For the last major conference I went to, I combined the science and travel reports together. This was easy because Cardiff to me is not an exotic destination so it hardly needs much description. But this year's EAS conference was in Padova, and my explorations of the city itself and neighbouring Venice easily required their own dedicated post over on Physicists of the Caribbean. That, however, is a sideshow. Here I should say something about the main reason I was there, i.e. the SCIENCE !
This will only be brief. I have a number of things I want to look up in detail, but for now, here's what I leaned from the conference itself.
Euclid Is Mega Awesome. Like, seriously awesome. I have this chronic bad habit of not paying attention to new telescopes when they're being proposed or even during construction : who know when they'll launch or whether they'll reach their design spec ? Worst of all, and perhaps a better reason, is that their marketing is often terrible. It's all public outreach about their main target mission. For example as far as I knew Euclid was some specialist thing for probing dark energy presumably by measuring something very specific and niche, which just goes to show how little attention I ever give new instruments.
Actually it's more like Hubble on steroids. It's got comparable resolution (though not quite Hubble level) but with a vastly larger field of view and exquisite optics that gives a uniformly high quality image across the whole field. This makes it a fantastic, game-changing instrument for the low surface brightness universe. Want to find faint stellar streams, tiny dwarf galaxies and other exotic phenomena ? Euclid to the rescue ! If they'd sold it as more of a survey instrument and not this dark energy thing... well, quite possibly they did and I just wasn't listening.
Whoops ! Luckily for me, its main survey will be almost all-sky and openly available, so I won't have to do anything except look at the data when it's available.
Cosmology Isn't Dead Yet. There are lots of press releases about the discovery of disc-like and massive galaxies in the early Universe, only a few hundred million years after the Big Bang, which is supposedly not long enough for them to have formed. The picture according to the experts is a bit more subtle than this popular description though. Some of these objects might be a problem - they really might, and not in a "but probably not" way : there's every chance there's something going on here that we don't understand. Whether that's the cosmology itself, i.e. the structure and nature of the universe, or just the detailed physics of the gas and star formation... that's where things get more suspect.
For example, it's not, it turns out, that simulations don't predict such objects. They do. It's just that it appears that they predict fewer than the numbers observed. In the Illustris simulations apparently about 10% of the relevant comparison sample are discy in the early simulated Universe, which isn't a lot but isn't insignificant either. The problem is that it's unclear if this is in conflict with the observations because the numbers are still small, and we aren't sure about the observational biases. For mass the situation is much worse : when deriving the mass using synthetic observational procedures (that is, transforming the simulations into the kind of data observers would process, and then using their methods to guestimate the stellar mass), results vary by up to three orders of magnitude away from the true value. So claims that there are too many massive galaxies in the early Universe can be safely ignored.
Except, there's a major caveat. Mass is a derived parameter with many uncertainties, but straightforward luminosity (i.e. brightness) is a direct observable. And here there does appear to be a conflict (sorry, tension) with observations. There are potential solutions here as well though. For example a more "bursty" mode of star formation, rather than the more continuous process generally assumed, as well as accounting for nebular continuum emission and a top-heavy IMF (that is, forming more big stars in proportion to small ones than in the nearby universe, which could happen because their chemistry would be completely different), might be enough to solve all this. As some wise old sage commented, complicated problems often have complex, multi-parameter solutions rather than one big single "change this and all will be well" moment. Which unfortunately means that figuring this out is going to take time.
EDIT : I almost forgot. Back in 2017 there was a paper claiming that galaxies in the early Universe show declining rotation curves, indicating they were far less dark matter dominated than today. I was skeptical of this, as reported here with some follow-up here. And from conference results it seems that these results are heavily dependent on observation time : too short and indeed the result is declining curves, but observed for longer than they flatten considerable. Exactly why this should be I don't know, but it appears the original authors rowed back on their initial claims somewhat in a 2020 paper. It seems that the original results were something of an oversimplification, if not just simply wrong.
There Are No Dark Galaxies. New HI surveys are reaching lower column (or surface) densities, that is, how much gas they detect per unit area, than I was aware. In fact they're comparable to Arecibo but with the added benefit of much higher resolution. The penalty is that this takes enormous amounts of observing time but this is largely compensated for by large fields of view.
The results so far I think are still mainly "watch this space" except for individual objects. But one interesting finding is that there's a distinct lack of optically dark galaxies which have gas but no stars. This is something I've been working on for many years with Arecibo data, but MHONGOOSE's much improved resolution combined with its sensitivity means we would already have expected to see something if they exist in numbers of any significance. Thousands of detections of normal galaxies already (over 6,000 in fact - compare this to the 31,000 from ALFALFA which took many years to achieve !) but no hint of anything dark that isn't explicable by another mechanism... though of course, this has some caveats, but a significant population there appears none.
