Traditionally* the missing satellite problem refers to the lack of satellite galaxies detected in our Local Group compared to theoretical predictions. It's only in this nearest region that we can reach the sensitivity needed to be able to spot the faintest smudges of the smallest galaxies. Which means it's always been possible that our Local Group is in some way peculiar, with other galaxies having multitudinous swarms of tiny satellites that we just can't see. It would certainly be great to have better data of more distant galaxies so we can see how typical our little patch of the cosmos really is.
* Since 1999. That's traditional now.
This paper presents observations of the NGC 3175 group of galaxies, which is reasonably similar to our own group but about 14 Mpc (45 million light years) away. Using deep, but not extraordinarily so, 2.5 hour observations on a 2.2 m telescope, they claim to have detected ~550 candidate dwarf galaxies : far more than the 100-odd satellites in the Local Group. So, super interesting ?
I'd have to say no, not really. I'm not at all convinced by the methodology here. They don't state the surface brightness sensitivity of their observations and their detection method seems dicey. All (or almost all) of those recent "ultra diffuse galaxies" have been detected by basically the same method : doing deep observations of both the target field and a background control field. That way the search method can be tested. What it should do - and what the UDG papers demonstrate successfully - is find stuff only in the target field, so you can be pretty confident that most of the detections are really associated with the known group or cluster of galaxies. That means you can say they're probably at a similar distance, which is much easier to measure for the big bright galaxies (and damned hard for the faintest ones).
That's not what they do here : they just have the target field, and no control. They decide if the galaxies are likely to be in the group or not based on their structural parameters. The UDG papers do this too, but they don't rely on them this heavily. While I don't doubt that this can indeed remove lots of contaminating background objects, e.g. angularly tiny galaxies with spiral arms are almost certainly distant background objects, it's not at all clear it's a good way to remove all the background objects, much less how much foreground contamination there is. The discovery of large numbers of UDGs, which could easily be mistaken for dwarves except they're known to actually be large and far away, makes this kind of method questionable at best. It's also rather worrying that they dismiss UDGs as unimportant :
"...these UDGs are just a subset of dwarf elliptical galaxies found mostly in rich clusters of galaxies."
Oh well, one man's revolution in the field is another's blasé event of tedious mediocrity.
(The third author has mentioned the unimportance of UDGs before and is quite clearly bitter about his own similar discoveries being overlooked - to some extent quite understandably so, but it's the large numbers of these objects that's really got people interested.)
The other crucial thing lacking here is a discussion on the environment of the group. On the small scale, it would be extremely interesting to see the positions of the galaxies - with >500 objects we could perhaps see those damn planes I hate so much or if they were at least denser nearer to the centre of the group. And we need a discussion of the background environment - maybe there are background clusters or groups we might expect to host UDGs and so cause contamination. They could have at least have calculated the distance beyond which their candidates would be unfeasibly large. But they don't do any of that. The radii given in their (truncated - though it's possible the full version will be available when it's actually published in MNRAS) main table (which is not referenced in the text) are very much smaller than that of UDGs, and their Sersic indicies (a measure of the shape of their light profile) are similar, so it's entirely reasonable to suggest that many of them could be much more distant objects.
Assuming this result is correct, then the numbers they found are a bit less than the classical simulations predict, but a bit more than the more modern versions which have far more physics. But there are so many uncertainties at the moment that this could simply be meaningless, so there's not much point speculating about it.
http://adsabs.harvard.edu/abs/2018arXiv180809020K
Wednesday, 29 August 2018
Tuesday, 28 August 2018
Explain yourself !
Ooo-eck. Lots of implications here.
The next step of this shift away from purely mathematical modeling is already on the way: Physicists now custom design laboratory systems that stand in for other systems which they want to better understand. They observe the simulated system in the lab to draw conclusions about, and make predictions for, the system it represents.
