightforward to give their statistical significance and predict how many false streams should be expected in a data set this large (the answer, it turned out, was a healthy zero).
The other referees' comments being minor, Goldilocks soon found a way to measure the statistical significance objectively and clarify that the streams might have multiple formation mechanisms. He didn't really understand why anyone would be hung up on these points though, as it was hardly a breakthrough discovery and plenty of other much stupider papers were floating around in the literature. Surely, he thought, the results are at least solid enough that the rest of the community deserved a look at them. "And anyway," he said to himself, "it's well-known that if you provide enough details to recreate your results, which I bugger well have, it isn't necessary that the referee actually has to agree with your conclusions. They shouldn't reject it unless they can actually find a flaw in the analysis, or better yet they should correct it."
So Goldilocks sent off the report feeling cautiously optimistic that this time he'd succeed. The referee certainly seemed more familiar with radio astronomy, which seemed like a good sign.
Alas ! Some considerable time later, the Ghost re-appeared. "Woe !" she cried with a banshee wail, "I remain unduly skeptical ! This porridge is too cold. Thou hast not addressed my concerns, and I reject thine paper ! May it be cast into the pits of hell !"
Goldilocks was astonished and dismayed. He read the report with contempt. The Ghost had blathered about a few points that made little or no sense, but worst of all he hadn't responded to the correction on the main point - at least, not sensibly. Instead of addressing the whole new section dedicated to assessing statistical significance, which was objective and quantitative, she'd simply said she "understood" it, but thought that "the evidence should be in the images".
This didn't sit well with Goldilocks at all. "Fair enough," he thought to himself, "an objective analysis can absolutely be wrong if the wrong procedure is used or whatnot. But surely in this case someone needs to tell me what the blazes actually is wrong with it, rather than just saying they understand it. If they really understand it, they bloomin' well ought to be able to explain why it's wrong." And he was also more than a little annoyed that they wanted "evidence in the images". All this amounted to the Ghost wanting subjective proof in place of an objective one, without saying what was wrong with the method. Goldilocks was Not Happy.
(He thought about complaining to the editor but decided it would do little good. He also noted that the Ghost claimed to remain skeptical of the "majority" of the streams, but when you added up the number of individual streams they said they were happy with, found that they came to 60% of the total.)
What to do ? Goldilocks was not as despondent as you might think. He'd been working on the analysis for well over a year already, and every time someone had come up with a reason to doubt their existence, the tests had only strengthened the case for the streams. True, two ghastly shades hadn't been convinced, but both appeared to be quite bizarre. The other co-authors were all happy with the result, all of whom were more senior and more experienced than him.
"Right," said Goldilocks. "None of the objections raised by the referees make any sense. Therefore, strange as it is, the only reasonable conclusion is that I'm right and they're wrong. I'm not going to dump more than a year of work on the scrapheap because some spectral nit doesn't understand it. I'm going to submit it to a whole new journal."
Goldilocks did, however, accept that images can often be more persuasive than numbers. So he did a whole new analysis in which he injected fake sources into real data, not only measuring the very few false positives that appeared but also making the same contour plots of them as were presented for the real streams. It was pretty effin' clear that you just didn't get false positives that looked anything like the real streams, exactly as the earlier analysis had shown. And so Goldilocks submitted the paper and once again waited.
And waited.
And waited some more.
Well, actually not really, because this time the Ghost was very prompt.
"Ahhhwoooooo !" cried the spectre. "Behold, I am the Baby Ghost of Precise Instruction ! Read my report with the utmost care and all will be well !"
"Oh spirit," said Goldilocks, "I fear you more than any ghost I have yet witnessed. Can it really be true that you are indeed the Ghost of Precise Instruction ?"
But the miniscule phantom only pointed a spectral finger at the report and disappeared.
Now you must understand that Goldilocks was in a pretty strange mental state but this point. He'd been haunted by three strange spectres all questioning his spectra, and was both quite cross and trepidatious. He didn't doubt himself, but he was highly suspicious that the Ghost would actually do their dang job properly. He read the report quite nervously, and decided to avoid reaching any conclusions, knowing that you can't judge someone until you see how they respond a second time.
Still, it looked promising. There were no clear indications that this doubt thought the porridge was too hot or too cold, only that the height of the chair was a bit off and the window needed oiling. That is, the Ghost didn't seem concerned about whether the streams were real, only that the paper was too long and didn't have a good comparison sample.
"Well, fair enough really," thought Goldilocks. "A comparison sample is a great idea, but unfortunately just not practical. Hopefully the ghost will understand this if I explain in sufficient detail."
The Ghost had, however, provided very Precise Instruction indeed when it came to shortening the manuscript. Goldilocks didn't particularly want to do this, but instructions this clear were difficult to get wrong. Best of all, by the very simple direction to "concentrate on the new results", this made it trivial to extrapolate as to which other parts could be cut. Those few words transformed a task ordinarily fraught with problems into the work of a couple of a days. Soon the paper was five pages shorter and, Goldilocks had to admit, considerably more focused.
"I'm still in two minds about it," said he, "but overall this is probably better. I liked the more detailed original version, but more people are likely to actually read this shorter document."
So Goldilocks made the remaining changes and carefully explained why they couldn't provide a comparison sample, substituting this for a literature search of similar features instead. He poked and prodded his co-authors until they finally gave the go-ahead, and then he submitted the revised paper. And very soon the Ghost returned and said "this porridge is just right !", all was well, and there was dancing in the streets.
"Hooray !" said Goldilocks. "But we've learned some valuable lessons here. First, the rules of refereeing ought to be clearly spelled out and not just left to the referee to make them up however they see fit. You can't just go around saying, 'I don't agree' without providing any justification. Second, editors ought to actively check if both sides follow the rules, and not just act as postmen. Pretty much a year of valuable research time has been wasted dealing with this crap and that needn't have happened. I'm going home."
And with that he stomped off and had a lovely Christmas. The end.
Monday, 13 January 2020
Wednesday, 8 January 2020
Get Off My Fundamental Plane
Or at least off my Tully-Fisher relation, which is almost the same thing. Yes, it's another case of galaxies rotating more slowly than expected given their mass, and which might not even have any dark matter at all.
The worryingly astute reader may remember that I briefly mentioned this paper back in this post last year. I didn't give it its own post then because of various problems : confusing language, poor figure labels, lack of any statement about which journal it was in, and lack of citation of a similar analysis from the same data set. Much of that is now cleared up - it's accepted in Nature Astronomy and had a press release late last year. Some figures still aren't labelled as clearly as they should and "inclination angels" wins the Typo Of The Day award, but mostly it seems in much better shape than it was. I still think they should have cited the earlier analysis, even though this is admittedly a different sample.