Gas Accretion Definitely Happens. But we still don't know when or how ! Obviously galaxies acquire gas at some point or they'd never form any stars, but while there's much indirect evidence for this, direct observations remain hugely unconvincing. Mergers have been ruled out as a significant source of the gas, there just aren't enough objects to do this. Cold accretion, where cold atomic HI falls into galaxies along streams, remains a distinct possibility but unobserved, even with the newest and most sensitive instruments.
Hot accretion (from the hot gas large-scale cosmic web) also remains a possibility but while large-scale bridges of X-ray gas have been detected, everyone was much more circumspect about claiming these as detections of the web itself than they have been in the past. And quite properly too, because any individual detection can always be challenged as an interaction. To claim a detection of the web, we'd really need to see it ubiquitously, with multiple strands connecting multiple clusters. Interestingly, HI intensity mapping has thus far got a statistical detection, but not to the point of being able to do actual imaging yet.
Radio Halos Do Funny Things. Not only does the X-ray gas trace giant, megaparsec-scale structures linking entire galaxy clusters, but so does the much lower-energy radio emission. There are different kinds of radio halos, with Mpc-scale giants to ~100 kpc scale "minihalos". And these aren't the same component, with the density profiles of the two showing a distinct change of slope. Then there are radio relics, the result apparently of shocks in the intracluster medium producing giant arcs of radio emission.
In one particularly noteworthy case, one of these giant radio lobes appears to be interacting with a galaxy. This shows a tail which is distinctly different from most. Where galaxies lose gas by ram pressure, the tails tend to be well-collimated and decrease in brightness at greater distances. This one instead gets both wider and brighter, indicating a different physical processes is at work. Like classical ram pressure, however, it seems to have caused an initial increase in the star formation rate of the galaxy. This is interesting to me because I've always assumed these features were of too low a density level to have an impact on anything, being an interesting way to trace the dynamics of an environment but being themselves no more than tracers, not interactors.
Ultra Diffuse Galaxies Are Still A Thing. There seems now to be a quite firm consensus : UDGs are two populations, one of "puffy" dwarves of low total mass that have become somehow extended, and the other of "failed", much more massive galaxies not predicted by any simulations. Several people made that last point and nobody raised any arguments, though exactly how massive they are (and how numerous this population is) wasn't clearly stated. Even so, to my mind if you want a serious challenge to cosmology, forget the attention-hogging results from JWST and look at these much nearer objects !
Likewise, the view seems to be that UDGs are indeed (following results from a few years ago) not actually all that large - but they are flatter : their light profiles are basically constant and then suddenly truncated at their edges, whereas normal galaxies have more complicated and varied profiles. There still seems to be some disagreement, however, as to whether UDGs therefore represent extreme examples of regular dwarf galaxies or are genuine outliers which are qualitatively different from the main galactic population.
The dynamics of the more massive objects to me suggests the latter, but there's still not a clear answer to this. Their globular cluster populations are also highly diverse, with some having none at all and others having far more than expected. One very interesting set of observations by Pierre-Alain Duc shows stellar tails from globular clusters in the inner regions of the UDGs, likely merging with the main body of the galaxy - that's how good our observations have become ! Considering their whole set of properties, it remains understandably difficult to decide what the hell UDGs actually are.
To me the most interesting individual object presented in the whole conference was a UDG by Pavel Mancera Piña, he of the "UDGs have no dark matter" fame. Regular readers will know I was at first enthusiastic about this result, then became a bit more skeptical, but finally I've settled (?) back to my original stance : the results seem secure enough that they can't be attributed to observational errors or improper corrections. Now Pavel has found an object which is especially weird. Like the others, it's isolated. Its HI map shows very neatly circular contours, and its kinematics are consistent with no dark matter at all. The only way it can have a dark halo is if the concentration is very low - much lower than standard cold dark matter predicts, but explicable with self-interacting dark matter...
And because it's isolated, explaining it with modified gravity is hard. If gravity rather than matter governs dynamics, then all isolated objects of the same mass and radius should show the same kinematic properties. That they don't might well argue that such notions are simply wrong, even as objects like this one indicate that the standard model of dark matter itself has flaws.
Well, of course much remains to be done in all those categories, especially the last. My own talk I'm pleased to say went down very well, I got lots of questions about the AGES dark clouds and had some nice discussions afterwards. My poster (should be accessible for a few months, I think) may have sunk without trace, but no matter. And the conference slogan "Where Astronomers Meet" may be the most blandest thing that ever did bland, but the sessions themselves were full of interesting stuff.
I'm old enough to remember conferences of a different era of more, shall we say, "robust conversations". I've not seen any actual arguments (except in some much smaller events) in many years now. Disagreements still arise but they're altogether gentler. Sometimes I miss the spectacle of hearing a good row from a safe distance, but perhaps, as long as we still do interesting science and still have fruitful discussions, this new way is better.
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