The best example may be the research area that goes by the name “quantum simulations.” These are systems composed of interacting, composite objects, like clouds of atoms. Physicists manipulate the interactions among these objects so the system resembles an interaction among more fundamental particles. For example, in circuit quantum electrodynamics, researchers use tiny superconducting circuits to simulate atoms, and then study how these artificial atoms interact with photons.
These simulations are not only useful to overcome mathematical hurdles in theories we already know. We can also use them to explore consequences of new theories that haven’t been studied before and whose relevance we don’t yet know.
Quantum simulations also make us wonder what it means to explain the behavior of a system to begin with. Does observing, measuring, and making a prediction by use of a simplified version of a system amount to an explanation?
But for me, the most interesting aspect of this development is that it ultimately changes how we do physics. With quantum simulations, the mathematical model is of secondary relevance. We currently use the math to identify a suitable system because the math tells us what properties we should look for. But that’s not, strictly speaking, necessary. Maybe, over the course of time, experimentalists will just learn which system maps to which other system, as they have learned which system maps to which math. Perhaps one day, rather than doing calculations, we will just use observations of simplified systems to make predictions.
I dunno, I'd say an explanation requires a description of the physical process. If you've just got a prediction (i.e. conventionally just a numerical value) you haven't got a physical understanding at all. Like how Maxwell had his crazy vortex theory that gave identical mathematical values for EM forces but gave way to something much easier for the rest of us to understand, the maths is not the model. If I've got a physical model that makes a prediction, but I don't understand how the physical model works, what use is that ? It's of no more benefit than a mathematical model I can't interpret. Do you need a physical description ? Perhaps not, but dammit, I want one. Can't see the point in making predictions I don't understand, that sort of defeats the whole purpose.
https://www.quantamagazine.org/the-end-of-theoretical-physics-as-we-know-it-20180827/?mc_cid=78b85a8581&mc_eid=ded24b5349
The next step of this shift away from purely mathematical modeling is already on the way: Physicists now custom design laboratory systems that stand in for other systems which they want to better understand. They observe the simulated system in the lab to draw conclusions about, and make predictions for, the system it represents.
The best example may be the research area that goes by the name “quantum simulations.” These are systems composed of interacting, composite objects, like clouds of atoms. Physicists manipulate the interactions among these objects so the system resembles an interaction among more fundamental particles. For example, in circuit quantum electrodynamics, researchers use tiny superconducting circuits to simulate atoms, and then study how these artificial atoms interact with photons.
These simulations are not only useful to overcome mathematical hurdles in theories we already know. We can also use them to explore consequences of new theories that haven’t been studied before and whose relevance we don’t yet know.
Quantum simulations also make us wonder what it means to explain the behavior of a system to begin with. Does observing, measuring, and making a prediction by use of a simplified version of a system amount to an explanation?
But for me, the most interesting aspect of this development is that it ultimately changes how we do physics. With quantum simulations, the mathematical model is of secondary relevance. We currently use the math to identify a suitable system because the math tells us what properties we should look for. But that’s not, strictly speaking, necessary. Maybe, over the course of time, experimentalists will just learn which system maps to which other system, as they have learned which system maps to which math. Perhaps one day, rather than doing calculations, we will just use observations of simplified systems to make predictions.
I dunno, I'd say an explanation requires a description of the physical process. If you've just got a prediction (i.e. conventionally just a numerical value) you haven't got a physical understanding at all. Like how Maxwell had his crazy vortex theory that gave identical mathematical values for EM forces but gave way to something much easier for the rest of us to understand, the maths is not the model. If I've got a physical model that makes a prediction, but I don't understand how the physical model works, what use is that ? It's of no more benefit than a mathematical model I can't interpret. Do you need a physical description ? Perhaps not, but dammit, I want one. Can't see the point in making predictions I don't understand, that sort of defeats the whole purpose.
https://www.quantamagazine.org/the-end-of-theoretical-physics-as-we-know-it-20180827/?mc_cid=78b85a8581&mc_eid=ded24b5349
Monday, 27 August 2018
To be cored or not to be cored, that is the question....
