This is a very nice, readable article challenging the myth that Einstein never called the cosmological constant his biggest blunder. In brief, the popular story goes (and still dominates today) that Einstein introduced this as an otherwise-unjustified term in his equations : he preferred a static universe, which the equations didn't allow. Later, on Hubble's discovery (simplifying) that the Universe was actually expanding, he changed his mind and regretted missing out on an amazing prediction that observations would have proven, famously using the expression that it was his "biggest blunder".
The now rather popular skeptical position is that actually he never used that term, and may not even have regarded it as such an important failure (https://www.theatlantic.com/technology/archive/2013/08/einstein-likely-never-said-one-of-his-most-oft-quoted-phrases/278508/). The quote is incredibly widely reported, so if nothing else it'd be nice to know if he really used it or not. The skeptical argument rests on there being only one source for the original quote : George Gamow, who wrote it down with a condescending sneer that Einstein was old and befuddled.
This article challenges this quite strongly, finding two other, independent sources of the quote. That alone makes it far more plausible. Less important for exact history, but much more interesting for the context, are their examinations of Gamow's character and the intent of his remark that "of course the old man agrees with anything nowaday". Gamow, they say, was underrated as a physicist precisely because of his mischevous humour, and they say this remark could be interpreted as Gamow actually being self-deprecating - essentially saying that Einstein's agreement with Gamow's theory wasn't the great honour one would normally assume it to be. A sort of backhanded insult, but more jovial and without the viciousness evident by reading the quote without context.
The most interesting part for me was a slightly tangential look at Gamow as a science populariser and how this wasn't seen as the desirable activity it's generally regarded as today. Paying tributed to Gamow, Wolfgang Yourgrau remarked :
Gamow committed an unforgivable sin. He wrote popular books on physics, biology and cosmology. Moreover, the books were bestsellers because they enabled the uninitiated not only to understand scientific discoveries and theories, but also to understand the human, often humorous facets of the researching men engaged in all of these mysterious ventures…. Most scientists do not fancy the oversimplifying, popularizing of our science…it is tantamount to a cheapening of the sacred rituals of our profession… many of us considered him washed up, a has-been, an intemperate member of our holy order.
http://adsabs.harvard.edu/abs/2018arXiv180406768O
Tuesday, 24 April 2018
How could a galaxy lose all its dark matter ?
This paper attempts to simulate the formation of that galaxy lacking in dark matter. It's somewhat taken for granted that while you can fake the appearance of additional dark matter through gravitational encounters - streaming motions along the line of sight are hard to distinguish from rotation, making it look as though a galaxy is rotating very quickly when actually it's just being disrupted. The opposite case, where a galaxy is made to appear deficient in dark matter but actually still has a lot of it, is probably not possible. So the authors try and get a tidal encounter to really remove as much dark matter from their target galaxy as possible.
As we all know, if you do this suddenly and completely, a galaxy will explode. But that requires magic. If you remove it gradually and leave a bit in the centre, there's no reason a galaxy can't survive quite happily. Oh, it'll be more vulnerable to more encounters in the future, but if it's left well alone it'll be fine.
Under the implicit assumption that this particularly weird object legitimately requires a weird progenitor, they therefore start with a target galaxy of unusually, but not exceptionally, low concentration. This means its dark matter halo is unusually extended and therefore easier to remove as it orbits around a more massive galactic thief. No doubt someone will calculate exactly how improbable this is and argue that the chances of detecting a galaxy that has a 1% chance of existing are a million to one, such are the dodgy statistics that seem to be in vogue in astronomy.
Anyway, their target galaxy has stars and dark matter, whereas the burglar galaxy (my term) is a purely analytic, fixed potential. This is a reasonable way to begin, although eventually I'd hope they add in a particle model for the second galaxy as well as including gas and star formation. Gas, in particular, could change things dramatically because it's collisional, but it's reasonable to speculate the target galaxy might have already depleted its gas supply. Using particles for the burglar's halo could also be important, since dynamical friction can increase the chance of a merger.
The authors find that indeed large amounts of dark matter can be stripped by this simple tidal process of close encounters, and the galaxy still survives. This is an important point, but what the paper does not yet adequately demonstrate (it's only submitted, not accepted) is how well the results compare to observations. NGC1052-DF2 is interesting because (criticism notwithstanding) it seems to have little or no dark matter not just in its outer halo, but everywhere. The authors say they reproduce the object's mass halo, but don't give a figure to demonstrate this or quantifiably compare the velocity dispersion of the stars in the simulations with the observations. Without this, the result that the most unbound dark matter can be removed is neither surprising nor novel. It's a decent beginning but the main claim hasn't been made very convincingly yet.
https://arxiv.org/abs/1804.06421
As we all know, if you do this suddenly and completely, a galaxy will explode. But that requires magic. If you remove it gradually and leave a bit in the centre, there's no reason a galaxy can't survive quite happily. Oh, it'll be more vulnerable to more encounters in the future, but if it's left well alone it'll be fine.