Previously we've seen a slew of claims that some Ultra Diffuse Galaxies (large, fluffy galaxies that are very spread out and usually quite faint) have weird dynamics. Whereas most galaxies are rotating so quickly that they should fly apart without dark matter, some UDGs are rotating exactly as their baryonic mass (that is, ordinary gas and stars) predicts. It seems that they don't need any dark matter at all, or at least, much less than other galaxies of comparable baryonic mass. One early worry was that there might be a problem correcting the inclination angles (not angels), meaning that the rotation speeds could have been underestimated, but recent data seems to make that less likely.
The galaxies in this sample are quite different. The deviant UDGs are extremely gas-rich, optically faint objects, whereas the ones in this sample are neither*. There gas content looks entirely normal compared to typical galaxies, so their star formation is - presumably - ticking over just fine. Nor are they especially faint. Indeed, their self-imposed brightness cutoff stands out quite clearly in their data, tentatively suggesting that there could be eve more massive galaxies hidden in the sample. So in essence these appear to be relatively normal galaxies chock-full of gas that's forming stars at a normal rate, but don't have nearly as much dark matter as normal galaxies (if they have any at all). And most of them aren't in groups or clusters, so interactions are probably not responsible.
* Strictly speaking we can't say if these galaxies are UDGs or not without more precise measurements of the radial stellar profile, but looking at some of the images, it's clear that they're pretty normal-looking objects.
This is getting very strange indeed. If you don't have much dark matter, feedback from stars and supernovae should make it much easier to blast apart any luckless galaxies that happened to form like that. But this hasn't happened in these cases. Why not ? The two options they suggest are either that there is an inclination angle estimation error here (if not in the other cases), so their true velocity width is actually much higher and they have dark matter after all, or that the feedback helps "flatten out" the dark matter potential during formation. So they'd still have dark matter, it would just be distributed in an unusual way.
The second option may or may not work but it would take detailed numerical studies to properly investigate (so far as I know, no such objects have shown up in existing simulations). The first option doesn't look likely either. Although they'd like to have resolved gas observations to measure the gas disc angle directly, looking at the optical images it seems very unlikely that there could be a big error in the estimated angle. They say that perhaps the presence of strong bars may have screwed things up, but again, inspecting the optical images, that doesn't seem likely either. Look, here's a few :
Those are as normal a set of galaxies that you could ever hope to meet. Except, apparently, they're just very slow.
Unfortunately they only plot the optical Tully-Fisher relation (optical brightness as a function of rotation speed). Baryonic mass would have been better, but since they're brighter than expected, there's no way including the gas mass can solve the problem.
Finally, they note that if you plot the distribution of the ratio between dynamic and baryonic matter, you get a nice Gaussian but with a tail at the low end where these galaxies live. They say this indicates they're a truly separate population rather than outliers. I'm not sure I'm convinced by that - maybe the normal population just has a non-Gaussian distribution. And in terms of their deviation from the Tully-Fisher, they look like a continuation of the general scatter, not outliers. But it's an interesting thing to plot even so.
So what's going on here ? Damned if I know. Apparently some otherwise normal galaxies just don't have any dark matter. How many more objects like this could there be ? Well, their main sample was 324 objects, of which 19 are weirdos, so 6%. That used the now-obsolete 40% ALFALFA catalogue. From the 100% catalogue size, that's about 1,850 objects - possibly more due to their brightness limit. As for such deviants affect MOND, which predicts a low scatter in the TFR, I've no idea, though I assume it wouldn't be good news given that most of these objects are quite isolated.
Spare a thought for poor van Dokkum, who had to face a barrage of skepticism over the claimed distance to his "original" galaxy without dark matter. That may be a concern for some of the objects here, but not all of them - they're just too far away for there distance to be drastically over-estimated. So regardless of whether van Dokkum's original claim stands up, it really does seem as though there's a population of very strange objects out there. Fun times !
EDIT : A very short rebuttal paper claims that it is all due to inclination angle after all, essentially saying that the galaxies could simply be not circular - i.e. they might be close to face-on, just not-disc shaped (as is quite usual for dwarf galaxies). I'm not convinced by this. No dwarf I ever plotted in the TFR showed any deviation like this, and I didn't correct for their shape either. Nor are these particularly low-mass dwarfs, and again, there seems to be cut-off in mass that's purely an arbitrary choice, hinting at more massive objects. And looking at the images, it just doesn't feel right - if this was the explanation, we ought to have heard previous similar claims, and to my knowledge we never have. Finally, the claim that the sample is biased because of their large axial ratios doesn't make any sense to me, since the authors specify they deliberately chose the sample in this way (since this is a necessary criteria for edge-on galaxies). Still, I could be persuaded with a longer paper : show me some comparable objects and simulations showing that such objects would be stable.
EDIT 2 : And because these are big, bright objects, whether they are rotating or not would be relatively easy to test. High resolution HI observations or spectroscopy with an IFU ought to give a definitive answer with little or no wriggle room.
The worryingly astute reader may remember that I briefly mentioned this paper back in this post last year. I didn't give it its own post then because of various problems : confusing language, poor figure labels, lack of any statement about which journal it was in, and lack of citation of a similar analysis from the same data set. Much of that is now cleared up - it's accepted in Nature Astronomy and had a press release late last year. Some figures still aren't labelled as clearly as they should and "inclination angels" wins the Typo Of The Day award, but mostly it seems in much better shape than it was. I still think they should have cited the earlier analysis, even though this is admittedly a different sample.
Previously we've seen a slew of claims that some Ultra Diffuse Galaxies (large, fluffy galaxies that are very spread out and usually quite faint) have weird dynamics. Whereas most galaxies are rotating so quickly that they should fly apart without dark matter, some UDGs are rotating exactly as their baryonic mass (that is, ordinary gas and stars) predicts. It seems that they don't need any dark matter at all, or at least, much less than other galaxies of comparable baryonic mass. One early worry was that there might be a problem correcting the inclination angles (not angels), meaning that the rotation speeds could have been underestimated, but recent data seems to make that less likely.
The galaxies in this sample are quite different. The deviant UDGs are extremely gas-rich, optically faint objects, whereas the ones in this sample are neither*. There gas content looks entirely normal compared to typical galaxies, so their star formation is - presumably - ticking over just fine. Nor are they especially faint. Indeed, their self-imposed brightness cutoff stands out quite clearly in their data, tentatively suggesting that there could be eve more massive galaxies hidden in the sample. So in essence these appear to be relatively normal galaxies chock-full of gas that's forming stars at a normal rate, but don't have nearly as much dark matter as normal galaxies (if they have any at all). And most of them aren't in groups or clusters, so interactions are probably not responsible.