Dark matter density profiles in simulations tend to be "cusped" - they increase very rapidly towards their centre. Unfortunately reality tends to disagree, with at least some galaxies having cored profiles : below some radius, the dark matter density seems to be roughly flat. Measurements of the density profile are difficult, relying on precision measurements of the motions of stars and gas, but they can be done, and the results are pretty clear. Assuming that dark matter exists at all, that is, and that the standard model of cold dark matter (CDM) is correct. Modifications to CDM are possible but they tend to make all dwarves have cored profiles, which is rather extreme.
In this very interesting paper submitted to MNRAS (not yet accepted), the authors show that at a given total mass, the central density of the dark matter is sufficient to determine if there's a core or a cusp. This still needs good data, but it's much easier to get than a whole density profile. They demonstrate that it gives results in good agreement with previous findings, though their modelling is surely the weakest section of the paper. I'm not qualified to say if their model is sensible, but they seem quite careful to point out potential weaknesses, address concerns where possible, and highlight where further research is needed. They're limited to a very small sample (16) of dwarves in the Local Group, but even so their findings strongly suggest this is one to watch.
According to this research, a galaxy's inner dark matter is either cored or cusped depending on its star formation history. How does this work if dark matter can only interact with normal matter through gravity ? Well, star formation is known to be able to disturb gas, even removing it entirely from dwarf galaxies, e.g. through supernovae explosions and stellar winds of hot, short-lived stars. And the mass of the gas can be significant, so having all that gas moving around effectively drags the dark matter around too. Each incidence of star formation doesn't do much, but if it goes on for long enough, it can slosh enough gas around to destroy the dark matter cusp.
The data here seems remarkably clear. Yes, it's a small sample, and yes there are uncertainties. But the difference in core or cusp based on star formation history is awfully good, and still with enough scatter that I wouldn't be suspicious. It's compelling stuff.
Even more interesting, it's tough to explain this in theories of modified gravity. Most claims of refuting modified gravity tend to run into the problem of the External Field Effect of MOND, where proximity to a large nearby mass (i.e. another galaxy) sends everything back into the Newtonian regime. For example, recent claims that dwarf satellites lie off the standard radial acceleration relation have intriguingly noted that their acceleration might depend on that of their parent galaxy. And that galaxy without dark matter has been criticised since the main study didn't account for the EFE correctly.
So what you want, really, are two identical galaxies in an identical environment. If MOND (or other non-CDM theories) are correct, then such objects ought to have very similar dynamical properties. If CDM is correct, then it's at least possible that they won't - star formation is one way to change the dark matter distribution, but there are others. It's not guaranteed, but possible that the two galaxies would have different dynamics, because the dark matter is essentially independent of the visible matter. If, however, MOND or some such is correct, then it shouldn't be possible at all : gravity is gravity, and if the mass and mass distribution are the same, then the dynamics should be the same.
Remarkably, such an ideal test system may have been found. The authors note that for the Draco and Carina dwarfs :
These two galaxies require different dynamical mass profiles for almost the same radial light profile. This is a challenge not only for MOND, but for any weak-field gravity theory that seeks to fully explain DM.
There is of course a caveat. They could have different dynamics if at least one of them wasn't in equilibrium. There is some observational evidence for this for Carina, but it's weak, and models, they say, suggest that MOND couldn't explain it anyway. It's just about possible that these two galaxies actually have very different orbits, but to get a result that would be compatible with MOND "is likely to require significant fine-tuning". And the latest data from Gaia suggest that if anything, it's Draco which should be experiencing the greatest disruption, yet none is evident. More models are definitely needed, but it's undoubtedly one to watch.
http://adsabs.harvard.edu/abs/2018arXiv180806634R
In this very interesting paper submitted to MNRAS (not yet accepted), the authors show that at a given total mass, the central density of the dark matter is sufficient to determine if there's a core or a cusp. This still needs good data, but it's much easier to get than a whole density profile. They demonstrate that it gives results in good agreement with previous findings, though their modelling is surely the weakest section of the paper. I'm not qualified to say if their model is sensible, but they seem quite careful to point out potential weaknesses, address concerns where possible, and highlight where further research is needed. They're limited to a very small sample (16) of dwarves in the Local Group, but even so their findings strongly suggest this is one to watch.
