To twirl or not to twirl, that is the question

There's two ways to stop a galaxy from collapsing. Either it can spin around, which flattens the gas and stars into a disc, or everything can be on chaotic, random orbits. The latter doesn't really work for gas, which is collisional and tends to do unpleasant things like shock and lose energy.

What about all those recent ultra diffuse galaxies ? If we want to know if they're dwarfs or giants, which will affect theories of galaxy formation, we'll need to know if they're spinning or supported by dispersion. This paper looks at a sample from simulations.

The paper gives a very good overview of the current state of play for UDGs, noting the interesting shenanigans going on with deviations from the Tully Fisher relation, the diversity of the sample, the different ways to measure their dimensions, and the different formation mechanisms proposed. Essentially there are two main ideas : they could form due to internal processes, or they could form in clusters (where most UDGs have been found) due to unique environmental processes. The later doesn't seem likely, or at least it certainly isn't sufficient, since we now know of isolated UDGs that have never even see a cluster.

The one major thing I don't like about this cluster is that they're unclear about their simulations and sample selection. I don't even like reading these bits of simulation papers and I normally just skim them, but even I found this too brief. It's important to know what the mass resolution is and what they mean when they say they selected an "isolated" sample - how you define isolation is basically arbitrary. They don't even say how many galaxies were in their simulation in total or how large a cosmological volume they simulated. Sure, I could consult the original simulation papers, but I shouldn't have to : these parameters are fundamental to this paper.

That aside, they find that almost exactly half of their sample of 38 isolated UDGs are dispersion dominated and the other half rotation supported. They tend to be gas rich (though "rich-most" is an ugly phrase indeed), though dispersion dominated galaxies are more gas poor. This is partly a selection effect, in that rotation is better at supporting more extended systems at any given mass. Also, their objects are not part of the high-spin tail of normal galaxies, as one of the most popular early papers claimed.

Interestingly, at these masses, supernovae feedback is expected to be highly efficient, but they find that this plays only a secondary role in determining whether a UDG is rotation or dispersion supported. The major driver appears to be how the gas accretes. If it happens to align with the existing rotation of a proto-galaxy, the accreting gas helps increase the rotation. This lets it maintain a steady accretion of gas without any destructive starbursts. If, however, the accreting gas is misaligned, then chaotic accretion leads to bursts of star formation and supernovae feedback that quickly reduces any further gas inflow. So rotationally supported systems tend to have later gas accumulation.

Prior to the lockdown I was writing a paper on gas observations of UDGs. Although the deviation from the Tully-Fisher of some objects is weird and interesting, quite a few don't show this : they look like normal rotating discs. So this paper makes a lot of sense to me, but I would have liked more information. How extended are these galaxies using different radius measurements ? What's their typical star formation rate - do they have much molecular gas ? How are they still accumulating gas when they're in isolation ? On the other hand, if they'd given all that I might have dismissed the paper for being too long and not read it at all...

NIHAO XXIV: Rotation or pressure supported systems? Simulated Ultra Diffuse Galaxies show a broad distribution in their stellar kinematics

In recent years a new window on galaxy evolution opened, thanks to the increasing discovery of galaxies with a low surface brightness, such as Ultra Diffuse Galaxies (UDGs). The formation mechanism of these systems is still a much debated question, and so are their kinematical properties.

The slightly surprising survival of spirals

Galaxy evolution is very similar to that of large, ruthless corporations : it's driven by merger after merger, until what started off as a small family shop becomes a huge international conglomerate that sells weapons specifically designed for killing babies living in third-world countries. Or something. A slightly more popular analogy is that galaxies are cannibals, but this is no less grisly as it still involves fat, bloated monster galaxies that mercilessly devour their smaller, weaker siblings.

Look, I've been stuck inside for a month. I've got to make my own entertainment. Stop judging me !

All that horror and questionable politics aside, there's a bit of a puzzle here. Merging is thought to be the most common way to turn spiral galaxies into ellipticals. But if that's so, how can there be any massive spirals around today ? Shouldn't their formation necessitate lots of mergers, making them far more likely to become ellipticals ?

The authors of this study look at one of the all-singing, all-dancing, "everything on" cosmological simulations to trace the evolution of the most massive spiral galaxies. This simulates a large cosmological volume, so unlike smaller simulations (which have to be focused on specific environments) it includes a whole range of evolutionary processes. It's also of extremely high mass resolution, enough to simulate even very modest dwarf galaxies. This means that from the perspective of the most massive spirals, which they concentrate on here, it should be complete.

They find that giant spirals are indeed rare. Of galaxies with more than 100 billion solar masses in stars, about 11% are spirals, in rough agreement with observations.