* Strictly speaking we can't say if these galaxies are UDGs or not without more precise measurements of the radial stellar profile, but looking at some of the images, it's clear that they're pretty normal-looking objects.
This is getting very strange indeed. If you don't have much dark matter, feedback from stars and supernovae should make it much easier to blast apart any luckless galaxies that happened to form like that. But this hasn't happened in these cases. Why not ? The two options they suggest are either that there is an inclination angle estimation error here (if not in the other cases), so their true velocity width is actually much higher and they have dark matter after all, or that the feedback helps "flatten out" the dark matter potential during formation. So they'd still have dark matter, it would just be distributed in an unusual way.
The second option may or may not work but it would take detailed numerical studies to properly investigate (so far as I know, no such objects have shown up in existing simulations). The first option doesn't look likely either. Although they'd like to have resolved gas observations to measure the gas disc angle directly, looking at the optical images it seems very unlikely that there could be a big error in the estimated angle. They say that perhaps the presence of strong bars may have screwed things up, but again, inspecting the optical images, that doesn't seem likely either. Look, here's a few :
Those are as normal a set of galaxies that you could ever hope to meet. Except, apparently, they're just very slow.
Unfortunately they only plot the optical Tully-Fisher relation (optical brightness as a function of rotation speed). Baryonic mass would have been better, but since they're brighter than expected, there's no way including the gas mass can solve the problem.
Finally, they note that if you plot the distribution of the ratio between dynamic and baryonic matter, you get a nice Gaussian but with a tail at the low end where these galaxies live. They say this indicates they're a truly separate population rather than outliers. I'm not sure I'm convinced by that - maybe the normal population just has a non-Gaussian distribution. And in terms of their deviation from the Tully-Fisher, they look like a continuation of the general scatter, not outliers. But it's an interesting thing to plot even so.
So what's going on here ? Damned if I know. Apparently some otherwise normal galaxies just don't have any dark matter. How many more objects like this could there be ? Well, their main sample was 324 objects, of which 19 are weirdos, so 6%. That used the now-obsolete 40% ALFALFA catalogue. From the 100% catalogue size, that's about 1,850 objects - possibly more due to their brightness limit. As for such deviants affect MOND, which predicts a low scatter in the TFR, I've no idea, though I assume it wouldn't be good news given that most of these objects are quite isolated.
Spare a thought for poor van Dokkum, who had to face a barrage of skepticism over the claimed distance to his "original" galaxy without dark matter. That may be a concern for some of the objects here, but not all of them - they're just too far away for there distance to be drastically over-estimated. So regardless of whether van Dokkum's original claim stands up, it really does seem as though there's a population of very strange objects out there. Fun times !
EDIT : A very short rebuttal paper claims that it is all due to inclination angle after all, essentially saying that the galaxies could simply be not circular - i.e. they might be close to face-on, just not-disc shaped (as is quite usual for dwarf galaxies). I'm not convinced by this. No dwarf I ever plotted in the TFR showed any deviation like this, and I didn't correct for their shape either. Nor are these particularly low-mass dwarfs, and again, there seems to be cut-off in mass that's purely an arbitrary choice, hinting at more massive objects. And looking at the images, it just doesn't feel right - if this was the explanation, we ought to have heard previous similar claims, and to my knowledge we never have. Finally, the claim that the sample is biased because of their large axial ratios doesn't make any sense to me, since the authors specify they deliberately chose the sample in this way (since this is a necessary criteria for edge-on galaxies). Still, I could be persuaded with a longer paper : show me some comparable objects and simulations showing that such objects would be stable.
EDIT 2 : And because these are big, bright objects, whether they are rotating or not would be relatively easy to test. High resolution HI observations or spectroscopy with an IFU ought to give a definitive answer with little or no wriggle room.
Further evidence for a population of dark-matter-deficient dwarf galaxies
In the standard cosmological model, dark matter drives the structure formation of galaxies and constructs potential wells within which galaxies may form. The baryon fraction in dark halos can reach the Universal value (15.7%) in massive clusters and decreases rapidly as the mass of the system decreases 1,2 .
Tuesday, 7 January 2020
Things are getting CHILE
The COSMOS HI Large Extragalactic Survey is an ambitious, 1000-hour VLA project to map the HI content in a bunch of galaxies at relatively high redshift (~0.4). Here they present some preliminary results from 178 hours of observing ten galaxies at a more modest redshift of 0.1 (a distance of about 430 Mpc, or a travel time of 1.2 Gyr at the speed of light). That's still impressive for HI studies, only a few of which have ever probed distances this large.
Although the abstract makes something of a song and dance about how the HI is aligned with filaments in the cosmic web, and even the title of the paper mentions this, in fact there's very little about this in the paper at all. And since they only have ten galaxies, I'm going to entirely discard their "main" result here.
In fairness, the authors say themselves, "any of the relations shown should be considered only a hint of a trend at most". This is very much a preliminary paper, but it does show some interesting possibilities for the future. Although they only have ten galaxies, they've also (don't ask me how) determined the large-scale structure of many other galaxies, i.e. the 3D geometry of the cosmic web. That means they can plot the properties of their sample as a function of true filamentary distance. Normally everyone uses projected distance, which assumes that any two objects are at the same distance from us and then just measures their distance across the sky as a proxy for their separation. Using true distance is obviously better, although just how accurate this I'm not at all sure.
But let's assume that it's fine. What do they find ?
Quite a few things. But not as much as they might, because they select their sample not only by HI detection but also Halpha (which traces star formation activity). So although they potentially had 33 galaxies to study, they limit themselves to 10. It would have been nice to use the rest for the HI-only trends, but oh well.
Anyway, even though they're only sensitive to dense gas, most galaxies in their sample show lopsidedness. They also have weird-looking spectra, including one with a triple peak instead of the usual double-horn (Batman-shaped) profile. They don't comment too much on this, though they may be signatures of interactions.
More interesting are the possible large-scale trends. There doesn't appear to be any relation at all between stellar mass and gas loss, but it looks to me that galaxies closer to the filament have lost less gas and may even be gas-rich. The authors disagree though, saying there's no trend, and admittedly this appearance is dominated by a single, weirdly gas-rich galaxy. It's still odd if you ask me.
On the other hand, HI mass quite clearly decreases with distance to the filament. Absolute mass is a tricky quantity, mind, and it probably would have been a good idea to also plot stellar mass. There seems to be hint that specific star formation rate (that's s.f.r. per unit mass, useful to correct for the fact that bigger galaxies tend to have more gas to form stars simply just because big things are bigger) increases closer to the filaments. Gas fraction shows no evidence of any trends at all.