According to this research, a galaxy's inner dark matter is either cored or cusped depending on its star formation history. How does this work if dark matter can only interact with normal matter through gravity ? Well, star formation is known to be able to disturb gas, even removing it entirely from dwarf galaxies, e.g. through supernovae explosions and stellar winds of hot, short-lived stars. And the mass of the gas can be significant, so having all that gas moving around effectively drags the dark matter around too. Each incidence of star formation doesn't do much, but if it goes on for long enough, it can slosh enough gas around to destroy the dark matter cusp.
The data here seems remarkably clear. Yes, it's a small sample, and yes there are uncertainties. But the difference in core or cusp based on star formation history is awfully good, and still with enough scatter that I wouldn't be suspicious. It's compelling stuff.
Even more interesting, it's tough to explain this in theories of modified gravity. Most claims of refuting modified gravity tend to run into the problem of the External Field Effect of MOND, where proximity to a large nearby mass (i.e. another galaxy) sends everything back into the Newtonian regime. For example, recent claims that dwarf satellites lie off the standard radial acceleration relation have intriguingly noted that their acceleration might depend on that of their parent galaxy. And that galaxy without dark matter has been criticised since the main study didn't account for the EFE correctly.
So what you want, really, are two identical galaxies in an identical environment. If MOND (or other non-CDM theories) are correct, then such objects ought to have very similar dynamical properties. If CDM is correct, then it's at least possible that they won't - star formation is one way to change the dark matter distribution, but there are others. It's not guaranteed, but possible that the two galaxies would have different dynamics, because the dark matter is essentially independent of the visible matter. If, however, MOND or some such is correct, then it shouldn't be possible at all : gravity is gravity, and if the mass and mass distribution are the same, then the dynamics should be the same.
Remarkably, such an ideal test system may have been found. The authors note that for the Draco and Carina dwarfs :
These two galaxies require different dynamical mass profiles for almost the same radial light profile. This is a challenge not only for MOND, but for any weak-field gravity theory that seeks to fully explain DM.
There is of course a caveat. They could have different dynamics if at least one of them wasn't in equilibrium. There is some observational evidence for this for Carina, but it's weak, and models, they say, suggest that MOND couldn't explain it anyway. It's just about possible that these two galaxies actually have very different orbits, but to get a result that would be compatible with MOND "is likely to require significant fine-tuning". And the latest data from Gaia suggest that if anything, it's Draco which should be experiencing the greatest disruption, yet none is evident. More models are definitely needed, but it's undoubtedly one to watch.
http://adsabs.harvard.edu/abs/2018arXiv180806634R
Thursday, 23 August 2018
Space Dragons
That moment when searching for "space dragons" in your inbox returns legitimate, meaningful, scientific results.
I mean, you know you've made it at that point, right ?
I mean, you know you've made it at that point, right ?
Friday, 17 August 2018
How to find interesting papers
This is a nice little paper on discovering similar papers without having to trawl through the literature. The most common technique is to look at papers citing any given paper, but this has problems of favouring highly cited papers and missing those that slip through the nets. Occasionally, papers languish in the doldrums for years before people suddenly notice them. And if someone is new to a particular field, they might not necessarily cite the expected literature. Often citations just reference a very specific aspect of a paper and actually focus on very different issues, so there's really no point in reading them if you wanted something similar. Basically the citation network is limited.