Under the implicit assumption that this particularly weird object legitimately requires a weird progenitor, they therefore start with a target galaxy of unusually, but not exceptionally, low concentration. This means its dark matter halo is unusually extended and therefore easier to remove as it orbits around a more massive galactic thief. No doubt someone will calculate exactly how improbable this is and argue that the chances of detecting a galaxy that has a 1% chance of existing are a million to one, such are the dodgy statistics that seem to be in vogue in astronomy.
Anyway, their target galaxy has stars and dark matter, whereas the burglar galaxy (my term) is a purely analytic, fixed potential. This is a reasonable way to begin, although eventually I'd hope they add in a particle model for the second galaxy as well as including gas and star formation. Gas, in particular, could change things dramatically because it's collisional, but it's reasonable to speculate the target galaxy might have already depleted its gas supply. Using particles for the burglar's halo could also be important, since dynamical friction can increase the chance of a merger.
The authors find that indeed large amounts of dark matter can be stripped by this simple tidal process of close encounters, and the galaxy still survives. This is an important point, but what the paper does not yet adequately demonstrate (it's only submitted, not accepted) is how well the results compare to observations. NGC1052-DF2 is interesting because (criticism notwithstanding) it seems to have little or no dark matter not just in its outer halo, but everywhere. The authors say they reproduce the object's mass halo, but don't give a figure to demonstrate this or quantifiably compare the velocity dispersion of the stars in the simulations with the observations. Without this, the result that the most unbound dark matter can be removed is neither surprising nor novel. It's a decent beginning but the main claim hasn't been made very convincingly yet.
https://arxiv.org/abs/1804.06421
Friday, 20 April 2018
Paper re-submitted with a slightly modified title...
Apparently the first one wasn't informative enough. So now we've gone with "Optically dark HI clouds in the Virgo cluster : will no-one rid me of this turbulent sphere ?"
We've kept the acknowledgement to Henry II though. I shall be thoroughly irked if they ask us to take that out.
We've kept the acknowledgement to Henry II though. I shall be thoroughly irked if they ask us to take that out.
Thursday, 12 April 2018
The life of a scientist explained in a simple flow chart
Actually, British universities exist to facilitate the drinking of tea. Any research that gets done is considered a bonus, or in extreme cases, detrimental. 9:30am - start off with a cuppa. That'll last you through to official tea time at 10:30, which can easily be extended until lunchtime, wherein you'll need another cup of tea to prepare you for the afternoon. Fortunately that could take so long that you end up in official afternoon tea, which will probably last until it's time to go home, full of delicious tea and with none of that pesky "research" having slowed you down.
Sunday, 8 April 2018
A grant lottery
Seems like a clear case of "let's try it and see" to me.
Implicit in this proposal is the idea that it isn’t possible to rank applications reliably. If a lottery approach meant we ended up funding weak research and denying funds to excellent project, this would clearly be a bad thing. But research rankings by committee and/or peer review is notoriously unreliable, and it is hard to compare proposals that span a range of disciplines. Many people feel that funding is already a lottery, albeit an unintentional one, because the same grant that succeeds in one round may be rejected in the next. Interviews are problematic because they mean that a major decision – fund or not – is decided on the basis of a short sample of a candidate’s behaviour, and that people with great proposals but poor social skills may be turned down in favour of glib individuals who can sell themselves more effectively.
