Galactic mess gets everywhere

Today's paper is a strong contender for "least exciting discovery I've ever blogged". It's still interesting though.

A typical spiral or irregular galaxy has three basic aspects to its gas distribution. There's the huge, hot, very thin outer halo that emits at X-rays (though rarely at levels strong enough to detect, except in the most massive galaxies). This million-Kelvin cloud extends well beyond the stellar disc. Then there's the much smaller, cooler (a mere 10,000 K) disc of atomic hydrogen, which extends to about twice the stellar radius. Finally, within the stellar disc there's more atomic hydrogen but at considerably higher densities than in the outskirts, along with a heavy smattering of massively denser, more compact, much colder (few hundred K) molecular hydrogen clouds where star formation happens.

Galaxy clusters posses their own hot X-ray gas. So galaxies falling through them build up a ram pressure as they move through the intracluster medium. This can easily remove their own outermost gas, which does nothing much except starve them of gas for future star formation on very long timescales. Stronger ram pressure can strip the atomic hydrogen directly, and in extreme cases can remove some of the molecular hydrogen itself.

All this is well-known in galaxy clusters, and generally reckoned to be one of the main drivers of galaxy evolution in that environment. But galaxies well outside clusters also frequently show signs that their star formation has been affected too. Could this "pre-processing" be the result of ram pressure stripping in seemingly more passive environments than clusters ?

To figure this out, ideally you need a direct signature of ram pressure, rather than just looking for the side-effects like a change in stellar colour or whatever. A one-sided gas tail is a pretty darn good indication of this, since gravitational encounters usually produce two-sided structures. The problem is that such tails tend to be quite short, so you need good resolution to detect them. Since ram pressure is expected to be weaker in groups (they're less massive than clusters, so they have less gas and the galaxies in them move more slowly) you also need a large sample to stand a chance of detecting a significant number of candidates.

That's where this paper comes in. They use the shiny new LOFAR telescope, which has both excellent resolution and area coverage. Instead of detecting the gas directly, it's sensitive to emission from cosmic rays - which, like the gas, can also be stripped into long one-sided tails by ram pressure.

They have an enormous sample of both groups, clusters, and isolated field galaxies. They use a combination of automated plus visual inspection of the radio images to search for one-sided features, with their field sample acting as a control. And the result is, for once, as clear as day. Only about 2% of field galaxies have tails, whereas it's about 10% in groups and 20% in clusters.

There's also strong evidence that these are truly ram-pressure tails and not the result of some other mechanism. The orientation of the tails tends to be only away from the cluster centre, whereas for groups it could be either away from or towards the group centre. This is exactly what you'd expect. Simplifying slightly, a tail pointing away indicates the galaxy is still on its first infall, whereas if it points towards the centre then the galaxy is now moving back out. Since ram pressure is so much stronger in clusters, the whole gas content can be stripped on first infall - hence most few cluster galaxies with tails pointing outwards. But being much weaker in groups, it can take a lot longer to strip the gas, hence a bimodality in the tail directions.

This also fits perfectly with the position of the tailed galaxies relative to their groups of clusters. Galaxies close to the physical centre but offset in velocity from the cluster centre are expected to be dominated by recent arrivals, and indeed, this is exactly where most of the tails in clusters are found. That's not the case for groups, with tailed galaxies having no particular preferred location in this "phase space".

What of the 2% with tails in the field ? Those could be false positives, since some galaxies do have weird asymmetries from internal processes anyway (strong star formation, active galactic nuclei) or past interactions with other galaxies. That the fraction is so much lower here is a good indication that their sample in groups and clusters doesn't suffer too much from this and that they're genuinely examining the effects of environment. A more interesting possibility is that there could be some weak ram pressure happening even in large-scale filaments of galaxies, with even isolated galaxies sometimes being stripped in this way. That requires further research.

Finally, the only point they've left unexamined is the structure of the tails themselves. That could potentially give more clues to the differences in ram pressure between different environments, e.g. the different length of the tails, morphological features, etc. But for now, this is a very satisfying result, like assembling a piece of IKEA furniture and finding that you don't have any screws left over. It's not exciting. It's not unexpected. It shouldn't be controversial in any way. It's just damned neat.