This is a nice way to illustrate how difficult it is to interpret the results. Other studies have variously found evidence of gas depletion or enhancement within filaments. It may also depend on whether you look at the gas fraction of individual galaxies as opposed to the fraction of galaxies which have gas detections at all. If instead you plot distance not from filament but from nearest neighbour, things get different again, with the trends changing considerably (they say they are no obvious relations at all - I'm not sure about that, but with ten galaxies... well...).
Finally, they note that galaxies tend to have a constant gas density, which agrees well with previous claims. There's no obvious trend in any deviation from the Tully-Fisher relation either, although one interesting object does look at though it might be rotating too slowly.
This is all very inconclusive. It does demonstrate the potential of the full survey quite well, but I would have liked it more if they'd used their full HI sample where possible. It's a nice enough analysis, but the final result is very definitely "watch this space".
Although the abstract makes something of a song and dance about how the HI is aligned with filaments in the cosmic web, and even the title of the paper mentions this, in fact there's very little about this in the paper at all. And since they only have ten galaxies, I'm going to entirely discard their "main" result here.
In fairness, the authors say themselves, "any of the relations shown should be considered only a hint of a trend at most". This is very much a preliminary paper, but it does show some interesting possibilities for the future. Although they only have ten galaxies, they've also (don't ask me how) determined the large-scale structure of many other galaxies, i.e. the 3D geometry of the cosmic web. That means they can plot the properties of their sample as a function of true filamentary distance. Normally everyone uses projected distance, which assumes that any two objects are at the same distance from us and then just measures their distance across the sky as a proxy for their separation. Using true distance is obviously better, although just how accurate this I'm not at all sure.
But let's assume that it's fine. What do they find ?
Quite a few things. But not as much as they might, because they select their sample not only by HI detection but also Halpha (which traces star formation activity). So although they potentially had 33 galaxies to study, they limit themselves to 10. It would have been nice to use the rest for the HI-only trends, but oh well.
Anyway, even though they're only sensitive to dense gas, most galaxies in their sample show lopsidedness. They also have weird-looking spectra, including one with a triple peak instead of the usual double-horn (Batman-shaped) profile. They don't comment too much on this, though they may be signatures of interactions.
More interesting are the possible large-scale trends. There doesn't appear to be any relation at all between stellar mass and gas loss, but it looks to me that galaxies closer to the filament have lost less gas and may even be gas-rich. The authors disagree though, saying there's no trend, and admittedly this appearance is dominated by a single, weirdly gas-rich galaxy. It's still odd if you ask me.
On the other hand, HI mass quite clearly decreases with distance to the filament. Absolute mass is a tricky quantity, mind, and it probably would have been a good idea to also plot stellar mass. There seems to be hint that specific star formation rate (that's s.f.r. per unit mass, useful to correct for the fact that bigger galaxies tend to have more gas to form stars simply just because big things are bigger) increases closer to the filaments. Gas fraction shows no evidence of any trends at all.
This is a nice way to illustrate how difficult it is to interpret the results. Other studies have variously found evidence of gas depletion or enhancement within filaments. It may also depend on whether you look at the gas fraction of individual galaxies as opposed to the fraction of galaxies which have gas detections at all. If instead you plot distance not from filament but from nearest neighbour, things get different again, with the trends changing considerably (they say they are no obvious relations at all - I'm not sure about that, but with ten galaxies... well...).
Finally, they note that galaxies tend to have a constant gas density, which agrees well with previous claims. There's no obvious trend in any deviation from the Tully-Fisher relation either, although one interesting object does look at though it might be rotating too slowly.
This is all very inconclusive. It does demonstrate the potential of the full survey quite well, but I would have liked it more if they'd used their full HI sample where possible. It's a nice enough analysis, but the final result is very definitely "watch this space".
CHILES VI: HI and H${\alpha}$ Observations for z < 0.1 Galaxies; Probing HI Spin Alignment with Filaments in the Cosmic Web
We present neutral hydrogen (HI) and ionized hydrogen (H${\alpha}$) observations of ten galaxies out to a redshift of 0.1. The HI observations are from the first epoch (178 hours) of the COSMOS HI Large Extragalactic Survey (CHILES). Our sample is HI biased and consists of ten late-type galaxies with HI masses that range from $1.8\times10^{7}$ M$_{\odot}$ to $1.1\times10^{10}$ M$_{\odot}$.
Tuesday, 10 December 2019
One of our arms is still missing and it's not getting better
Remember that galaxy with the great big twirling loops around it that recently disappeared ? Well, there's this normal-looking spiral galaxy that in really deep images has these spectacular double-loops of stellar streams. Except recently there was a claim that one of those loops might not exist, and might be due to the data reduction by some unspecified mechanism.
I find it difficult to believe that any data reduction procedure could give rise to an artifact that nice and coherent, but here the independent authors do their own observations and find that there is indeed only one loop. Things are getting strange.
Ahem. Right, no, the point is this should encourage the need for more dialogue and co-operation between amateurs and professionals. Perhaps the amateurs have been over-zealous in searching for fainter structures, in which case it's important for us to understand what went wrong. But equally, maybe they've found some clever way to reveal faint structures that professionals have missed. After all, it would hardly be the first time that amateurs have made important contributions to astronomy, and I daresay it wouldn't be the last.
I find it difficult to believe that any data reduction procedure could give rise to an artifact that nice and coherent, but here the independent authors do their own observations and find that there is indeed only one loop. Things are getting strange.
An argument raised in flavor of the existence of a double loop is the fact that many amateur astronomers have repeatedly detected it. However, the data reduction procedures adopted for the amateur images are not always transparent. The fact that now two completely independent professional teams could not confirm the double loop puts some doubts on at least some of the previously found very low-surface brightness features... It seems that professionally handled data always yields a single stream, while data processed by amateur astronomers uncovers more features.It's surely fair to say that amateur observations do not have such clearly-described data reduction procedures as the professionals, but does that mean they're wrong ? It's a bit strong to say that professionals "always" detect only one stream if only two professional teams have looked at it, but reasonable to raise doubts. Surely this points not to pitting amateurs against professionals, in a bloodthirsty battle royale in which everyone is armed with spiked clubs and wears only loincloths, but... sorry, what was I saying ?
Ahem. Right, no, the point is this should encourage the need for more dialogue and co-operation between amateurs and professionals. Perhaps the amateurs have been over-zealous in searching for fainter structures, in which case it's important for us to understand what went wrong. But equally, maybe they've found some clever way to reveal faint structures that professionals have missed. After all, it would hardly be the first time that amateurs have made important contributions to astronomy, and I daresay it wouldn't be the last.
Hunting ghosts: the iconic stellar stream(s) around NG5907 under scrutiny
Stellar streams are regarded as crucial objects to test galaxy formation models, with their morphology tracing the underlying potentials and their occurrence tracking the assembly history of the galaxies. The existence of one of the most iconic stellar streams, the double loop around NGC5907, has recently been questioned by new observations with the Dragonfly telescope.