My view is that there are advantages for the lottery approach over and above the resource issues. First, Avin’s analysis concludes that reliance on peer review leads to a bias against risk-taking, which can mean that novelty and creativity are discouraged. Second, once a proposal was in the pool, there would be no scope for bias against researchers in terms of gender or race – something that can be a particular concern when interviews are used to assess. Third, the impact on the science community is also worth considering. Far less grief would be engendered by a grant rejection if you knew it was that you were unlucky, rather than that you were judged to be wanting. Furthermore, as noted by Marina Papoutsi, some institutions evaluate their staff in terms of how much grant income they bring in – a process that ignores the strong element of chance that already affects funding decisions. A lottery approach, where the randomness is explicit, would put paid to such practices.
http://deevybee.blogspot.com/2018/04/should-research-funding-be-allocated-at.html
Implicit in this proposal is the idea that it isn’t possible to rank applications reliably. If a lottery approach meant we ended up funding weak research and denying funds to excellent project, this would clearly be a bad thing. But research rankings by committee and/or peer review is notoriously unreliable, and it is hard to compare proposals that span a range of disciplines. Many people feel that funding is already a lottery, albeit an unintentional one, because the same grant that succeeds in one round may be rejected in the next. Interviews are problematic because they mean that a major decision – fund or not – is decided on the basis of a short sample of a candidate’s behaviour, and that people with great proposals but poor social skills may be turned down in favour of glib individuals who can sell themselves more effectively.
My view is that there are advantages for the lottery approach over and above the resource issues. First, Avin’s analysis concludes that reliance on peer review leads to a bias against risk-taking, which can mean that novelty and creativity are discouraged. Second, once a proposal was in the pool, there would be no scope for bias against researchers in terms of gender or race – something that can be a particular concern when interviews are used to assess. Third, the impact on the science community is also worth considering. Far less grief would be engendered by a grant rejection if you knew it was that you were unlucky, rather than that you were judged to be wanting. Furthermore, as noted by Marina Papoutsi, some institutions evaluate their staff in terms of how much grant income they bring in – a process that ignores the strong element of chance that already affects funding decisions. A lottery approach, where the randomness is explicit, would put paid to such practices.
http://deevybee.blogspot.com/2018/04/should-research-funding-be-allocated-at.html
Friday, 30 March 2018
How to write a pseudoscience email
Heh, that's every pseudoscience email ever.
Found on the internet :
[AUTHOR] is [BASICALLY, ENTIRELY, PRECISELY] [WRONG / RIGHT].
To be fair, [CONCESSION]. But [UNCHARITABLE RETRACTION OF THAT CONCESSION]. In this case, [SPECIAL PLEADING].
Presuming [THING WHICH IT IS INSANE TO PRESUME], then [OVERSTATED CONCLUSION.] But if [INSANE MISSTATEMENT OF OPPOSING ARGUMENT], then [UNCONDITIONAL RETRACTION OF MY BASIC POINT].
When [THING WHICH WILL NEVER HAPPEN] happens, then I will be proven conclusively right. But until that time, no one can criticize me for having made unfalsifiable conclusions, because they were merely conditional at the time I made them.
[SHORT STATEMENT WHICH IS LESS PITHY THAN I INTENDED]
Found on the internet :
[AUTHOR] is [BASICALLY, ENTIRELY, PRECISELY] [WRONG / RIGHT].
To be fair, [CONCESSION]. But [UNCHARITABLE RETRACTION OF THAT CONCESSION]. In this case, [SPECIAL PLEADING].
Presuming [THING WHICH IT IS INSANE TO PRESUME], then [OVERSTATED CONCLUSION.] But if [INSANE MISSTATEMENT OF OPPOSING ARGUMENT], then [UNCONDITIONAL RETRACTION OF MY BASIC POINT].
When [THING WHICH WILL NEVER HAPPEN] happens, then I will be proven conclusively right. But until that time, no one can criticize me for having made unfalsifiable conclusions, because they were merely conditional at the time I made them.
[SHORT STATEMENT WHICH IS LESS PITHY THAN I INTENDED]
Thursday, 29 March 2018
The Coolest Galaxy In The Universe
The press releases about a galaxy without any dark matter are doing the rounds, but are they accurate ? For once, yes ! This is a really genuinely strange object.
"To our knowledge this is the coolest known galaxy outside of the Local Group", say the authors. Sadly this galaxy isn't decked out with bling - they just mean it has a low velocity dispersion (which is sort-of equivalent to temperature). Measuring individual stars at this 20 Mpc (65 million light years) distance isn't possible, so they use globular clusters instead. These are much brighter, so they're easier to measure. Galaxies with large dark matter components (which is pretty nearly all of them) have their globular clusters orbiting around them like a swarm of normally very fast-moving bees.
In this case the bees are much more lethargic. The galaxy is of comparable size to the Milky Way, which rotates at about 220 km/s, but the globular clusters in this object are moving at only about 10 km/s (or even less). Speed measurements let you estimate total mass without needing to know anything about how bright the stars are, and usually masses measured in this way are 10x more than estimated by their brightness (because of the invisible dark matter). Not in this case though : its total mass is about 400 times less than typical galaxies of this size, and the motions can be explained entirely from the stellar mass. It doesn't seem to have any dark matter at all, which is almost unprecedented.