There are two different ways they can form. About 70% result from mergers... but not just any mergers. The mass ratio of the progenitors doesn't seem to matter much, but their gas content does. Since the gas is collisonal, the merger causes it to shock and collapse to a disc supported by the angular momentum imparted by the merging galaxy.

Even more persuasively, 98% of all massive spheroids which experience a gas-rich merger subsequently become discs, so gas-rich mergers appear to be a highly effective route for disc formation. And the discs tend to have had their last major merger more recently than spheroids, meaning there hasn't been much time for other, unsuitable mergers (or other processes) to destroy them. In addition, as the gas content of the simulated universe decreases over time, so does the fraction of galaxies which are massive discs. It's pretty clear that it's gas what done it.

Apart from these "rejuvenated discs", the remaining 30% are always discs. Presumably they initially form by mergers, but subsequently they get lucky and have a very quiet merger history. They mention an interesting idea was that such galaxies could be near to even more massive galaxies whose enormous gravity could protect their smaller brethren from incoming would-be mergers, but this doesn't appear to be the case for their sample : they seldom have a more massive companion. So for the most massive objects, at least, it's simply a matter of luck.

Whether this has any bearing on runaway capitalism or the culinary practises of uncontacted jungle tribes is left as an exercise for the reader.

Why do extremely massive disc galaxies exist today?

Galaxy merger histories correlate strongly with stellar mass, largely regardless of morphology. Thus, at fixed stellar mass, spheroids and discs share similar assembly histories, both in terms of the frequency of mergers and the distribution of their mass ratios.

Filament finding fun

Where do galaxies get beaten to death ? Is a galaxy falling into a cluster about to be torn limb from limb, or is it likely to be half-dead already ?

Of course, it probably varies considerably. We know that the processes at work in clusters - mainly tidal harassment and ram-pressure stripping - certainly can cause enormous damage. And we know that at least some galaxies falling into clusters look, for the time being, perfectly healthy. But of course the real question is what happens in a typical case, if there even is such a thing.

Since we can't wait around for billions of years to track individual galaxies, like most extragalactic problems we have to tackle this statistically. As we generally have rather small samples - a few hundred galaxies per cluster - we plot radial trends in galaxy properties (colour, gas content, morphology and so on) as clues to the general trends at work. The problem is that this biases us to looking for cluster-induced changes. What about processes at work more locally, or even occurring to galaxies before they've fallen into the cluster proper ? Such "pre-processing" might be very important, but would be easy to miss in radially-averaged plots.

Being outright annoyed by such seductively simple but potentially misleading techniques, my postdoc* Boris Deshev decided to try something more sophisticated and plot Voronoi maps. We can't avoid the need to bin the data due to our small sample size, but this method allows us to really see the 2D map of various properties instead of the 1D radial average. It's a sort of adaptive-gridding that's good for dealing with points that vary widely in density from place to place, whereas a uniform gridding would wipe out a lot of fine details.

* Can one own a postdoc ? Native Americans say you can't own land, but as far as I know they don't say anything about postdocs.

The main map in the paper is a Voronoi map of the fraction of star-forming galaxies. This does not show the neat, circular trend that you might expect if the cluster was dominating galaxy evolution. In earlier drafts, we saw a clear north-south filament of galaxies dominated by non star-forming objects, but sadly this largely faded from view with more accurate star formation measurements. But it re-appeared when looking at the specific star formation rate, that is, the star formation activity accounting for the mass of each galaxy.

It's even possible to show that those objects haven't had their star formation reduced recently - it must have happened in the relatively distant past (> 500 Myr ago). So it does appear that pre-processing is having a significant influence : part of the cluster is assembling along a filament in which galaxies have already had their star-formation activity reduced. This Voronoi mapping is a neat way to show things that radial plots cannot, though the disappearing filament is an important reminder that small differences can sometimes make a big difference to the result.

This cluster is also one of only two at this distance (about 2.6 billion light years) which have HI measurements. At that distance, the resolution of the observations isn't great. Even so, there are some intriguing hints of galaxies being caught in the act of gas loss, with their HI noticeably displaced, stretched, and shifted in velocity from their most likely parent galaxy. The main problem is identifying the parent galaxy : we can't be sure that it isn't actually some barely-visible little blob that's just about visible within the beam of the telescope. But a few cases look intriguing enough that it was certainly worth reporting, as no-one's detected this at this redshift before (two of them look pretty darn convincing to me). They'll be good targets for future observations at higher resolution, and there might be more candidates hiding in the data. Developing new ways to dig them out is ongoing.

Mapping the working of environmental effects in A963

We qualitatively assess and map the relative contribution of pre-processing and cluster related processes to the build-up of A963, a massive cluster at z=0.2 showing an unusually high fraction of star forming galaxies in its interior.