The method here uses some basic searches for keywords and their importance, which is measured (I think) by the fraction of the word count. It then trawls the literature (they downloaded the whole or arXiv for this !) for papers with similar keywords of similar importance. They say that this method, which is open and public, gives better results than the similar (closed) method now available on NASA ADS. It sounds like a useful way to discover new and related papers, especially ones that might not be well cited, when starting work in a slightly different sub-topic.
http://adsabs.harvard.edu/abs/2017arXiv170505840K
The method here uses some basic searches for keywords and their importance, which is measured (I think) by the fraction of the word count. It then trawls the literature (they downloaded the whole or arXiv for this !) for papers with similar keywords of similar importance. They say that this method, which is open and public, gives better results than the similar (closed) method now available on NASA ADS. It sounds like a useful way to discover new and related papers, especially ones that might not be well cited, when starting work in a slightly different sub-topic.
http://adsabs.harvard.edu/abs/2017arXiv170505840K
Monday, 13 August 2018
Before the rain hits
The view from the roof of the Prague institute. After an insanely hot summer, the temperature has well and truly broken now. It's still getting hot, it's just not hot all the time. 12 C overnight temperatures on the weekend was absolute bliss. And there's rain - sweet blessed rain ! - at long last.
It Simply Is
Gravity simply is, and it's merely that the equations that describe General Relativity are geometric in nature. The idea that mass-and-energy curves space can be right, even though this naive visualization must be wrong.
Statements that say, "this simply is" invariably mean, "we do not yet understand why this happens." This is, however, very different from saying that we have no knowledge at all.
But under no circumstances should you conceive of space as though it's a material, physical thing; it isn't. This is only a mathematical structure that we can write down equations to describe: the equations of Einstein's General Relativity. The fact that matter and radiation respond to that curvature in the exact ways that the equations predict validates this theory, but it doesn't mean that space is actually a fabric.
We also talk about the expanding Universe in the context that 'the fabric of space is stretching,' even though there is no fabric and it isn't really stretching, or for that matter, changing in any way. What's happening is simply that the distance between any two points in the Universe is changing according to a particular set of rules in the context of General Relativity. Galaxies, like raisins embedded in a loaf of baking bread, expand away from one another. The wavelength of radiation gets longer too, as though the length of the wave crests and troughs expanded away from one another too.
I dunno... are non physical things still things ? If they are, then I have no problem with saying that space is expanding and thus changing; space is a relational thing. And if space isn't a thing, how can gravity be described as the curvature of space ?
One of the most paradoxical ideas to wrap your head around in all of physics is that the equations that describe the Universe are just that: equations describing things we can physically observe. We can no more observe the 'fabric of space' than we can observe the nothingness of empty spacetime; it simply exists. Any visualization we attempt to assign to it, whether it's a 2D fabric, a 3D grid, or a baking ball of dough, is just that: a human-inspired creation. The theory itself doesn't demand it.
I think I'll stick to pretending that space is a sort of non-physical fabric, accept that I don't understand what that is, and "shut up and calculate" as a wise man once said. Easier that way. Otherwise I'm likely to explode in a shower of Zeno's Paradoxes as I struggle to contemplate the formless, infinite nothingness that apparently divides me from measurable reality.
Science is hard.
https://www.forbes.com/sites/startswithabang/2018/08/11/ask-ethan-is-spacetime-really-a-fabric/
Statements that say, "this simply is" invariably mean, "we do not yet understand why this happens." This is, however, very different from saying that we have no knowledge at all.
But under no circumstances should you conceive of space as though it's a material, physical thing; it isn't. This is only a mathematical structure that we can write down equations to describe: the equations of Einstein's General Relativity. The fact that matter and radiation respond to that curvature in the exact ways that the equations predict validates this theory, but it doesn't mean that space is actually a fabric.
We also talk about the expanding Universe in the context that 'the fabric of space is stretching,' even though there is no fabric and it isn't really stretching, or for that matter, changing in any way. What's happening is simply that the distance between any two points in the Universe is changing according to a particular set of rules in the context of General Relativity. Galaxies, like raisins embedded in a loaf of baking bread, expand away from one another. The wavelength of radiation gets longer too, as though the length of the wave crests and troughs expanded away from one another too.