Unfortunately even the number of available globular clusters is small, just ten. Normally I wouldn't put much stock in such small numbers, but in this case the velocity of the clusters is so close to that of the galaxy it's difficult to believe this could happen by chance. There's no obvious reason for any selection effects here : globular clusters should be at random positions and velocities around their parent galaxy.
Of course there are some caveats. Not quite all galaxies lack dark matter. Tidal dwarf galaxies, produced by galaxies tearing bits off each other, seem to be pretty nearly devoid of the stuff. So could that explain this oddball ?
Probably not. Tidal dwarfs are well-accepted when you can actually see the tearing in action, e.g. denser clumps within long stellar streams, but much more controversial for other objects. This galaxy is in an unhappy middle ground : at only 80 kpc (260,000 light years) from the nearest bright galaxy, with other objects nearby, there's certainly scope for tidal interactions to have occurred. It's not close enough to say it's probably tidal, nor isolated enough to say that it probably isn't.
But against this, it's a very smooth, regular looking object, with no signs of long extended features. The encounters could have happened in the distant past and the fainter streams dispersed, but this galaxy is very large and very low-mass : it should be vulnerable to being disrupted itself by further encounters. It's tough to see how it could survive for very long. Actually this is true regardless of its origins, which would make for some fun simulations.
Another possibility is that the distance has been measured incorrectly, but that doesn't seem plausible either : the distances look solid, and if it was significantly different then it would have a very strong peculiar velocity. That can happen in massive groups and clusters, where the gravity of the group as a whole can accelerate individual galaxies to tremendous speeds. But this group is little, so that shouldn't happen. It would be weird, but, as the authors put it, "it is difficult to argue that it is less likely than having a highly peculiar globular cluster population and a lack of dark matter." Weird objects do tend to require weird explanations, after all... the key point is to check if this weird explanation is at least possible in this environment.
Could it just be a galaxy that's rotating but we're seeing it face-on ? In that case we wouldn't be able to measure the rotation since we can only do that for line of sight movement. The authors don't like this because discs normally have visible structures like spiral arms and whatnot, whereas this one doesn't. I'd caution that some lenticular galaxies look very smooth indeed - although they often do have some structure, it can be very low contrast.
And then there's the second, less-reported paper on the globular clusters themselves - they're a bit odd as well. Like other UDG's, the galaxies got literally tonnes* of globular clusters, but in this case they're generally more elongated and about four times brighter. So a weird galaxy surrounded by weird star clusters. Fun times !
* OK, lots of tonnes.
So what's going on ? Does this point to galaxy formation occurring by multiple mechanisms, with some requiring huge amounts of dark matter but some without ? Dunno. To answer that, we need more statistics both on this object and others. Lots of ultra-diffuse galaxies are known, but dynamical mass estimates are still very rare.
Does it challenge our ideas of gravity ? Well, the authors say it disfavours popular alternatives to dark matter. They rely on reproducing the dark matter evidence just from the stars and gas alone, so in those models all galaxies should appear to have dark matter. There's no reason some galaxies should have a fake appearance of missing mass while others don't. In contrast, if dark matter does exist then it's a bit easier to believe that maybe some galaxies just don't have very much of it - there's no problem in principle here, at least.
A caveat is that some alternative theories of gravity are bloody complicated, and the external gravity of nearby galaxies can change the dynamics. This "external field effect" is routinely invoked to explain just about any problems with such theories, which is extremely irritating. But in this case, since there are nearby massive galaxies, the effect can probably be at least estimated.
So for once the press release matches the paper, and neither seem unreasonable. But, as usual, this raises more questions than answers.
Related reading :
https://arxiv.org/abs/1803.10237
https://arxiv.org/abs/1803.10240
http://astrorhysy.blogspot.cz/2017/07/ultra-diffuse-galaxies-revenge-of-ghosts.html
https://arxiv.org/abs/1803.10237
"To our knowledge this is the coolest known galaxy outside of the Local Group", say the authors. Sadly this galaxy isn't decked out with bling - they just mean it has a low velocity dispersion (which is sort-of equivalent to temperature). Measuring individual stars at this 20 Mpc (65 million light years) distance isn't possible, so they use globular clusters instead. These are much brighter, so they're easier to measure. Galaxies with large dark matter components (which is pretty nearly all of them) have their globular clusters orbiting around them like a swarm of normally very fast-moving bees.