A stream or not a stream, that is the question
Have our two largest nearest neighbours interacted in the past ? It's widely believed so, although some people think that M33 (Triangulum) and M31 (Andromeda) are experiencing their first encounter. There's a well-known HI stream linking the two, which has sometimes been claimed to be part of the "cosmic web", though I'd bet money most people think it's just a signature of an interaction.
This new study is mainly about modelling the orbital history of the two galaxies. To be honest my eyes glazed over for most of this; long story short, they think the two galaxies have previously interacted (you can watch their simulations here). More interesting is that they make a distinct, testable prediction : as well as the stream linking it to M31, M33 should have a counter-stream of much more diffuse material extending in the opposite direction.
"But Rhys !", I hear you cry, "Weren't you involved in a super-sensitive survey of the M33 region, which was like, totally the shizzle ? Didn't this take you bloody years to complete ? Wouldn't that be something to check ?"
Well, yes, indeed. It took us five years and we got the highest sensitivity ever reached in this region. We found a bunch of new clouds never before seen, including a giant ring-shaped feature that we still can't explain (none of which are seen in these simulations). It was, in short, very cool. But we didn't find anything terribly odd about M33 itself - at least nothing that wasn't known before. True, it has a weird warp in its disc and a funny gas distribution to the north-east, but people knew that already. And true, the gas disc is much more extended than the optical, but we didn't find gas out to significantly higher distances than other surveys.
That said, we couldn't properly identify the edge of the gas disc. Its density profile continuously decreases right until it hits the noise level, so it doesn't look at though we reached the real edge - for that, we'd expect to see a sudden drop. Yet the density the authors predict for the stream is high enough that it should be readily detectable with AGES, and though sensitivity calculations are not always straightforward, I'd be a bit surprised if AGES couldn't see something if it was there.
Still, while they speculate that such a feature might eventually be detectable with FAST, they neglect the existing deep observations. That's a bit odd. I emailed the first author and sent him a link to our paper and data, but I didn't get any response yet.
This new study is mainly about modelling the orbital history of the two galaxies. To be honest my eyes glazed over for most of this; long story short, they think the two galaxies have previously interacted (you can watch their simulations here). More interesting is that they make a distinct, testable prediction : as well as the stream linking it to M31, M33 should have a counter-stream of much more diffuse material extending in the opposite direction.
"But Rhys !", I hear you cry, "Weren't you involved in a super-sensitive survey of the M33 region, which was like, totally the shizzle ? Didn't this take you bloody years to complete ? Wouldn't that be something to check ?"
Well, yes, indeed. It took us five years and we got the highest sensitivity ever reached in this region. We found a bunch of new clouds never before seen, including a giant ring-shaped feature that we still can't explain (none of which are seen in these simulations). It was, in short, very cool. But we didn't find anything terribly odd about M33 itself - at least nothing that wasn't known before. True, it has a weird warp in its disc and a funny gas distribution to the north-east, but people knew that already. And true, the gas disc is much more extended than the optical, but we didn't find gas out to significantly higher distances than other surveys.
That said, we couldn't properly identify the edge of the gas disc. Its density profile continuously decreases right until it hits the noise level, so it doesn't look at though we reached the real edge - for that, we'd expect to see a sudden drop. Yet the density the authors predict for the stream is high enough that it should be readily detectable with AGES, and though sensitivity calculations are not always straightforward, I'd be a bit surprised if AGES couldn't see something if it was there.
Still, while they speculate that such a feature might eventually be detectable with FAST, they neglect the existing deep observations. That's a bit odd. I emailed the first author and sent him a link to our paper and data, but I didn't get any response yet.
The Andromeda System: A new orbital history and its implications
We revisit the orbital history of the Triangulum galaxy (M33) around the Andromeda galaxy (M31) in view of the recent Gaia Data Release 2 proper motion measurements for both Local Group galaxies. Earlier studies consider highly idealized dynamical friction, but neglect the effects of dynamical mass loss.
Tuesday, 19 November 2019
The Radial Acceleration Relation is just not a thing so let's all shut up about it
A long time ago the Radial Acceleration Relation was quite the thing for galactic dynamic studies. There's a tight correlation between the acceleration predicted by the observed material and the actual observed acceleration, which is unexpected because of the enormous amount of invisible dark matter. MOND supporters danced with joy at a successful prediction, only for standard model advocates to shoot them down days later, pointing out that the exact same thing was observed in their own simulations.
It took quite some time, though, before anyone gave a credible explanation as to why this effect was actually seen in dark matter-dominated models, where naively one wouldn't expect a connection between normal and dark matter. Plenty of people waved their hands with loud cries of, "SCALING RELATIONS !", but no-one really understood what they were on about.
Finally some clever chaps managed to explain these scaling relations in a reasonably convincing way. In brief, galaxies can't form in the largest (because of feedback and other effects) or smallest (because they don't have enough gravity to accrete much gas) dark matter halos. And galactic dark matter halos have a characteristic density profile in the standard model, giving rise to the observed relation.
But the others of the current paper aren't satisfied. They want to know just what it is that's happening physically to cause this. What exactly is it about the observed baryonic matter that could lead to this behaviour ? Why does it match the MOND predictions ? "This connection", they say, "must be in some sense “universal.” " I won't say I fully understand it, but their proposed solution feels quite elegant to me.
Normally astronomers think of star formation as being related to the gas density. More gas => more self-gravity => more stars. We normally assume that the density of the dark matter is low so that we can neglect it. Here the authors implicitly say, "yes, that's usually true, but it's not technically accurate, now is it ?". After all, what drives the collapse of the gas is the total gravitational field, not only the mass of gas. We can't possibly expect to discover a dark matter-baryon connection without including the dark matter.
Their model goes like this. Star formation proceeds efficiently if the total acceleration is above some threshold, whereas if it's below this threshold then there won't be much star formation at all. In regions initially dominated by dark matter, and below the threshold, few baryons will ever be accreted : the feedback from the few stars which do form will easily suppress further star formation by blowing the gas away. So these regions remain dominated by dark matter, i.e. dwarf galaxies and galaxy outskirts. In the opposite case of regions initially dominated by baryons and above the threshold, star formation will proceed much more efficiently. Feedback will be less effective at disrupting the gas inflow due to the stronger gravitational field, so these regions will always be dominated by baryons. Thus the acceleration threshold marks the boundary between regions dominated by baryons or dark matter.