Ram pressure stripping in groups versus clusters

We compare the group jellyfish galaxies identified in this work with the LoTSS jellyfish galaxies in clusters presented in Roberts et al. (2021), allowing us to compare the effects of ram pressure stripping across three decades in group/cluster mass. We find that jellyfish galaxies are most commonly found in clusters, with the frequency decreasing towards the lowest mass groups. Both the orientation of observed radio continuum tails, and the positions of group jellyfish galaxies in phase space, suggest that galaxies are stripped more slowly in groups relative to clusters. Finally, we find that the star formation rates of jellyfish galaxies in groups are consistent with `normal' star-forming group galaxies, which is in contrast to cluster jellyfish galaxies that have clearly enhanced star formation rates.

The orphan of the stars

Today's paper is unusual in that I hardly ever read anything about X-rays. Gas that's hot enough to emit X-rays tends to be incredibly hot, low density, smooth and featureless. It's useful for measuring the mass of galaxy clusters, but it's not interesting to look at and the physics sounds horrible.

The feature described here, however, is sufficiently weird for me to venture a comment. The bulk of the X-ray gas in the Abell 1367 cluster is, as you might expect, a big boring blob, filling the whole cluster and probably responsible for all the gas stripping of galaxies which fall into it. Perfectly normal. But just a little way outside the smooth  main body is a so-called "orphan cloud", a distinct, crescent-shaped overdensity of gas. Now large individual galaxies do tend to have their own X-ray gas, but this one isn't clearly associated with any galaxy.

As well as the X-rays, the cloud also has a H-alpha component : much cooler than the X-ray material, but still hotter than the cooler gas normally found in galactic discs (atomic and molecular hydrogen). The H-alpha morphology is quite complex. Within the Orphan, it's found mainly in the tips of the crescent, but nearby it's seen in a stream extending from a galaxy roughly in the direction of the X-ray cloud.

The H-alpha material is interesting because its kinematics can be measured. There's a velocity gradient across the cloud of about 200 km/s, but there's no clear pattern of rotation. The velocity gradient and the size of the H-alpha region are not that different to some of the optically dark HI clouds seen in Virgo, but given that this one is embedded in hot gas of a mass a thousand times greater, they could well be completely different phenomena. On the other hand, since the Orphan has clouds of multiple components, perhaps the Virgo clouds also have hitherto undetected hotter gas.

The most likely explanation seems to be that the Orphan has lost its parent by some result of stripping, as most galaxies do when falling through the hot gas. But exactly how this happened, why it produced this one particular and quite unique feature, is unknown. Why don't we see more features like this ? The mass and kinematics of the cloud suggest a massive parent galaxy, which should be easy to spot, but they don't suggest any particular candidate. It's also unclear how it could have survived as such an intact, coherent feature for long enough to become well-separated from its parent, or why it's such a discrete feature and not part of a nice long stream.

I wonder if the nearby galaxy with the H-alpha might actually be not the source but a consequence of the cloud. Perhaps it passed through the overdensity of X-ray gas and thus had a much stronger amount of ram pressure than a galaxy normally would this far outside the cluster centre. That would take some detailed modelling to properly answer.

What else could the cloud be ? Being optically dark and with negligible or no star formation activity, it's unlikely to be a galaxy - it would have to be a really massive feature to hold this much hot gas, and it's very hard to see how it would prevent star formation. Nor is there enough star formation occurring to explain the cloud as an excitation of the general intracluster medium. So "stripped out of a galaxy" seems the most likely explanation, even if that hardly answers all the questions as yet.

An H α/X-ray orphan cloud as a signpost of intracluster medium clumping

We present the discovery of the first and still the only known isolated cloud (or orphan cloud [OC]) detected in both X-rays and H-alpha in the nearby cluster A1367.This example shows that stripped ISM, even long after the initial removal from the galaxy, can still induce ICM inhomogeneities. We suggest that the magnetic field can stabilize the OC by suppressing hydrodynamic instabilities and thermal conduction. This example also suggests that at least some ICM clumps are multiphase in nature and implies that the ICM clumps can also be traced in H α. Thus, future deep and wide-field H-alpha surveys can be used to probe the ICM clumping and turbulence

EAS 2021 : The Conferencing

This time last year I went to my first purely virtual conference, EAS 2020. I happened to spend all my spare time that week mucking around with my shiny new VR headset on account of living my entire life digitally.

One year later, history repeats.

Well, almost. I don't have another new headset, but I did by sheer coincidence decide to spend all my spare time mucking around with Virtual Desktop to get the PC wireless working very much better than it used to. I also strapped a powerbank to the back as a counterweight that also charges it up, prolonging the battery life by (I reckon) at least threefold whilst making it a lot more comfortable to wear. So that's nice. What about the conference ?