The dirt in the discs

What, another paper on dust ? "But Rhys", I hear you say, "you said you hated dust !". Well, I do - it's awful. Outside of His Dark Materials, dust is not the least bit interesting. I just can't bring myself to become interested in where it comes from or what it does. Come on - it's feckin' dust. Dust is something you get rid of. Every time someone mentions it, I immediately remember an advertising slogan for a hoover : "compress your dust, compress your worries". Almost as good as a jingle on a children's magazine, "I love horses, they're my friends !"... but I digress.

Anyway, what caught my eye about this paper was not the dust, but the detection of spiral structure in elliptical galaxies. Since such structures have been found in early-type galaxies in Virgo, this is a potentially interesting avenue for exploring galaxy evolution as a function of environment.

This paper looks at a sample of field ellipticals, using two deliberately similar populations : one with and one without gas (both HI and CO). They use the MegaCam instrument on the CFHT to produce colour maps, using reddening as an indication of dust content after controlling for the underlying stellar population (I also find myself wondering why both this and the previous paper never mention Herschel or other direct dust measurements). This shows a variety of different dust morphologies. Here's their main figure using the SDSS optical images :

And here's the same sample (with no attempt at all at keeping the scaling constant) using their dust maps, taken directly from the paper :

N = no dust; D = disc; R = ring; Ir = Irregular

Now to be fair, you can see some hints of these structures in the conventional images, but they're far clearer in the dust maps. NGC 3626 I find particularly interesting : although it certainly doesn't look like a typical elliptical in the optical map, the spiral dust structure is quite different again. And if you'd just shown me it's dust map, I'd have assumed it was a quite typical spiral shown with a weird colour scheme.

The main trend they note is that there are clear differences in dust morphology depending on gas content. The presence of any gas anywhere in the galaxy seems to be an excellent predictor of dust in the central regions, which it would be nice to know a bit more about... what's the physical connection between outer gas and inner dust ? Perhaps they say something in the appendix, but that's a billion pages long so I'm not going to read it. They also show that gas-rich galaxies are dominated by dust morphologies of spiral and irregular types, with no rings and a very few discs. Gas-poor galaxies have dust morphologies dominated by discs, rings and irregulars. Gas-rich galaxies tend to have large dust structures, whereas small dust structures are found in gas-poor galaxies.

Understandably, they put a lot of effort into comparisons of their study with others, both in terms of similarities and differences. In particular, their sample only deals with field galaxies, so cluster galaxies (which are almost unanimously gas-poor) may be different. But what does this tell us about galaxy evolution ? When some apparently early-type galaxies look so strikingly similar to late-types by looking at their dust, I think we have to wonder if this indicates a connection between the two. They speculate that the origin of the dust could be due to internal production (by stars, despite the low rate of star formation) and brought in externally, but honestly, that's boring. I think this result is sufficient to warrant asking bigger questions about the connections between galaxies of different morphologies. Does one type evolve into the other ? Do they have more similar formation mechanisms than previously suspected ? I dunno, but that to me is way more interesting than any amount of bloomin' dust.

Cold gas and dust: Hunting spiral-like structures in early-type galaxies

Observations of neutral hydrogen (HI) and molecular gas show that 50% of all nearby early-type galaxies (ETGs) contain some cold gas. Molecular gas is always found in small gas discs in the central region of the galaxy, while neutral hydrogen is often distributed in a low-column density disc or ring typically extending well beyond the stellar body.

The Virgo Cluster is a dirty, dirty place

And now we return to regular boring old science with nary a space vampire in sight.

One of the many, many wonderful things about the Virgo Cluster is that it lies behind a great big hole in Galactic dust. So we can measure the galaxies there without worrying too much that the foreground dust has made them appear significantly redder and fainter than they really are*. That's especially nice for me, because I can't stand dust. I just cannot bring myself to get interested in it. Sure, it may be an important component of star formation, but... come on, it's dust. Don't expect an insightful commentary, is what I'm saying.

* This is called extinction, presumably just to be confusing.

Anyway, through some very sophisticated modelling it is possible to correct for the reddening due to foreground dust. Once that was done, the authors looked at the extinction variation of Virgo galaxies that must be caused by extragalactic sources. Normally I'd probably be skeptical, but the trend is so darn clear - more reddening near the cluster centre with a very rapid drop off with radius - that
it looks to be pretty convincing, at least to a naive dust-aversion person like me.

They're even able to make a map of the extinction, albeit at low resolution, but you can clearly see something. Dunno what it is, but it's definitely there (the issue is you can only measure the dust by looking at galaxies which have been reddened by it, rather than detecting it directly). Broadly, they say it seems to follow the same distribution of the intracluster light, thought to result from stars that have been thrown out of their galaxies by tidal interactions. They estimate the total mass of dust at just 3 billion solar masses, which is about the same as the gas mass of a single large dwarf galaxy. Being able to detect this when spread over such an enormous area is pretty darn impressive.