This means there's a physical reason to expect the observed change in the RAR at low masses. They go further, saying that this also predicts the characteristic scaling behaviour at the different extremes, and even why galaxies generally have flat rotation curves : it's not a "conspiracy", as MONDian advocates worry, but an entirely natural effect of stellar feedback in regions dominated by dark matter - a flat rotation curve (if I understand them correctly) is the natural form of a dark matter halo, and it's only the presence of baryons that disrupts this. They also say that this all in no way implies that they expect the relation to be perfect or perfectly universal : variations in the stellar mass function will alter the feedback, so there could certainly be deviations from galaxy to galaxy; they can't make detailed predictions with this hugely simplified model - the point is to predict basic scalings, not develop a new and exact model of galaxy formation.
Again, I need to think a lot more about this. At first glance it seems like a neat way to universally connect the microphysics of star formation with the more global parameter of acceleration. I wonder how far this can be pushed to explain star formation despite its limitations : what happens in the other cases, if dark matter dominates but the acceleration is above the threshold, or if baryons dominate but the acceleration is low ? What, if anything, does this suggest for ultra diffuse and dark galaxies ? Definitely interesting, and this is a case where I wholly support the authors putting the paper on arXiv and asking for feedback* before publications.
* "Feedback is welcome", they say, leaving it to the reader to decide if that's a pun or not.
It took quite some time, though, before anyone gave a credible explanation as to why this effect was actually seen in dark matter-dominated models, where naively one wouldn't expect a connection between normal and dark matter. Plenty of people waved their hands with loud cries of, "SCALING RELATIONS !", but no-one really understood what they were on about.
Finally some clever chaps managed to explain these scaling relations in a reasonably convincing way. In brief, galaxies can't form in the largest (because of feedback and other effects) or smallest (because they don't have enough gravity to accrete much gas) dark matter halos. And galactic dark matter halos have a characteristic density profile in the standard model, giving rise to the observed relation.
But the others of the current paper aren't satisfied. They want to know just what it is that's happening physically to cause this. What exactly is it about the observed baryonic matter that could lead to this behaviour ? Why does it match the MOND predictions ? "This connection", they say, "must be in some sense “universal.” " I won't say I fully understand it, but their proposed solution feels quite elegant to me.
Normally astronomers think of star formation as being related to the gas density. More gas => more self-gravity => more stars. We normally assume that the density of the dark matter is low so that we can neglect it. Here the authors implicitly say, "yes, that's usually true, but it's not technically accurate, now is it ?". After all, what drives the collapse of the gas is the total gravitational field, not only the mass of gas. We can't possibly expect to discover a dark matter-baryon connection without including the dark matter.
Their model goes like this. Star formation proceeds efficiently if the total acceleration is above some threshold, whereas if it's below this threshold then there won't be much star formation at all. In regions initially dominated by dark matter, and below the threshold, few baryons will ever be accreted : the feedback from the few stars which do form will easily suppress further star formation by blowing the gas away. So these regions remain dominated by dark matter, i.e. dwarf galaxies and galaxy outskirts. In the opposite case of regions initially dominated by baryons and above the threshold, star formation will proceed much more efficiently. Feedback will be less effective at disrupting the gas inflow due to the stronger gravitational field, so these regions will always be dominated by baryons. Thus the acceleration threshold marks the boundary between regions dominated by baryons or dark matter.
This means there's a physical reason to expect the observed change in the RAR at low masses. They go further, saying that this also predicts the characteristic scaling behaviour at the different extremes, and even why galaxies generally have flat rotation curves : it's not a "conspiracy", as MONDian advocates worry, but an entirely natural effect of stellar feedback in regions dominated by dark matter - a flat rotation curve (if I understand them correctly) is the natural form of a dark matter halo, and it's only the presence of baryons that disrupts this. They also say that this all in no way implies that they expect the relation to be perfect or perfectly universal : variations in the stellar mass function will alter the feedback, so there could certainly be deviations from galaxy to galaxy; they can't make detailed predictions with this hugely simplified model - the point is to predict basic scalings, not develop a new and exact model of galaxy formation.
Again, I need to think a lot more about this. At first glance it seems like a neat way to universally connect the microphysics of star formation with the more global parameter of acceleration. I wonder how far this can be pushed to explain star formation despite its limitations : what happens in the other cases, if dark matter dominates but the acceleration is above the threshold, or if baryons dominate but the acceleration is low ? What, if anything, does this suggest for ultra diffuse and dark galaxies ? Definitely interesting, and this is a case where I wholly support the authors putting the paper on arXiv and asking for feedback* before publications.
* "Feedback is welcome", they say, leaving it to the reader to decide if that's a pun or not.
Stellar feedback sets the universal acceleration scale in galaxies
It has been established for decades that rotation curves deviate from the Newtonian gravity expectation given baryons alone below a characteristic acceleration scale $g_\dagger \sim 10^{-8}\,\rm{cm\,s^{-2}}$, a scale promoted to a new fundamental constant in MOND-type theories.
Twinkle, twinkle, little Starlink...
Well this is worrying.
Last night, they were taking about 40 exposures of the night sky, looking towards the small and large magellanic clouds, two dwarf galaxies that neighbor the Milky Way. But during one set of those observations, 90 minutes before sunrise, the train of SpaceX’s Starlink satellites moved into view, glinting in the early morning sunlight and taking five minutes to pass across the telescope's line of sight.
"This happened just before astronomical twilight," says Johnson. "By almost any observing standard this was still the heart of the night, exactly when you want to be taking data. And especially when you want to use every minute of observing time you get on these telescopes and these facilities."There are several factors that could affect how much worse this will get :
- The number of satellites : "So far, SpaceX has launched just 0.14 percent of its total planned Starlink constellation.... Bassa has calculated that up to 140 satellites could be visible at any one time if all the planned satellites launch.". Doesn't specify what's meant by "visible" here, i.e. in what field of view.
- The altitude of the satellites : "The satellites were deployed in a long train at an altitude of 280 kilometers... but are in the process of being raised to their operational altitude of 550 kilometers". The higher they are, the longer they'll reflect sunlight, though they will also be fainter.
- The albedo of the satellites : "...painting future Starlink satellites black to reduce their reflectivity, although it’s not believed this was done for this latest batch – while the glinting of the large solar panels on each satellite still poses a problem."
[Walks away angrily singing the Firefly theme tune...]
'This Is Not Cool!' - Astronomers Despair As SpaceX Starlink Train Ruins Observation Of Nearby Galaxies
In the early hours of the morning today, November 18, two astronomers checked in on their remotely operated telescope in Chile, expecting to see images of distant stars and galaxies. Instead, they saw a train of SpaceX satellites crossing the night sky, a worrying sign of what might be to come for astronomy.