I dunno... are non physical things still things ? If they are, then I have no problem with saying that space is expanding and thus changing; space is a relational thing. And if space isn't a thing, how can gravity be described as the curvature of space ?
One of the most paradoxical ideas to wrap your head around in all of physics is that the equations that describe the Universe are just that: equations describing things we can physically observe. We can no more observe the 'fabric of space' than we can observe the nothingness of empty spacetime; it simply exists. Any visualization we attempt to assign to it, whether it's a 2D fabric, a 3D grid, or a baking ball of dough, is just that: a human-inspired creation. The theory itself doesn't demand it.
I think I'll stick to pretending that space is a sort of non-physical fabric, accept that I don't understand what that is, and "shut up and calculate" as a wise man once said. Easier that way. Otherwise I'm likely to explode in a shower of Zeno's Paradoxes as I struggle to contemplate the formless, infinite nothingness that apparently divides me from measurable reality.
Science is hard.
https://www.forbes.com/sites/startswithabang/2018/08/11/ask-ethan-is-spacetime-really-a-fabric/
Thursday, 9 August 2018
Phwwwwwhoopslsssssh !
That's the sound a galaxy would make as it falls into a cluster. Pressure from the hot cluster gas builds up until the galaxy's own gas disc can't take it any more and it streams out into a disintegrating wake, filling the cluster with hundreds of little gas clumps.
Explaining the origin and nature of certain little clumps is a long-running obsession of mine, particularly ones that look like they're rotating. Directly measuring rotation isn't possible, but we can compare what rotation and other motions look like with the velocity field of the object. Currently the alternative explanations aren't doing very well. If they are rotating, then the clouds would need a lot of dark matter to avoid going splat. That would make them genuine dark galaxies and would make a lot of people very upset.
This paper simulates the effect of ram pressure stripping on a set of galaxies falling into a cluster, and finds that they produce quite a lot of little clumps as the gas from the galaxy and cluster mix together. Their numbers are quite high, but very variable - typically 50-150. The tidal encounter simulations we were playing with were giving much lower numbers. It seems that ram pressure is a much more efficient way of producing clumps, which is very interesting.
Except, of course, we need to have a fair comparison...
The models in this paper are a major improvement over the simplified ones I was doing. The most important difference is that they have gas in both the galaxies and in the cluster, which is easy with grid-based hydrocodes but very computationally expensive with the particle models I was running. So our models just had gravitational interactions, whereas we think (with some confidence) ram pressure is the major mechanism of gas loss. We showed that tidal effects were important, but we were still missing the most important process.
What's also really neat is that they simulate a bunch of galaxies falling in all at the same time, whereas ours only had one galaxy at a time (again because of computational cost). I would wound small children for the chance to combine gravitational and hydrodynamic physics. Unfortunately, as the author pointed out to me when I emailed him, this isn't really the case yet. The density of galaxies in the simulation is just too low - none of them have close encounters with each other. So this is a good approximation to a set of independent examples of pure ram pressure.
What they find is that the clumps can sometimes be so dense that they form stars, but not enough to make up a significant fraction of the intracluster light. Even these few star-forming clumps are very rare, only 3% of the total. That's interesting in itself, and predicts lots of little clumps that could be detectable with future (gas) surveys. And in reality there'd be at least an order of magnitude more clumps, since they're only using a dozen or so galaxies. But what about those "rotating" clumps ?
Well, it's kindof neat and unexpected how many dense clumps ram pressure produces, so that's cool. But on the specific sorts of clumps I'm interested in, they can't say much yet. The ones they form are generally much less massive, by about a factor ten, though they do have a few which are as massive. They report slightly longer lifetimes (100-300 Myr) than in our simulations (< 100 Myr) due to mixing with the intracluster gas. But the key point is the velocity structure of the clouds - can you form something which looks like it's rotating (but isn't really) by this mechanism ? Alas, they don't have the resolution to tell. Also, while the title says "molecular", they can't really distinguish molecular from atomic gas in these simulations.
