Ghostly giants versus huge dwarves - the battle continues
Ultra-diffuse galaxies are currently the trendiest thing in extragalactic astronomy. The same size as the Milky Way but a thousand times fainter, these things may be extremely boring to look at but they break all the rules of galaxy formation. Or do they ? Two years after first hitting the headlines, I review the current state of the art on the most important question : how massive are they ? Are they actually just huge dwarves or are they genuinely ghostly giants ? What have we really learned so far and where are we going ?
With star-studded cast of a giant squid, Father Ted, Gimli, Ghostbusters and for some reason James Corden...
Placeholder post intended to be replaced with a better summary.
Monday 31 July 2017
Thursday 27 July 2017
Yet another Dark Matter versus MOND paper
Well... partially. Mainly this paper is about whether the standard dark matter model can explain some of the recently discovered "ultra faint" dwarf galaxies in the Local Group. Cold Dark Matter (CDM) models predict that really small dark matter halos, which have low rotational speeds, shouldn't be able to form any stars at all. They shouldn't have enough mass to pull in enough gas to form stars... but that's just what these new galaxies seem to have done.
In this paper, the authors use new simulations with much higher resolution and more complex physics than previous efforts. They show that similar objects can form through tidal stripping (i.e. gravitational effects), which can remove large fractions of the dark matter and stars but, interestingly, doesn't change the size of the galaxy. So apparently weird new objects (like the "giant" Crater II) may make sense after all.
They also show more quantitatively that the main objections to this idea aren't as fatal as might be thought. First, tidal stripping is thought to be very disruptive, with stars and gas and dark matter flying all over the place. And it is... but not for very long. The damage is done only when the dwarf galaxy flies closest to its bigger, scarier neighbour. The rest of the time the tidal effects don't really do a lot, and afterwards the galaxy can settle down into a nice round shape, like those that are observed. In essence it's like that bit in Jaws where the decapitated head suddenly appears - everyone jumps, but you don't go running off down the street screaming your head off for ten minutes afterwards. Usually.
The second objection is that these tidally-disrupted galaxies should only be found near their larger neighbours. But because the scary moment of disruption is short-lived, the galaxy can continue on its merry way back out to the hinterlands. So it's no surprise that the observed galaxies are found quite far away from the Milky Way.
All well and good, but now we reach the controversial stuff....
First, there were several recent papers claiming the discovery of a "new law of nature", where the gravity from dark matter appeared to be closely correlated with the gravity from normal matter. Which seemed a bit odd, but then there were several other papers which said, "yeah, but we see this is dark matter simulations anyway, shut your ugly face, it's not very interesting." Or words to that effect.
This paper shows that the scatter in this "mass discrepancy acceleration relation" is much higher for faint galaxies than was previously reported. They also claim that their simulations explain this. I think this is true, but the agreement is not that great. It's OK, but not particularly impressive. More interesting is simply the claim about the size of the scatter, which I find fairly convincing. Our resident tame MOND expert says that the original papers already discussed this, but I don't think this is true. However, they do mention that they only use the highest-quality data available. A better objection might be whether these new galaxies are rotating at all or if the stars are just moving on random orbits - this might not change the conclusions, but it should be discussed.
The second controversial point was the claim that they have discovered something which poses a "possibly insurmountable challenge" to Modified Newtonian Dynamics, the main alternative to dark matter. Strong words. Strong, angry words - which have been used before against MOND... but with admirable if cockroach-like tenacity the bloody thing keeps coming back.
MOND has this funny thing called the "external field effect". If there's another galaxy nearby, acceleration in a smaller galaxy reverts back to standard Newtonian behaviour. When you account for this, it seems that the velocities in the faint galaxies are in strong disagreement with MOND's predictions (they're also in strong disagreement even if you ignore it).
The tame MOND expert agrees with the method used, but disputes the conclusion. He's probably right that the conclusion is too strong, although personally I find claims of "a new law of nature" to be far worse on that front. His objection is that the fraction of binary stars in a galaxy can change the overall measured velocity dispersion, or it could be that even those the galaxies aren't in the process of being tidally disrupted right now, they're still out of equilibrium. It's possible, but I'm not convinced this can really explain the strong systematic offset seen in the data. It needs to be quantified though.