Quite queerly quiescent
Why do some galaxies burn through their gas like a fire in an un-raked forest while some are content to sit back and relax ? In hand-wavy terms it's probably some combination of internal and environmental effects. In more massive galaxies the gas density can trigger more star formation throughout the disc, while in smaller ones the gas density tends to be lower. At the same time, when stars from in low-mass galaxies, their feedback effects can be more effective at suppressing further star formation by lowering the gas density still further, since there's less gravity to work again. Then again, high mass galaxies won't be much effected by encounters with other galaxies unless they're similarly massive, whereas low mass galaxies are much more vulnerable. So this simple question is not easy to answer properly.
What would be nice to have is so basic you'd think we'd already have it : some kind of census of the gas content of galaxies in different environments at different distances, masses, and star formation rate. Getting this is more difficult than you might think, because gas measurements are difficult except in the relatively nearby Universe. Getting a statistically significant sample, and understanding whether each galaxy is being affected by its own internal process of environment, is not an easy task.
The authors of this paper attempt to simplify things by looking at how the gas content varies in massive galaxies as a function of star formation rate. They have a sample of about 9,500 galaxies from the ALFALFA HI survey, matched with SDSS data to get star formation rates and stellar masses. They correct for their sample incompleteness and other statistical biases, so their final results should be an accurate representation of what's really going on.
Remarkably, they find that the atomic gas content of disc galaxies doesn't vary much at all even as star formation rate varies by a factor of a hundred. The proportion of disc galaxies with HI detections doesn't vary with star formation rate either (though it's not clear to me if this is the case for elliptical galaxies, which are anyway hugely biased towards low star formation activity). Nor does their gas fraction vary as a function of star formation rate either. And even the average HI spectrum of the star-forming and quiescent galaxies look incredibly similar.
It's a different story for the molecular gas. This shows a very clear, neat, strong trend, increasing in mass with star formation rate. That's not too surprising, as the consensus has been building for a quite a while that molecular gas correlates much more strongly with star formation activity than atomic, but it's nice to see. But why does this happen ? They say, "These galaxies are quenched because of their significantly reduced molecular gas and dust content and lower star formation efficiency", but this is a tautologous description and not an explanation. Galaxies which are quenched have low star formation activity by definition !
More interestingly, they note that the similar spectra imply that both quiescent and star-forming galaxies in their sample are likely rotating discs. They suggest that once the inner gas (which is denser) has been consumed by star formation, it takes a long time for the outer, less dense gas to either form stars or migrate inwards. So it just sits their, slowly rotating and generally doing sod all.
What would be nice to see next is a more detailed look at some of those individual galaxies. They make a testable prediction that the HI should be found in rings in quiescent galaxies, since the innermost gas will have been consumed. It would also be nice to describe their environments in a lot more detail : they say they work with "central" galaxies, but this doesn't help much. If they're central cluster galaxies then I'd be very surprised indeed if the quenching happened inside-out, since that's the exact opposite of how ram pressure normally removes gas in such galaxies. And I'd like to know a lot more about how they stacked the spectra - I'm very surprised that the velocity widths of the galaxies are apparently all so similar. It would also be nice to see what happens if they use specific (instead of global) star formation rates - that is, the star formation rate per unit mass.
The other thing I wonder about is this paper. Really massive disc galaxies have less baryonic mass than expected given how fast they rotate, which the authors there suggested might point to an upper limit to galaxy formation : above a certain mass, gas may be unable to cool and form stars. But this current paper says you can have really quite large gas reservoirs that disdain star formation, so perhaps things at the high mass end get more complicated. It would be interesting to see how the two samples compare, at least. It's nice to see such very clear evidence that it's molecular gas that matter most for star formation, but there's a lot more left to do to understand as to how atomic gas is converted into molecular.
What would be nice to have is so basic you'd think we'd already have it : some kind of census of the gas content of galaxies in different environments at different distances, masses, and star formation rate. Getting this is more difficult than you might think, because gas measurements are difficult except in the relatively nearby Universe. Getting a statistically significant sample, and understanding whether each galaxy is being affected by its own internal process of environment, is not an easy task.
The authors of this paper attempt to simplify things by looking at how the gas content varies in massive galaxies as a function of star formation rate. They have a sample of about 9,500 galaxies from the ALFALFA HI survey, matched with SDSS data to get star formation rates and stellar masses. They correct for their sample incompleteness and other statistical biases, so their final results should be an accurate representation of what's really going on.
Remarkably, they find that the atomic gas content of disc galaxies doesn't vary much at all even as star formation rate varies by a factor of a hundred. The proportion of disc galaxies with HI detections doesn't vary with star formation rate either (though it's not clear to me if this is the case for elliptical galaxies, which are anyway hugely biased towards low star formation activity). Nor does their gas fraction vary as a function of star formation rate either. And even the average HI spectrum of the star-forming and quiescent galaxies look incredibly similar.
It's a different story for the molecular gas. This shows a very clear, neat, strong trend, increasing in mass with star formation rate. That's not too surprising, as the consensus has been building for a quite a while that molecular gas correlates much more strongly with star formation activity than atomic, but it's nice to see. But why does this happen ? They say, "These galaxies are quenched because of their significantly reduced molecular gas and dust content and lower star formation efficiency", but this is a tautologous description and not an explanation. Galaxies which are quenched have low star formation activity by definition !
More interestingly, they note that the similar spectra imply that both quiescent and star-forming galaxies in their sample are likely rotating discs. They suggest that once the inner gas (which is denser) has been consumed by star formation, it takes a long time for the outer, less dense gas to either form stars or migrate inwards. So it just sits their, slowly rotating and generally doing sod all.
What would be nice to see next is a more detailed look at some of those individual galaxies. They make a testable prediction that the HI should be found in rings in quiescent galaxies, since the innermost gas will have been consumed. It would also be nice to describe their environments in a lot more detail : they say they work with "central" galaxies, but this doesn't help much. If they're central cluster galaxies then I'd be very surprised indeed if the quenching happened inside-out, since that's the exact opposite of how ram pressure normally removes gas in such galaxies. And I'd like to know a lot more about how they stacked the spectra - I'm very surprised that the velocity widths of the galaxies are apparently all so similar. It would also be nice to see what happens if they use specific (instead of global) star formation rates - that is, the star formation rate per unit mass.
The other thing I wonder about is this paper. Really massive disc galaxies have less baryonic mass than expected given how fast they rotate, which the authors there suggested might point to an upper limit to galaxy formation : above a certain mass, gas may be unable to cool and form stars. But this current paper says you can have really quite large gas reservoirs that disdain star formation, so perhaps things at the high mass end get more complicated. It would be interesting to see how the two samples compare, at least. It's nice to see such very clear evidence that it's molecular gas that matter most for star formation, but there's a lot more left to do to understand as to how atomic gas is converted into molecular.