I find the result that large numbers of clumps can be produced by ram pressure very interesting. Potentially, given a realistic number of galaxies, that might, and I stress might, be enough to explain the number of massive clouds that are detected in real clusters. However, it's not yet clear if such clouds ought to be embedded in long streams (unlike the real ones), and the crucial measurement of their dynamics is still unknown.
The research continues.
https://arxiv.org/abs/1807.09771
Explaining the origin and nature of certain little clumps is a long-running obsession of mine, particularly ones that look like they're rotating. Directly measuring rotation isn't possible, but we can compare what rotation and other motions look like with the velocity field of the object. Currently the alternative explanations aren't doing very well. If they are rotating, then the clouds would need a lot of dark matter to avoid going splat. That would make them genuine dark galaxies and would make a lot of people very upset.
This paper simulates the effect of ram pressure stripping on a set of galaxies falling into a cluster, and finds that they produce quite a lot of little clumps as the gas from the galaxy and cluster mix together. Their numbers are quite high, but very variable - typically 50-150. The tidal encounter simulations we were playing with were giving much lower numbers. It seems that ram pressure is a much more efficient way of producing clumps, which is very interesting.
Except, of course, we need to have a fair comparison...
The models in this paper are a major improvement over the simplified ones I was doing. The most important difference is that they have gas in both the galaxies and in the cluster, which is easy with grid-based hydrocodes but very computationally expensive with the particle models I was running. So our models just had gravitational interactions, whereas we think (with some confidence) ram pressure is the major mechanism of gas loss. We showed that tidal effects were important, but we were still missing the most important process.
What's also really neat is that they simulate a bunch of galaxies falling in all at the same time, whereas ours only had one galaxy at a time (again because of computational cost). I would wound small children for the chance to combine gravitational and hydrodynamic physics. Unfortunately, as the author pointed out to me when I emailed him, this isn't really the case yet. The density of galaxies in the simulation is just too low - none of them have close encounters with each other. So this is a good approximation to a set of independent examples of pure ram pressure.
What they find is that the clumps can sometimes be so dense that they form stars, but not enough to make up a significant fraction of the intracluster light. Even these few star-forming clumps are very rare, only 3% of the total. That's interesting in itself, and predicts lots of little clumps that could be detectable with future (gas) surveys. And in reality there'd be at least an order of magnitude more clumps, since they're only using a dozen or so galaxies. But what about those "rotating" clumps ?
Well, it's kindof neat and unexpected how many dense clumps ram pressure produces, so that's cool. But on the specific sorts of clumps I'm interested in, they can't say much yet. The ones they form are generally much less massive, by about a factor ten, though they do have a few which are as massive. They report slightly longer lifetimes (100-300 Myr) than in our simulations (< 100 Myr) due to mixing with the intracluster gas. But the key point is the velocity structure of the clouds - can you form something which looks like it's rotating (but isn't really) by this mechanism ? Alas, they don't have the resolution to tell. Also, while the title says "molecular", they can't really distinguish molecular from atomic gas in these simulations.
I find the result that large numbers of clumps can be produced by ram pressure very interesting. Potentially, given a realistic number of galaxies, that might, and I stress might, be enough to explain the number of massive clouds that are detected in real clusters. However, it's not yet clear if such clouds ought to be embedded in long streams (unlike the real ones), and the crucial measurement of their dynamics is still unknown.
The research continues.
https://arxiv.org/abs/1807.09771
Friday, 3 August 2018
I want air conditioning
Working in astronomy institute is nice and all but sometimes not even a TARDIS and some astronauts can save you from the heat.
Wednesday, 1 August 2018
The faintest galaxy ?