In this case the bees are much more lethargic. The galaxy is of comparable size to the Milky Way, which rotates at about 220 km/s, but the globular clusters in this object are moving at only about 10 km/s (or even less). Speed measurements let you estimate total mass without needing to know anything about how bright the stars are, and usually masses measured in this way are 10x more than estimated by their brightness (because of the invisible dark matter). Not in this case though : its total mass is about 400 times less than typical galaxies of this size, and the motions can be explained entirely from the stellar mass. It doesn't seem to have any dark matter at all, which is almost unprecedented.
Unfortunately even the number of available globular clusters is small, just ten. Normally I wouldn't put much stock in such small numbers, but in this case the velocity of the clusters is so close to that of the galaxy it's difficult to believe this could happen by chance. There's no obvious reason for any selection effects here : globular clusters should be at random positions and velocities around their parent galaxy.
Of course there are some caveats. Not quite all galaxies lack dark matter. Tidal dwarf galaxies, produced by galaxies tearing bits off each other, seem to be pretty nearly devoid of the stuff. So could that explain this oddball ?
Probably not. Tidal dwarfs are well-accepted when you can actually see the tearing in action, e.g. denser clumps within long stellar streams, but much more controversial for other objects. This galaxy is in an unhappy middle ground : at only 80 kpc (260,000 light years) from the nearest bright galaxy, with other objects nearby, there's certainly scope for tidal interactions to have occurred. It's not close enough to say it's probably tidal, nor isolated enough to say that it probably isn't.
But against this, it's a very smooth, regular looking object, with no signs of long extended features. The encounters could have happened in the distant past and the fainter streams dispersed, but this galaxy is very large and very low-mass : it should be vulnerable to being disrupted itself by further encounters. It's tough to see how it could survive for very long. Actually this is true regardless of its origins, which would make for some fun simulations.
Another possibility is that the distance has been measured incorrectly, but that doesn't seem plausible either : the distances look solid, and if it was significantly different then it would have a very strong peculiar velocity. That can happen in massive groups and clusters, where the gravity of the group as a whole can accelerate individual galaxies to tremendous speeds. But this group is little, so that shouldn't happen. It would be weird, but, as the authors put it, "it is difficult to argue that it is less likely than having a highly peculiar globular cluster population and a lack of dark matter." Weird objects do tend to require weird explanations, after all... the key point is to check if this weird explanation is at least possible in this environment.
Could it just be a galaxy that's rotating but we're seeing it face-on ? In that case we wouldn't be able to measure the rotation since we can only do that for line of sight movement. The authors don't like this because discs normally have visible structures like spiral arms and whatnot, whereas this one doesn't. I'd caution that some lenticular galaxies look very smooth indeed - although they often do have some structure, it can be very low contrast.
And then there's the second, less-reported paper on the globular clusters themselves - they're a bit odd as well. Like other UDG's, the galaxies got literally tonnes* of globular clusters, but in this case they're generally more elongated and about four times brighter. So a weird galaxy surrounded by weird star clusters. Fun times !
* OK, lots of tonnes.
So what's going on ? Does this point to galaxy formation occurring by multiple mechanisms, with some requiring huge amounts of dark matter but some without ? Dunno. To answer that, we need more statistics both on this object and others. Lots of ultra-diffuse galaxies are known, but dynamical mass estimates are still very rare.
Does it challenge our ideas of gravity ? Well, the authors say it disfavours popular alternatives to dark matter. They rely on reproducing the dark matter evidence just from the stars and gas alone, so in those models all galaxies should appear to have dark matter. There's no reason some galaxies should have a fake appearance of missing mass while others don't. In contrast, if dark matter does exist then it's a bit easier to believe that maybe some galaxies just don't have very much of it - there's no problem in principle here, at least.
A caveat is that some alternative theories of gravity are bloody complicated, and the external gravity of nearby galaxies can change the dynamics. This "external field effect" is routinely invoked to explain just about any problems with such theories, which is extremely irritating. But in this case, since there are nearby massive galaxies, the effect can probably be at least estimated.
So for once the press release matches the paper, and neither seem unreasonable. But, as usual, this raises more questions than answers.
Related reading :
https://arxiv.org/abs/1803.10237
https://arxiv.org/abs/1803.10240
http://astrorhysy.blogspot.cz/2017/07/ultra-diffuse-galaxies-revenge-of-ghosts.html
https://arxiv.org/abs/1803.10237
Wednesday, 28 March 2018
The giants have failed... or have they ?