Nearly all Massive Quiescent Disk Galaxies Have a Surprisingly Large Atomic Gas Reservoir
The massive galaxy population above the characteristic Schechter mass M * ≈ 10 10.6 {M} ☉ contributes to about half of the total stellar mass in the local universe. These massive galaxies usually reside in hot dark matter halos above the critical shock-heating mass ̃10 12 {M} ☉ , where the external cold gas supply to these galaxies is expected to be suppressed.
Stripping with jelly
You might remember a paper back in September that found tails of gas from a couple of galaxies around 6 billion light years (3 Gpc) away. Here's another, similar paper looking at a galaxy quite a bit closer (1.2 Gpc). Which makes it a bit odd that the first line declares this to be the first such study at intermediate redshifts. Either :
- Their paper completed the submission process before publication of the other one
- They submitted later but both the authors and referee were unaware of the other paper
- They're doing a Donald Trump, like when he declared that impeachment required both high crimes and misdemeanours. Perhaps they deem the other galaxies to be at high redshift, so it's okay that this one is only at intermediate redshift.
I also found their introduction to oddly imply that mergers between galaxies are not an effect of environment, but that's nick-picking. Otherwise it's nice.
They note that the effects of ram pressure stripping, the primary way of forming long one-sided tails, are still unclear. Given long enough and it will strip all the gas in a galaxy, quenching its star formation. But what happens during that process is potentially much more complicated : it might immediately reduce star formation by lowering the gas density, but it might also trigger star formation by compressing the gas (at least at the point of collision with the external gas). So a galaxy could potentially have its star formation reduced in some areas but temporarily increased in others.
Even more complex and controversial is what happens to the gas that gets stripped. Molecular gas is so dense that it probably doesn't get stripped directly, though some people think it might be possible in extreme circumstances. A few galaxies - not many mind - seem to have stellar tails as well as gaseous ones. It's unclear if these star-forming wakes result from direct stripping of molecular gas or if the molecular gas forms in the tail from the stripped atomic gas. And goodness only knows what happens to the stars formed in the tails after all the hullabaloo is over.
The galaxy they study here is a particularly dramatic case, forming stars as much as five times faster than other galaxies this massive. It's also a very massive galaxy, making material harder to strip. Yet it shows very clear tails and tentacles extending quite neatly and continuously from its disc - it is, as they say, a textbook jellyfish galaxy. What seems quite clear in this case is that the star-forming regions they see in the ultra-violet (which traces hot young stars) aren't just found in the tentacles, suggesting that ram pressure can indeed trigger star formation within the disc as well as in the stripped wake. Of course it's hard to be sure, since we don't know what the galaxy was like before stripping began, but the visual is awfully convincing.
Some of their other points are less clear. I don't say they're wrong, only that it's not obvious to me how they reach certain conclusions. They say, for example, that the stripped gas lags behind in velocity compared to the disc gas, but it doesn't look like that to me in the figure. They also say that if they account for this lag then the galaxy is rotating exactly as expected, but again I don't see how they actually do this. They also try a simple simulation to work out the galaxy's trajectory, finding that it's consistent with infall along a filament from another nearby cluster, but their description seems unnecessarily geared towards experts in orbital dynamics.
It's still a nice paper. Perhaps in a few years we'll have statistically significant samples of such objects, and then things will get a lot more interesting.
Jellyfish: Resolving the kinematics of extreme ram-pressure stripping at $z\sim0.3$
We present and discuss results from the first spatially resolved kinematic study of ram-pressure stripping of a massive late-type galaxy at intermediate redshifts. Our target, the spectacular "jellyfish" galaxy A1758N\_JFG1, was previously identified as a fast-moving member of the equal-mass merger A1758N ($z=0.28$) with a star-formation rate of 48 M$_\odot$ yr$^{-1}$, far above the galaxy main sequence.
Monday, 18 November 2019
Wibbly-wobbly spacey-wacey
A few weeks ago I posted some first tests of displaying isosurfaces in Blender. This got interrupted by the receipt of a referee report, so now back to the wibbly-wobbly spacey-wacey stuff.
I used to poo-pooh isosurfaces as being inherently inferior to volumetric renders because they lose information. They also feel somewhat like cheating, because rendering a surface is easier and uses a lot less memory. While all this is true, I'm somewhat reconciled to their uses : isosurfaces depend far less strongly on viewing angle than volumetric renders. That makes them much easier to highlight features in an objective way, and contrary to my intuition, this can make them better for finding faint structures rather than worse. The eye tends to get very confused when you have really bright and faint sources together, but slap on a fixed level surface and BAM you can see if what you're looking at is significant or not. You might still miss the very faintest stuff, but there can be a surprising amount to see at relatively bright levels.
The other nice thing about isosurfaces is that because they're very fast to generate (typically seconds per surface in these examples, if not less), they're easy to animate. Last time I showed some fixed-level turntable animations. Here's M33 from the AGES HI survey shown at a variable flux level, starting with the brightest gas and ending with the faintest. Each frame decreases the flux to 97% of the previous value.
I used to poo-pooh isosurfaces as being inherently inferior to volumetric renders because they lose information. They also feel somewhat like cheating, because rendering a surface is easier and uses a lot less memory. While all this is true, I'm somewhat reconciled to their uses : isosurfaces depend far less strongly on viewing angle than volumetric renders. That makes them much easier to highlight features in an objective way, and contrary to my intuition, this can make them better for finding faint structures rather than worse. The eye tends to get very confused when you have really bright and faint sources together, but slap on a fixed level surface and BAM you can see if what you're looking at is significant or not. You might still miss the very faintest stuff, but there can be a surprising amount to see at relatively bright levels.
The other nice thing about isosurfaces is that because they're very fast to generate (typically seconds per surface in these examples, if not less), they're easy to animate. Last time I showed some fixed-level turntable animations. Here's M33 from the AGES HI survey shown at a variable flux level, starting with the brightest gas and ending with the faintest. Each frame decreases the flux to 97% of the previous value.
There's a whole bunch of clouds around M33, most of which show up quite well in the animation. It's not as good as manually tweaking the level of each region, but it basically works. I probably should have frozen the final level and rendered a full rotation, but never mind.
Here's the same sort of animation but using NGC 4361 from the WSRT HALOGAS survey, which also has a bunch of weird stuff going on around it :
And finally, here's a test of a true time series from a simulation. This is an old one from when I was trying to learn the FLASH hydrocode and my galaxy got all unstable because of a bug in the boundary conditions. Four fixed levels all animated, showing how the nice stable galaxy gradually goes fully wibbly-wobbly and eventually gives up. Does it help analyse what went wrong ? Not really, but it looks nice.
Subscribe to:
Posts (Atom)