EAS 2021 : The Zoomening

As before, let's do the experience and the science separately. First off, the niggles, which remain unfortunately much the same. There were once again too many people giving multiple talks, and too many plenary sessions. This promotes inequality and I don't like it. Yes, there are worse problems in the world, but it's still an annoyance : why do I get a mere poster (this time not even with a token 1 minute presentation slot) but other people get two or three talks and plenary sessions take two hours per day ? Seems unfair if you ask me.

In a rather different niggle from last year, I found this time that the session titles were often misleading.  "Galaxy clusters and AGN" didn't feature all that much about AGN, while "Satellite galaxies" didn't have that much even about satellite galaxies, etc... why people decide to give their sessions ultra-specific titles but then include very liberal content, I don't know. It's a minor irritation, but it makes it harder to know which session to submit an abstract to, and harder to plan which sessions to attend : you really had to check the scheduled talks quite carefully and not just go by the session titles.

On that note, I found an interesting and unexpected use for AdBlock. Here's the conference schedule as it appears by default :

Which is hideous. Probably less than a quarter of the screen is taken up with actual useful content, i.e. clickable schedule blocks. I got so frustrated with this obscene conference scheduling censorship that I used AdBlock's "hide something on this page" feature to cancel out the annoying extraneous faff and blam :

Much better ! There was actually a decent enough interface hidden under there - less is indeed more.

The final niggle I have to mention is e-posters. While they offer many advantages over traditional conference posters, their major weakness is that the interface is essentially useless for browsing. In a real conference you can casually walk by and look at the titles and/or biggest pictures, but it's bloody tedious to do that virtually. Not only because there's no good way to quickly search for potentially interesting posters, but also because they just don't look nice. Here's mine as it appears in the gallery :

Basically pointless. The poster is mainly images, but the preview shows almost nothing but text. Granted things improve if you deliberately order the images nearer the top of each panel, but this would make the content incoherent so I didn't. Even if you click through to the poster itself (the link works at the time or writing but won't last forever), all you initially see is a bigger, higher resolution version of the above. There's got to be a better interface than this. For example, if you could upload a preview image to display in the gallery, that could easily be much more eye-catching that the scaled-down view that's generated automatically. Yes, there was the option to upload a traditional PDF as well, but... the thing is, e-posters are two hundred times easier to build. So I took the lazy option and made a hideous poster. Oh well.

The Sciencening

Let's move on to the talks. There were quite a lot of good review talks but not that much in the way of interesting new discoveries. That said, I have to give an honourable mention to Heloise Stevance, whose talk on public speaking should probably be required viewing. Although there weren't any "oh god just shoot me instead" talks this year (unlike last time), everything tended towards a certain... mediocrity. Stevance's talk was the only one where the speaker conveyed real energy and enthusiasm. Granted, this is exceptionally difficult to do when talking to a monitor, especially when you don't see any of the other participants, but even in real-life presentations too many speakers are positively sullen bordering on depressed. I have to say that my own institute is particularly bad at this, as though everyone is positively afraid of sticking their head above the metaphorical parapet in case it gets metaphorically lopped off for some reason.

As well as the essential but apparently not-so-obvious points that you should be enthusiastic and not monotonic, Stevance made the interesting case for designing talks and papers very differently. Instead of trying to make a talk a micro-version of a paper, she says that instead you should build the talk explicitly around the take-home points. I think this is generally good advice, especially for short talks to specialist audiences. In a longer seminar or to a broader audience, you can and should afford to spend a disproportionate time on a lengthy introduction to get everyone up to speed. In a 15 minute presentation to specialists, you can practically reduce the introduction to a sentence or two, and spend the rest of the time discussing the science. Not the methodology, but the results. A lot of speakers seem obsessed with the methods to the point that what is is they actually found - the thing we've all come to learn about - gets almost tacked-on at the end. 

Come to that, too many talks were about planned projects or results at a preliminary stage. Frankly, that's a bit daft. Overwhelmingly what I want from a talk is an interesting new result, so if someone wants to present a new survey they haven't even started yet, they should bloody well do a poster instead. This seemed particularly problematic this year, or perhaps I've just lost patience because I feel badly in need of a holiday.

Sigh.