Quite a nice little letter about ultra diffuse galaxies, those huge ghostly galaxies with hardly any stars. Do they have a crapload of dark matter, as their huge size suggests, or not that much at all, in proportion to their pathetic stellar mass ?
Thus far the issue has been pretty one-sided. Van Dokkum et al. keep claiming that they're probably massive, "failed" galaxies that do a lousy job at forming stars (which would pretty much break the current paradigm of galaxy formation); everyone else seems to think they're not very massive objects but somehow became very spread out. This paper sees a new team join in on the side of the failed giants.
The authors have spectroscopic measurements of globular clusters in and around 3 UDGs in the Virgo cluster (for some reason, Virgo doesn't seem to have as many UDGs as other clusters). Unfortunately the numbers of globular clusters is very low in each case, around 10. Still, the velocity dispersion seems high enough to indicate that all of them are heavily dark matter dominated. One of them looks like a reasonable candidate for a failed giant, another might be but it's more tentative. With this low number of data points, constructing nice rotation curves just isn't possible yet, although they do try.
Another possibility for these objects is that they could just be tidal debris stripped out of galaxies and not galaxies themselves at all. In this case their high velocity dispersion would just mean that they're disintegrating rather than being bound by a dark matter halo. They say this is particularly likely for one of the (probably less massive) objects, which looks disturbed. Personally I'd be a bit more cautious about the others too, but in general they do a nice job of describing the alternative explanations.
The problem with these super-faint smudges is that you can't really get a proper mass measurement even in the relatively small regions where the globular clusters are detected - with a high error, it's more of an estimate. They're not even sure if the objects are rotating. But this isn't too bad; what I really wish they'd state more directly is that the inferred total mass (assuming the dark matter halo extends like in normal galaxies) is about 100x the mass estimate from observations.
It's great to see more people examining the possibility that these things are really weird objects that don't fit the standard models (and this team is an eminently reputable one, so let's here no more talk of scientific closed-mindedness, thank you). But realistically, we're going to need much better data to say anything more definitive about them.
https://arxiv.org/abs/1803.09768
Thus far the issue has been pretty one-sided. Van Dokkum et al. keep claiming that they're probably massive, "failed" galaxies that do a lousy job at forming stars (which would pretty much break the current paradigm of galaxy formation); everyone else seems to think they're not very massive objects but somehow became very spread out. This paper sees a new team join in on the side of the failed giants.
The authors have spectroscopic measurements of globular clusters in and around 3 UDGs in the Virgo cluster (for some reason, Virgo doesn't seem to have as many UDGs as other clusters). Unfortunately the numbers of globular clusters is very low in each case, around 10. Still, the velocity dispersion seems high enough to indicate that all of them are heavily dark matter dominated. One of them looks like a reasonable candidate for a failed giant, another might be but it's more tentative. With this low number of data points, constructing nice rotation curves just isn't possible yet, although they do try.
Another possibility for these objects is that they could just be tidal debris stripped out of galaxies and not galaxies themselves at all. In this case their high velocity dispersion would just mean that they're disintegrating rather than being bound by a dark matter halo. They say this is particularly likely for one of the (probably less massive) objects, which looks disturbed. Personally I'd be a bit more cautious about the others too, but in general they do a nice job of describing the alternative explanations.
The problem with these super-faint smudges is that you can't really get a proper mass measurement even in the relatively small regions where the globular clusters are detected - with a high error, it's more of an estimate. They're not even sure if the objects are rotating. But this isn't too bad; what I really wish they'd state more directly is that the inferred total mass (assuming the dark matter halo extends like in normal galaxies) is about 100x the mass estimate from observations.
It's great to see more people examining the possibility that these things are really weird objects that don't fit the standard models (and this team is an eminently reputable one, so let's here no more talk of scientific closed-mindedness, thank you). But realistically, we're going to need much better data to say anything more definitive about them.
https://arxiv.org/abs/1803.09768
Friday, 23 March 2018
A huge hydrogen stream in an galaxy group no-one's ever heard of
Very interesting paper today about the discovery of an enormous, 500 kpc (about 1.6 million light years) hydrogen stream in an otherwise obscure galaxy group. Earlier observations with ATCA (Australian Telescope Compact Array) had found something, but observations with the shiny new Karoo Array Telescope (KAT-7) revealed a much larger, fainter structure. KAT-7 is only a precursor to the larger MeerKAT, which is itself a prototype for the even larger Square Kilometre Array, giving some indication of what we'll find with the next generation of radio telescopes.