The paper concludes by worrying that this model requires some pretty dramatic tidal disruption to explain the galaxies - they've have to have lost 99% of their original mass (whether this is common in their simulations is not clear). But with measurements of the 3D velocities of the galaxies it would be possible to trace their orbits back in time and work out whether they really have experienced close encounters with the giant galaxies of the Local Group. So, as usual, watch this space.
https://arxiv.org/abs/1707.03898
In this paper, the authors use new simulations with much higher resolution and more complex physics than previous efforts. They show that similar objects can form through tidal stripping (i.e. gravitational effects), which can remove large fractions of the dark matter and stars but, interestingly, doesn't change the size of the galaxy. So apparently weird new objects (like the "giant" Crater II) may make sense after all.
They also show more quantitatively that the main objections to this idea aren't as fatal as might be thought. First, tidal stripping is thought to be very disruptive, with stars and gas and dark matter flying all over the place. And it is... but not for very long. The damage is done only when the dwarf galaxy flies closest to its bigger, scarier neighbour. The rest of the time the tidal effects don't really do a lot, and afterwards the galaxy can settle down into a nice round shape, like those that are observed. In essence it's like that bit in Jaws where the decapitated head suddenly appears - everyone jumps, but you don't go running off down the street screaming your head off for ten minutes afterwards. Usually.
The second objection is that these tidally-disrupted galaxies should only be found near their larger neighbours. But because the scary moment of disruption is short-lived, the galaxy can continue on its merry way back out to the hinterlands. So it's no surprise that the observed galaxies are found quite far away from the Milky Way.
All well and good, but now we reach the controversial stuff....
First, there were several recent papers claiming the discovery of a "new law of nature", where the gravity from dark matter appeared to be closely correlated with the gravity from normal matter. Which seemed a bit odd, but then there were several other papers which said, "yeah, but we see this is dark matter simulations anyway, shut your ugly face, it's not very interesting." Or words to that effect.
This paper shows that the scatter in this "mass discrepancy acceleration relation" is much higher for faint galaxies than was previously reported. They also claim that their simulations explain this. I think this is true, but the agreement is not that great. It's OK, but not particularly impressive. More interesting is simply the claim about the size of the scatter, which I find fairly convincing. Our resident tame MOND expert says that the original papers already discussed this, but I don't think this is true. However, they do mention that they only use the highest-quality data available. A better objection might be whether these new galaxies are rotating at all or if the stars are just moving on random orbits - this might not change the conclusions, but it should be discussed.
The second controversial point was the claim that they have discovered something which poses a "possibly insurmountable challenge" to Modified Newtonian Dynamics, the main alternative to dark matter. Strong words. Strong, angry words - which have been used before against MOND... but with admirable if cockroach-like tenacity the bloody thing keeps coming back.
MOND has this funny thing called the "external field effect". If there's another galaxy nearby, acceleration in a smaller galaxy reverts back to standard Newtonian behaviour. When you account for this, it seems that the velocities in the faint galaxies are in strong disagreement with MOND's predictions (they're also in strong disagreement even if you ignore it).
The tame MOND expert agrees with the method used, but disputes the conclusion. He's probably right that the conclusion is too strong, although personally I find claims of "a new law of nature" to be far worse on that front. His objection is that the fraction of binary stars in a galaxy can change the overall measured velocity dispersion, or it could be that even those the galaxies aren't in the process of being tidally disrupted right now, they're still out of equilibrium. It's possible, but I'm not convinced this can really explain the strong systematic offset seen in the data. It needs to be quantified though.
The paper concludes by worrying that this model requires some pretty dramatic tidal disruption to explain the galaxies - they've have to have lost 99% of their original mass (whether this is common in their simulations is not clear). But with measurements of the 3D velocities of the galaxies it would be possible to trace their orbits back in time and work out whether they really have experienced close encounters with the giant galaxies of the Local Group. So, as usual, watch this space.
https://arxiv.org/abs/1707.03898
Saturday 22 July 2017
There... are... THREE... clouds !
Thanks everyone who voted on yesterday's poll (https://plus.google.com/u/0/+RhysTaylorRhysy/posts/h4ePJaDsHXf). The final score was :
1 group : 0%
2 groups : 7%
3 groups : 71 %
4 groups : 16%
More : 7%
... and the correct answer is.... there areFOUR LIGHTS three groups !
