What's this ? A second post on the highlights of the EAS conference ? Yes ! This year I've been unusually diligent in actually watching the online talks I didn't get to see in person. Thankfully these are available for three months after the conference, long enough to actually manage to watch them but also, crucially, short enough to provide an incentive to bother. And I remembered a couple of interesting things from the plenaries that I didn't mention last time but which may be of interest to a wider audience.
Aliens ? There are hardly any talks which dare mention the A-word at astronomical conferences, but one of the plenaries on interstellar asteroids dared to go there. The famous interstellar visitor with the unpronounceable name of ʻOumuamua (which is nearly as bad as that Icelandic volcano that shut down European airspace a few years ago) got a lot of attention because Avi Loeb insists it must be an alien probe. He's wrong, and his claims to have found bits of it under the ocean have been utterly discredited. Still, our first-recorded visitor on a hyperbolic trajectory did do some interesting things. After accounting for the known gravitational forces, its rotation varies in a way that's inconsistent with gravity at the 10-sigma level. The speaker said that the only other asteroids and comets known to do this have experienced obvious collisions or have obvious signs of outgassing, neither of which happened here. He took the "alien" idea quite seriously.
Ho hum. No comment.
Time-travelling explosions. The prize lecture was by best-PhD student Lorenzo Gavassino, who figured out that our equations for hydrodynamics break down at relativistic velocities. Normally I would find this stuff incomprehensible but he really was a very good speaker indeed. And the main results is that they break down in a spectacular way. You might be familiar with simultaneity breaking, where events look different to observers at different speeds. Well, says Lorenzo, this happens to fluids moving at relativistic speeds in dramatic fashion : one observer should see a small amount of heat propagating at faster than the speed of light while another would see some energy travelling backwards through time. The result should be massive (actually, infinite) instabilities and the spontaneous formation of singularities. Accretion discs in simulations ought by rights to explode, and God knows what should happen to neutron stars.
The reason that this doesn't happen appears to be a numerical artifact which effectively smooths over this (admittedly small) amount of leakage. But what we need to do to make the equations rigorously correct, and how that would affect our understanding of these systems, isn't yet known.
Ultra Diffuse Galaxies may be tidal dwarfs in disguise. Another really interesting PhD talk was on how UDGs might form in clusters. When galaxies interact in low density environments, they can tear off enough gas and stars to form so-called tidal dwarfs. The key features of these mini-galaxies is that they don't have any dark matter (which is too diffuse to be captured in an interaction like this) and short-lived, usually re-merging with one of their parents in, let's say, 1 Gyr or so. But what if the interaction happens near the edge of a cluster ? Well, then the group can disperse and its members separated as they fall in, so the TDG won't merge with anything. Ram pressure will initially increase its star formation, increasing the stellar content in its centre and making it more compact, before eventually quenching it due to simple lack of gas. So there should be a detectable trend in these galaxies, from more compact to more diffuse going outwards from the cluster centre, all lacking in dark matter.
Of course this doesn't explain UDGs in isolated environments, but there's every reason to think that UDGs might be formed by multiple different mechanisms. A bigger concern was that the simulations didn't seem to include the other galaxies in the cluster, so the potentially very destructive effects of the tidal encounters weren't included. But survivorship bias was very much acknowledged : all galaxies, she said, get more compact closer to the centre, but not all survive at all. It's a really intriguing idea and definitely one to watch.
Even more about UDGs ! These were a really hot topic this year and whoever decided to schedule the session to be in one of the smaller rooms was very foolish, because it was overflowing. A few hardy souls stood at the back, but most gave up due to the poor air conditioning. Anyway, a couple of extra points. You might remember that I wasn't impressed by early claims than NGC 1052-DF4, one of the archetypes of galaxies without dark matter, had tidal tails. Well, I was wrong about that. New, deeper data clearly shows that it does have extended features beyond its main stellar disc. Whether that really indicates tidal disruption... well, I'll read the paper on that. And its neighbour DF2 remains stubbornly tail-less.
The other point is a new method for measuring distances to UDGs by looking at the stellar velocity dispersion of their globular clusters. This was the work of a PhD student who found that there's a relationship between this dispersion and the absolute brightness of the parent galaxy. Getting dispersion of the clusters is still challenging, requiring something like 20 hours on the VLT... but this is a far cry from the 100 HST orbits needed for the dispersion of the main stellar component of the galaxy itself. Apparently this work on the dispersion within individual clusters, so even one would be enough. They tested this on DF2 and DF4 and found a distance of....16 Mpc, right bang in the middle of the 13 and 20 Mpc claims that have been plagued with so much controversey.
Ho hum. No comment.
Fountains of youth and death. Some galaxies which today are red and dead appear to have halted their star formation very early on, but why ? One answer presented here quite decisively was due to AGN – i.e. material expelled from the enormous energies of a supermassive black hole in the centre of the galaxy. Rather unexpectedly it seems that most of this gas is neutral with only a small fraction being ionised, and detections of these neutral outflows are now common. In fact this may even be the main mechanism for quenching at so-called "cosmic noon" (redshifts of 1-2) when star formation peaked. Well, we'll see.
The other big talking point about fountains of ejected material was how galaxies replenish their gas. Here I learned two things I wish someone had told me years ago because they're very basic and I should probably have known them anyway. First, by comparing star formation rates with the mass of gas, one can estimate the gas depletion time, which is just a crude measure of how long the gas should last. And at low redshift this is suspiciously low, about a billion years. Does this mean we're in the final stages of star formation ? This is still about 10% or so of the lifetime of the Universe so it's never seemed all that suspicious to me.
The problem is that this depletion time has remained low at all redshifts. It's not that galaxies are suspiciously close to the end, it's that they should have already stopped forming stars and run out of gas long ago. Star formation can be estimated in different ways with no real constraint on distance, though gas content is a bit harder – we can't do neutral hydrogen in the distant Universe, but we can absolutely do molecular and ionised gas. Despite the many caveats of detail there's a very strong consensus that galaxies simply must be refuelling from somewhere.
One of those models has been the so-called galactic fountain. Galaxies expel gas due to stellar winds and supernovae, some of which escapes but most of which falls back to the disc. Now this is obvious as to how it explains why star formation keeps going in individual, local parts of the disc where the depletion time is too short, but how this explains the galaxy overall has never been clear to me. What might be going on is that the cold clouds of ejected gas (which look like writhing tendrils in the simulations) act as condensation sites as they move through the hot corona and fall back. Here gas in the hot, low density corona of the galaxy can cool, with the simulations saying that this mass of gas can be very significant. So the galaxy tops up its fuel tank from its own wider reservoir. It will of course eventually run out completely, but not anytime soon.
This is a compelling idea but there are two major difficulties, one theoretical and one observational. The theoretical problem is that the details of simulations really matter, especially resolution. If this is too low, clouds might appear to last much longer than they do in reality. One speaker presented simulations showing that this mechanism worked very well indeed while another showed that actually the clouds should tend to evaporate before they ever make it back to the disc, so this wouldn't be a viable mechanism at all. On the other hand, neither used a realistic corona : if it's actually not the smooth and homogenous structure they assume it to be, this could totally change the results.
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