For comparison, as far as extended gas goes Arecibo is about ten times more sensitive than KAT-7, which is itself about ten times more sensitive than the VLA. The other advantage of KAT-7 and other SKA "pathfinder" instruments is that the field of view is much larger than in the older facilities. Getting that sensitivity increased by another factor of 10, though, is a formidable challenge indeed, and new instrumentation on Arecibo could expand its field of view by a factor of a few. The VLA also possesses superior resolution. In short, radio instrumentation is fiendishly complicated, and anyone who thinks it isn't does not know what they're talking about.
Anywho, this new giant gas stream (see their figure 2) is in a fairly small galaxy group. In galaxy clusters, the main gas loss mechanism is believed to be ram pressure stripping, where galaxies move through the hot, thin surrounding gas. That can't really be the explanation here, since a) there isn't much in the way of external gas; b) the galaxies are moving too slowly. Which means the most likely explanation is that the galaxies are gravitationally interacting. Unlike ram pressure, that's actually more effective because of the low velocities, since it gives the gravitational forces more time to act.
This is sort of plausible for this case, but there are some interesting oddities. Their figure 4 shows higher resolution (but lower sensitivity) observations with the VLA of the likely parent galaxy of the stream. It does seem to be interacting with a nearby companion, in that they both have one-sided gaseous extensions. But normally such interactions produce two tails, characteristically on opposite sides. That doesn't seem to be the case here. The authors say there's a twin-tail structure, but I don't see it.
A lot of the other galaxies in the group also seem to be interacting. Taken together they form a single, coherent, giant structure. The velocity of the gas shows pretty convincingly (figure 3) that this is likely a single entity, not a chance alignment of different features. I suppose that's possible though, if the galaxies were all falling into the group along a filament. It also shows a strange bifurcation, with one part of the stream at one velocity but (at the same spatial location) other parts are at quite different velocities. And the velocity changes look to be pretty sharp. They also say that there's a cloud nearby with no optical counterpart, though it isn't clear to me which feature they mean by this or its velocity gradient.
In context this is especially interesting to me because we've shown how it's possible to make this "kinky" velocity structures in galaxy clusters (http://astrorhysy.blogspot.cz/2017/01/check-out-my-kinky-curves.html). But clusters are much more massive than groups, so I wouldn't necessarily expect the same effects in groups. Naively, I would expect the lower speed of the interactions to be better at drawing out long gas streams, but worse at causing kinks. Only more simulations will answer that one.
https://arxiv.org/abs/1803.08263
For comparison, as far as extended gas goes Arecibo is about ten times more sensitive than KAT-7, which is itself about ten times more sensitive than the VLA. The other advantage of KAT-7 and other SKA "pathfinder" instruments is that the field of view is much larger than in the older facilities. Getting that sensitivity increased by another factor of 10, though, is a formidable challenge indeed, and new instrumentation on Arecibo could expand its field of view by a factor of a few. The VLA also possesses superior resolution. In short, radio instrumentation is fiendishly complicated, and anyone who thinks it isn't does not know what they're talking about.
Anywho, this new giant gas stream (see their figure 2) is in a fairly small galaxy group. In galaxy clusters, the main gas loss mechanism is believed to be ram pressure stripping, where galaxies move through the hot, thin surrounding gas. That can't really be the explanation here, since a) there isn't much in the way of external gas; b) the galaxies are moving too slowly. Which means the most likely explanation is that the galaxies are gravitationally interacting. Unlike ram pressure, that's actually more effective because of the low velocities, since it gives the gravitational forces more time to act.
This is sort of plausible for this case, but there are some interesting oddities. Their figure 4 shows higher resolution (but lower sensitivity) observations with the VLA of the likely parent galaxy of the stream. It does seem to be interacting with a nearby companion, in that they both have one-sided gaseous extensions. But normally such interactions produce two tails, characteristically on opposite sides. That doesn't seem to be the case here. The authors say there's a twin-tail structure, but I don't see it.
A lot of the other galaxies in the group also seem to be interacting. Taken together they form a single, coherent, giant structure. The velocity of the gas shows pretty convincingly (figure 3) that this is likely a single entity, not a chance alignment of different features. I suppose that's possible though, if the galaxies were all falling into the group along a filament. It also shows a strange bifurcation, with one part of the stream at one velocity but (at the same spatial location) other parts are at quite different velocities. And the velocity changes look to be pretty sharp. They also say that there's a cloud nearby with no optical counterpart, though it isn't clear to me which feature they mean by this or its velocity gradient.