This diagram shows the location of galaxies in a particularly complicated part of the Virgo cluster, where, in addition to the main cluster, it's believed there are two other infalling sub-groups. Distance measurements to individual galaxies are hard but velocity (redshift) measurements are much easier. The distance measurements which we do have indicate that these three different groups are at different distances. It's also possible to see this using the velocity measurements, with each group centred on a different overall velocity (though with lots of scatter).
A colleague of mine disputed whether you'd really pick out three distinct groups without already knowing the distances. I thought it looked clear enough, but heck, this is the age of social media so why not test it ? The diagram I showed you yesterday was the same as the one below, just stripped of all distance information and rotated to a random angle. I deliberately gave you absolutely no information on the problem and didn't specify what size features count as significant - I wanted to see what you'd naturally guess without over-thinking what counts as a group.
Turns out the answer's three. Since I told you absolutely nothing, that makes the conclusion much stronger - not only can you pick out the three groups, but that's what most people naturally actually do. So you definitely don't need the distance measurements to realise there are three distinct groups here.
Clusters are messy places, so it's completely understandable that you might think there are more than three groups from this limited information. Even with the distance assignments, you can see that some objects at similar velocities apparently below to different groups. That's probably not actually the case - more likely, the distance assignments just didn't take this velocity information into account. There's clearly a lot of scatter in all cases - these aren't nice spheroidalish distributions - this is the real universe, messy and ugly and with galaxies lying in awkward locations. It's quite possible that a few of them do lie at quite different distances to the others; whether they would constitute separate groups or not is another matter (though not a terribly interesting one).
1 group : 0%
2 groups : 7%
3 groups : 71 %
4 groups : 16%
More : 7%
... and the correct answer is.... there are
This diagram shows the location of galaxies in a particularly complicated part of the Virgo cluster, where, in addition to the main cluster, it's believed there are two other infalling sub-groups. Distance measurements to individual galaxies are hard but velocity (redshift) measurements are much easier. The distance measurements which we do have indicate that these three different groups are at different distances. It's also possible to see this using the velocity measurements, with each group centred on a different overall velocity (though with lots of scatter).
A colleague of mine disputed whether you'd really pick out three distinct groups without already knowing the distances. I thought it looked clear enough, but heck, this is the age of social media so why not test it ? The diagram I showed you yesterday was the same as the one below, just stripped of all distance information and rotated to a random angle. I deliberately gave you absolutely no information on the problem and didn't specify what size features count as significant - I wanted to see what you'd naturally guess without over-thinking what counts as a group.
Turns out the answer's three. Since I told you absolutely nothing, that makes the conclusion much stronger - not only can you pick out the three groups, but that's what most people naturally actually do. So you definitely don't need the distance measurements to realise there are three distinct groups here.
Clusters are messy places, so it's completely understandable that you might think there are more than three groups from this limited information. Even with the distance assignments, you can see that some objects at similar velocities apparently below to different groups. That's probably not actually the case - more likely, the distance assignments just didn't take this velocity information into account. There's clearly a lot of scatter in all cases - these aren't nice spheroidalish distributions - this is the real universe, messy and ugly and with galaxies lying in awkward locations. It's quite possible that a few of them do lie at quite different distances to the others; whether they would constitute separate groups or not is another matter (though not a terribly interesting one).
Monday 3 July 2017
The busy life of an astronomer
EWASS is over and I'm still alive; the 1200-strong horde of barbarous astronomers has been sent back to the hellish netherworld from whence they came. I finally booked my summer holiday to see the total solar eclipse from Grand Teton (hello darkness, bye bye money - also, Expedia seems designed to make the whole process as nerve-inducing as watching a horror movie while receiving random electric shocks). My flat has been restored to something approaching normality. Today we have a scientific visitor hanging around after EWASS. Tonight/early tomorrow morning a friend arrives for a week of holiday and on Wednesday I'm giving a 90 minute public talk.
Wheeee.....
Wheeee.....
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In the last batch of simulations, we dropped a long gas stream into the gravitational potential of a cluster to see if it would get torn... -
Another day, another paper on how exciting Ultra Diffuse Galaxies are. At first, these large, faint galaxies were just wholly unexpected, a...
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