The main thing I wonder about is survival. They say the expected lifetime for dust in the cluster environment is about 100 million years, which is not all that long. Are there likely to be enough tidal encounters pulling out sufficient dust to explain the observations ? There's not that much of it, but I would expect it to be pretty hard to remove since it should be mainly found in the inner regions of galaxies. I dunno. I might just be too naive on this, but it would certainly be interesting to know how much dust can be removed in an encounter and then get a handle on how often such encounters must occur. Which would likely end up as an exhausting project involving all kinds of ghastly physics with a result that may or may not be interesting to anyone. Such is life.

The GALEX Ultraviolet Virgo Cluster Survey (GUViCS) VIII. Diffuse dust in the Virgo intra-cluster space

We present the first detection of diffuse dust in the intra-cluster medium of the Virgo cluster out to $\sim$0.4 virial radii, and study the radial variation of its properties on a radial scale of the virial radius.

Behold April

In the words of the great Jeremy Paxman when he was forced to present a weather forecast, "Well what did you expect ? It's April", so it is for arXiv. Here's a brief round-up for those who missed it.


The Really Habitable Zone

Who cares if liquid water can exist on a planet orbiting a star ? The important thing is whether conditions are suitable for making a gin and tonic, a region "which might actually be worth existing on." Astronomers, they say, need alcohol, and the presence of astronomers is a good definition of civilisation. And a lack of gin might explain why a planet has a terrible atmosphere.

(They note also the possibility of "ginspermia", in which juniper bushes propagate through interstellar dispersal, but suggest that efficient harvesting for maximum gin productions means there are no spare bushes flying through space.)

How to define the parameters for the RHZ as opposed to the old BHZ (Boring Habitable Zone) ? They followed standard practise and made them up. This all involved the consumption of multiple "gins and tonic", which is not a typo : "You want multiple gins, not more tonic." Adorable.


Searching for Space Vampires

"It is a truth universally acknowledged, that a single human in possession of a good space telescope,
must be in search of a space vampire." A strong opener indeed. Apparently inspired by an xkcd webcomic which notes that reflecting telescopes can't see space vampires, the authors realised that not all telescopes these days use mirrors. The Transiting Exoplanet - sorry, Exovampire Survey Satellite uses lenses, so is an ideal tool for searching for spaceborne undead.

They note three prospects for space vampires : free-floating, to be examined in a forthcoming paper by Van Helsing et al.; already landed on Earth; tidally locked around M-dwarf stars. Why M dwarfs ? Because while vampires are susceptible to sunlight, they are clearly unaffected by firelight, which is typically not too much cooler than M dwarf stars. Presumably daylight on planets around such stars holds no fears for vampires there...

They then drew a profile of a vampire and a bat in Microsoft Paint and fed it through some transit photometry modelling package to see the expected dip in the light. Comparing to observations from TEvSS, they find there could be between 0 and 394400933 possible space vampire transit events, i.e. between 0 and 100% of all observations. They consider this to be a major breakthrough.


Conspiratorial Cosmology

Reality is just a conspiracy and "generally misleading", say the authors of this the most incoherent of the suggestions. Loosely based on the idea that if anyone figured out the meaning of existence, the Universe would disappear and be replaced with something more complex, and this has already happened, the authors develop the concept of inflationary imbecility. This is like regular inflation, only for stupidity instead of space.

But who are They who are behind it all ? Following the Chuck Norris theorem - that there's an easy way and a hard way - they suggest that They started with a simple Universe and have been gradually ramping up the complexity. Recent events comprise such things as the challenge of electing an unelectable president and controlling an uncontrollable virus.

And why is the Universe controlled by so much dark matter and dark energy and the like ? They solved this by employing occultism, in which a medium declared that it's due to the Fertile Neutrino (as opposed to sterile neutrinos, of course). This is, apparently, very fertile up to several "guinea pig units", and the ongoing production of fertile neutrinos causes the expansion of space. There follows a lengthy rant about the anthropic principle, philosophy, string theory, the importance of simulations, eventually concluded that we ourselves are the grand conspirators behind it all, somehow. I got as far as, "In order to make sure that we need all the energy we harvest, we will build our spaceships in form of giant SUVs which are constructed such that they have a decent wind-resistance even in the interstellar medium, and we will cause a greenhouse effect in the Galaxy so strong that Dyson-trees (Dyson, 1997) start to grow on molecular clouds" before deciding that the words, "please stop" had never been more appropriate and gave up.


And finally, overheard elsewhere :

Normal services will be resumed as soon as possible.