In context this is especially interesting to me because we've shown how it's possible to make this "kinky" velocity structures in galaxy clusters (http://astrorhysy.blogspot.cz/2017/01/check-out-my-kinky-curves.html). But clusters are much more massive than groups, so I wouldn't necessarily expect the same effects in groups. Naively, I would expect the lower speed of the interactions to be better at drawing out long gas streams, but worse at causing kinks. Only more simulations will answer that one.
https://arxiv.org/abs/1803.08263
Wednesday, 21 March 2018
I am super famous now
I haven't read the article yet. Stephen Hawking used one of my graphics !
http://www.rhysy.net/other-1.html
Confirmation 7 minutes in :
https://www.youtube.com/watch?v=QF9jAGyL1fg
Excuse me while I go and stroke my ego with a nice cup of tea.
“The questions that they’re attempting to answer are still valid, open questions, and the best this paper can do — if it’s correct and relevant, and it may be neither — is provide suggestions towards an answer. The approach is largely based off of work that Hartle, Hawking, and Hertog have done in the past, the dS/CFT connection pioneered by Chris Hull and others, along with string-inspired work done by Andrew Strominger and his collaborators. None of this is based off of any realistic cosmological models; these are toy models that they are calculating in, and then reasoning-by-analogy with what we actually know exists. Like most theoretical work in the very early stages, there are interesting ideas that are presented, the work and calculations are highly speculative, and there is not necessarily a connection with reality. But there’s a non-zero chance that one is real. And in theoretical physics, a novel idea with a chance is worth infinitely more than no new ideas at all.”
There have been a lot of incredible claims floating around the media about what’s going on with Stephen Hawking’s final paper, which was submitted earlier in March, less than two weeks before he died. Some are claiming it will help us detect the multiverse, others claiming that it will tell us how the Universe will end. The truth is much more sobering, however: it discusses issues involving the dynamics of inflation. There are incredible questions we’re trying to understand about the Universe, such as: did inflation begin, or was it eternal; will it continue indefinitely into the future; does it inevitably lead to a multiverse; did time and space begin with a singularity? These are very important, and Hawking’s final paper was the construction of a toy model that argued “yes” for the final question. But it has nothing to do with the hype surrounding it.
Let’s not deify our heroes; let’s allow their good work to stand on their own merits. And most importantly, let’s be honest about what they did. Here’s the truth."
https://www.forbes.com/sites/startswithabang/2018/03/21/i-am-an-astrophysicist-heres-what-stephen-hawkings-final-paper-was-actually-about/
http://www.rhysy.net/other-1.html
Confirmation 7 minutes in :
https://www.youtube.com/watch?v=QF9jAGyL1fg
Excuse me while I go and stroke my ego with a nice cup of tea.
“The questions that they’re attempting to answer are still valid, open questions, and the best this paper can do — if it’s correct and relevant, and it may be neither — is provide suggestions towards an answer. The approach is largely based off of work that Hartle, Hawking, and Hertog have done in the past, the dS/CFT connection pioneered by Chris Hull and others, along with string-inspired work done by Andrew Strominger and his collaborators. None of this is based off of any realistic cosmological models; these are toy models that they are calculating in, and then reasoning-by-analogy with what we actually know exists. Like most theoretical work in the very early stages, there are interesting ideas that are presented, the work and calculations are highly speculative, and there is not necessarily a connection with reality. But there’s a non-zero chance that one is real. And in theoretical physics, a novel idea with a chance is worth infinitely more than no new ideas at all.”
There have been a lot of incredible claims floating around the media about what’s going on with Stephen Hawking’s final paper, which was submitted earlier in March, less than two weeks before he died. Some are claiming it will help us detect the multiverse, others claiming that it will tell us how the Universe will end. The truth is much more sobering, however: it discusses issues involving the dynamics of inflation. There are incredible questions we’re trying to understand about the Universe, such as: did inflation begin, or was it eternal; will it continue indefinitely into the future; does it inevitably lead to a multiverse; did time and space begin with a singularity? These are very important, and Hawking’s final paper was the construction of a toy model that argued “yes” for the final question. But it has nothing to do with the hype surrounding it.
Let’s not deify our heroes; let’s allow their good work to stand on their own merits. And most importantly, let’s be honest about what they did. Here’s the truth."
https://www.forbes.com/sites/startswithabang/2018/03/21/i-am-an-astrophysicist-heres-what-stephen-hawkings-final-paper-was-actually-about/
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