Another one in a long list of weird hydrogen clouds that don't make much sense. This one is part of the Leo group, inside the huge Leo Ring which is about 650,000 light years across. How this formed isn't known for sure, but the best guess so far is that it was formed by the collision of two galaxies. Simulations have shown this is plausible, though I'm sure everyone is by now aware of the difference between consistency and evidence.
Anyway, this particular cloud, BST1047, is one small part of the Leo Ring. It was already known from the HI observations that this cloud is rather separate from the main Ring. It was also known that elsewhere in the Ring are several actively star-forming regions, well outside of the gas' likely parent galaxies. So you might think that this is just one more extragalactic star-forming blob - kinda interesting, but not especially new. And while this little dude isn't likely to win anyone a Nobel, it is weird.
The parameters of this blob are extreme. Its brightness per unit area ranks it a the faintest object ever discovered by visible light. The brightness profile is also strange. Rather than peaking in the centre and continuously declining (e.g. exponentially), as in normal galaxies, here the brightness is uniform over the central region and only declines beyond a certain distance. That's much more like a typical gas density profile than a stellar distribution. It's also quite blue and has UV emission, suggesting ongoing star formation, but the gas density appears much lower than expected if that's the case.
Although it's got a weird surface brightness profile, the shape of the object's stellar emissions is kinda boring. In the HI, however, it's weirder (see their figures 2 and 3), with two parallel tails on the same side of the object, pointing in the same direction. Galaxy encounters are complicated, but normally produce two tails on opposite sides pointing in opposite directions. So the gas structure is strange.
And the dynamics of the object are also confusing. It's got a small velocity gradient of about 16 km/s across it, which, if due to rotation, indicates it's got very little dark matter. In light of the recent furore about that other galaxy without dark matter, I'm surprised they don't make more of this. But they're careful to note that because of the object's faintness and the fact it's likely gravitationally interacting with other objects, it would be premature to jump to any conclusions here.
What is it ? Dunno. The most likely explanation is some sort of debris from the interacting galaxies, but we don't really know. Leo is a strange place that needs more HI and deep optical observations.
https://arxiv.org/abs/1807.11544
Anyway, this particular cloud, BST1047, is one small part of the Leo Ring. It was already known from the HI observations that this cloud is rather separate from the main Ring. It was also known that elsewhere in the Ring are several actively star-forming regions, well outside of the gas' likely parent galaxies. So you might think that this is just one more extragalactic star-forming blob - kinda interesting, but not especially new. And while this little dude isn't likely to win anyone a Nobel, it is weird.
The parameters of this blob are extreme. Its brightness per unit area ranks it a the faintest object ever discovered by visible light. The brightness profile is also strange. Rather than peaking in the centre and continuously declining (e.g. exponentially), as in normal galaxies, here the brightness is uniform over the central region and only declines beyond a certain distance. That's much more like a typical gas density profile than a stellar distribution. It's also quite blue and has UV emission, suggesting ongoing star formation, but the gas density appears much lower than expected if that's the case.
Although it's got a weird surface brightness profile, the shape of the object's stellar emissions is kinda boring. In the HI, however, it's weirder (see their figures 2 and 3), with two parallel tails on the same side of the object, pointing in the same direction. Galaxy encounters are complicated, but normally produce two tails on opposite sides pointing in opposite directions. So the gas structure is strange.
And the dynamics of the object are also confusing. It's got a small velocity gradient of about 16 km/s across it, which, if due to rotation, indicates it's got very little dark matter. In light of the recent furore about that other galaxy without dark matter, I'm surprised they don't make more of this. But they're careful to note that because of the object's faintness and the fact it's likely gravitationally interacting with other objects, it would be premature to jump to any conclusions here.
What is it ? Dunno. The most likely explanation is some sort of debris from the interacting galaxies, but we don't really know. Leo is a strange place that needs more HI and deep optical observations.
https://arxiv.org/abs/1807.11544
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