(Is it just me though ? Does everyone else long for detailed descriptions of upcoming surveys and planned instruments ? There doesn't seem much possibility for discussion about things that people are going to do but can't actually do yet. Compare that were data that's already been analysed - there you have the chance for actual productive conversations - which seems like something very important in a conference, to me.)

Anyway, jellyfish galaxies were much in vogue this year. In general they're the result of strong gas stripping occurring as a galaxy slams at high speed through the hot gas in a cluster, leaving behind tails that can sometimes look quite a lot like the tentacles of a jellyfish. Luca Cortese made the point that this phase is common but not ubiquitous, and in extreme cases it's not just the hot or warm outer gas that can be stripped but even the very cold, enormously denser gas as well. Annalisa Pillepich demonstrated from simulations that for a jellyfish to result in a permanent morphological change to a galaxy, the stripping has to actually remove the gas from the galaxy, not just cause a temporary disturbance - otherwise when the gas returns, the galaxy will reclaim its original structure.

There was also quite a lot about data science. Most of this was about machine learning and suchlike, but there was one very nice presentation by Lucia Marchetti about the iDaVIE visualisation tool. This is explicitly based around realtime rendering and analysis using virtual reality headsets, which is just scandalously cool. I'm hoping I'll have time to try out the code sometime this week. Though this is something that's long been on my Christmas list for the awesomeness alone, I'll confess to being just a little bit skeptical that doing analysis this way would actually offer much advantage over traditional methods. That said, some of the tools do look promising, and potentially easier to do in VR, which bodes well. And encouraging users to actually spend a long time just looking at their data is definitely a good thing. The more visualisation tools we have, the better.

A great deal was said about Ultra Diffuse Galaxies, particular by using their globular clusters to try and estimate their masses. An interesting controversy came up regarding DF44, the UDG that's the main candidate for being a "failed giant" with the mass of the Milky Way (the consensus seems quite clear now that most if not all UDGs are just dwarfs), with rival claims made using the same data that establish very different masses. The cause of this disagreement isn't yet known, so I'm going to wait and see what comes onto arXiv before commenting further. 

There was also quite a bit about "pre-processing", a term everyone seems to have become confused as to what it actually means. Certainly there are different evolutionary processes affecting galaxies inside and outside clusters, as well as conditions resulting from external influences acting on galaxies and those resulting from internal processes. To my mind, pre-processing means any external influence that acted on a galaxy prior to its infall into a cluster. Seems simple enough, and I'm not sure why this is at all problematic.

Rory Smith gave a very nice overview of the state of the art - mainly from the perspective of simulations, but with plenty of observational stuff too. Galaxies with lower star formation activity are seen too far from cluster centres for them to have experienced cluster processes, so something else must be at work. One plausible effect is that filaments - the largest-scale structures in the Universe - actually have their own hot gas which could cause ram pressure stripping. Probably not enough to remove the cool gas within galaxies, but enough to remove their outer reservoirs of hotter material and so eventually reduce their star formation. In support of this, clear trends are visible in filaments in age, metallicity and colour of their galaxies, which is pretty compelling evidence that something is happening. Less clear is whether this really is ram pressure or something else, as there's not much constraint on the gas density. 

Lyla Jung raised an interesting point about selection effects that seems obvious when you say it but wouldn't have occurred to me otherwise. That is, large clusters tend to be assembled from large groups. And large groups will cause more pre-processing, so there are selection effects as to where pre-processing is important. 

One potential example of this came from Tirna Deb, who presented evidence that within galaxy clusters it's actually the most isolated galaxies which are the most gas-deficient, the exact opposite of what you'd expect ! But from discussions afterward, Palo Serra suggested that galaxies which are still in sub-groups within clusters are likely recent arrivals, since groups are soon broken apart by the hungry cluster - and group effects are much weaker than clusters. Conversely, galaxies not in sub-groups are more long-term residents, so they've experience the full whack from the cluster already - hence the more isolated galaxies should be the most deficient. There does seem to be pretty decent evidence for this in that sub-groups tend to be found at higher distance from the cluster centres. So a very neat, elegant explanation for a counter-intuitive effect.

So that's years virtual overload over and done with. Mildly interesting, though disappointingly lacking in anything controversial. I'm almost tempted to say that the EAS should drop their official code of conduct policy just to encourage more fights to break out. Or perhaps we all need specialist training in How To Give An Enthusiastic Talk Over Zoom. Or holidays. Holidays are nice.