Red, dead, but very well-fed

What stops galaxies from forming stars ? Normally I'm most interested in this for objects which have hardly any stars at all : ultra diffuse galaxies, ultra faint dwarves, dark galaxies and suchlike. But there are also cases of objects which have managed to form tonnes and tonnes (literally) of stars in the past, still have oodles of gas today, but are doing diddly-squat right now. What's going on ?

Today's paper is an attempt to catalogue some of these "quenched HI-rich" galaxies, "quenched" just being standard jargon for "not forming many stars any more". It's not surprising that when galaxies run out of gas they stop forming stars, it's the whole "gas rich and quenched" thing that's surprising. The details get rapidly complicated. For instance, the atomic gas doesn't seem to (much) participate directly in star formation, but has to condense into a molecular state first, which makes the correlation between star formation and atomic gas hard to interpret. And most gas loss removal mechanisms involve initially compressing the HI, i.e. transforming it to molecular and so potentially triggering a burst of star formation before quenching.

We also know that in most normal galaxies the HI extends well beyond the stellar disc. Generally in the outskirts the gas density is very much less than in the inner regions, so low density can certainly mean a lack of star formation. But it seems extraordinarily rare to encounter cases where the gas density is low everywhere. After all, if you have a massive stellar disc, gravity ought to help the gas to collapse to higher densities in the central regions, ensuring star formation keeps ticking over.

The great bulk of this paper is given over to proving that such objects do in fact exist. Most gas-rich objects are spirals and irregulars, which are happily churning out stars, but it's not so unusual to find at least some gas and star formation in ellipticals. These are often known as "red and dead" because their (much older) stellar population tends to be reader, and star formation rates tend to be much lower than in spirals. Especially so in clusters, where most ellipticals are found, where they very often appear to have no gas at all.

So what they're looking for are gas-rich red galaxies. Previous studies, they note, have been widely criticised because they get the star formation rates wrong, so many claimed quenched gas-rich galaxies may simply not be as quenched as thought. They try and improve things by using more robust selection criteria, especially using UV emission to quantify colour as this is more sensitive to ongoing star formation than traditional optical wavelengths : UV photons are higher energy and preferentially emitted by hot, young stars. Most of their HI data comes from the large ALFALFA survey, plus a couple of other Arecibo projects.

This is a very nice, easy-to-follow, tremendously narratively linear paper that spells out what they did in great detail. Basically they define their sample in terms of HI mass and colour and make different cuts to have both a putative gas-rich red sample but also different comparison samples of the same stellar mass, colours, etc. And they show that yes, their main sample is weird, being mostly elliptical galaxies which are very red but have shedloads of gas. They're clearly outliers from the trends in a way that their control samples certainly aren't. Compared with previous findings these ones are definitely unusual in their gas content, as they confirm that most galaxies with similar stellar parameters tend to be very gas poor.

This did make me wonder why, if these red/dead/fed objects are so structurally similar to ordinary galaxies, what changed ? That is, they must have been forming stars in the usual way in the past, so why is the star formation in these objects only now being suppressed ? This isn't such a problem for objects which have never managed to form many stars at all, but I'll get back to that.

Because the paper is so concerned with establishing the existence of the objects, it doesn't go much into detail as to what might be causing the quenching. It can't be gas loss, but it might still be gas density. There are some tentative hints that the gas might be more extended in these objects than others, but this is based on scaling relations – only one galaxy in their sample actually has resolved HI observations, and this is only a bit more extended than normal. They suggest this could be the result of high angular momentum, though it's a bit disappointing that they don't do anything with the HI kinematic data to see if this is consistent. And they note that the galaxies tend to live in low-density environments and in relatively small dark matter halos, but this doesn't address what happened to stop them forming stars when they clearly used to have no such problems. Well, we all have problems as we get older...

Most of the paper is rife with statistics and endless graphs. Normally my eyes glaze over with papers like this but I found their presentation style really very good indeed : for a potentially dry statistical analysis, it's remarkably un-put-downable. 

Except... it contains not a single image of any galaxy [but see below for a major caveat !].

Here's where I get seriously worried. When you actually look at the sample, yes, some of them do indeed appear to be your classical ellipticals : red, smooth, structureless, and ultimately aesthetically boring. But even going through just the first ten, it's clear that these are almost the exception rather than the norm. Their very first object is clearly a spiral, with a dominant central bulge but enormous, blue spiral arms ! Their second has a funky interaction going on, their sixth and eighth appear to be basically normal spirals, and their tenth appears to be a freakin' polar ring galaxy. Some of them are really spectacular oddballs that defy description, like this one. Many of their objects have tens of references in the literature but they don't remark on any objects individually : it's purely statistical.

This has me concerned that all their very careful work is for nought. Rather than being the globally-quenched objects they claim, many of their sample might well just have little or no star formation in their central regions, which is not so unusual for spiral galaxies. Visually, at least, that's certainly what it looks like : big red smooth bulges surrounded by complex, blue, likely star-forming extended arms. Not mentioning the ring, in particular, is a bit of a red flag, because these are such interesting systems in their own right that you'd think this is surely worth reporting.

STOP PRESS : At the last minute I realised I'd read an older version of the paper prior to its passing peer review. The accepted version contains an appendix which has cutout images of their whole sample. I don't want to re-read the whole paper, but their section 4.3 does now acknowledge that about half their sample contains rings or other extended features. From what I can tell, though, they haven't changed their major conclusions or adjusted the morphological classifications, or made any significant changes to any of the other figures.

I find that very strange. When you look at the images, it's pretty obvious they cannot possibly be as quenched as they claim : there are some really striking blue structures here which just must be forming stars. For sure, some of their objects are indeed classical, smooth ellipticals, and those will be interesting. The rest may well be interesting as well but I doubt very much as examples of fully-quenched galaxies. My guess is that the gas is indeed extended, but it's experiencing local star formation in the usual way with the central regions being quenched through simple gas depletion. I'm also extremely skeptical of the deep learning paper that presented the original classifications, though I should try and read that. Another factor might be the relatively low sensitivity of the GALEX UV observations, which might not have picked up the faint outer star-forming regions.

In short, there may be a few interesting objects here, but most of them don't look all that impressive or unusual from a quenching point of view.