The galaxies that quenched backwards

Today's paper is about contrarian galaxies that don't play by the rules. Most galaxies, left to their own devices, build up a big bulge of stars in the middle as the gas density is initially highest there. But this high star formation rate burns through its fuel very quickly and soon pitters out, "quenching" the star formation in the centre. Meanwhile the disc happily ambles along forming stars at a stately, steady pace, unhurried but longer-lasting.

There are some, though, which appear to do just the opposite. In clusters this is easy. Ram pressure preferentially removes the outer gas first, leaving the most stubborn gas remaining in the centre to carry on forming stars while the disc slowly reddens and dies. What's weirder is that there appear to be some galaxies like this in environments where ram pressure can't be playing any role. Ram pressure requires a hot intergalactic medium, which is pretty much only found at any significant levels in massive clusters. Everywhere else it should be far too weak to cause this kind of damage*.

* It probably doesn't have zero role to play outside clusters though. Very small satellite dwarfs can experience significant ram pressure from the hot gas of their parent galaxies, and there's some evidence that larger galaxies can experience at least a little gas-loss due to ram pressure in large-scale filaments. But probably not anywhere enough to account for galaxies like this. What they more likely experience is only starvation, where their outermost, thinnest gas is removed but nothing from within their denser discs. This means they can't replenish their gas as it gets eaten up by star formation. 

This paper is about a particular sort of these kind of galaxies, which they give the ugly name of "BreakBRDs" : Break Bulge Red Discs. The "break" refers to a particular spectral line indicating that there was star formation very recently in the bulges. "Blue Bulges Red Discs" might have been easier, but technically the bulges aren't actually blue, so this wouldn't be right. Even so, BBRDS would be a better acronym. Or heck, I'll just call 'em backwards galaxies.

I have to say I both like and dislike this paper. On the one hand it's very careful, thorough, and doesn't draw any overblown conclusions from the limited data. On the other, some of the discussion is long-winded, non-committal, and rather tedious considering that in the end the conclusion is so indecisive. I think there's some really great discussion on each individual scenario proposed to explain the backwards galaxies, but the collective whole becomes at times very confusing. It feels a bit like a paper written by committee. The main problem is nothing to do with the science at all, but the structure : there isn't a good unifying framework to tie all the different scenarios together. 

Here I shall try and simplify and disentangle things a bit. 

They begin with a nice overview of the observational difficulties of establishing what's going on. Some results support this classical picture of outside-in quenching where ram pressure (or other environmental effects) strip the outer gas first. But others find that this happens more in low density environments, exactly the opposite of what's expected ! Still others show something entirely different, where quenching happens irrespective of position in the galaxy, i.e. the whole disc quenches everywhere, all at once. Even simulations aren't much help, with galaxies similar to their BBRDs being found but with no clear mechanism responsible for what happened.

The particularly interesting thing about BBRDs, i.e. backwards quenchers, is that they exist over a wide range of stellar masses and apparently in all environments. Unfortunately they don't say anything else about the environments(s) of their particular sample, which is my only scientific quibble with the paper. Anyway what they do here is look at a sample of about a hundred or so which have HI gas measurements, using a combination of existing and their own new observations.

Their main result is actually quite simple, but you need to be aware of the colour-magnitude diagram first. Very simply, there are :

• Galaxies which are blue, have lots of gas, and are forming stars as per usual (the so-called star formation main sequence, or more simply the blue cloud).
• Galaxies which are red, don't have any gas, and aren't forming any stars : the red sequence.
• Galaxies which are intermediate between red and blue, have a bit of gas, and are forming stars more slowly : the green valley (or transition region).

Of course, regular readers here will know that it isn't as simple as that, but this'll do for what's needed here.

They show that BBRDs and GV (green valley) galaxies have about the same gas fractions regardless of their stellar mass, both considerably lower than those in the blue cloud. But given their star formation rates, BBRDs would burn through their current gas content very much faster than GV galaxies. That is, for the same amount of gas, BBRDs are consuming their gas far more efficiently than GV galaxies. They're forming stars at rates typical of blue cloud galaxies, even though they've got way less gas. GV galaxies, by contrast, are forming stars even slowly than those on the main sequence.

How do they do this ? Or, conversely, what suppresses the star formation in GV galaxies ?

This is not at all easy to answer. For the GV galaxies, their gas depletion times seem to be low because of their extremely low star formation rates, which more than compensates for their lower gas contents. For the BBRDs, their rapid gas depletion times are easier to explain, as their star formation rate remains normal despite their low gas contents.

But what's behind it all ? There are three main options.

1) The final stages of quenching

These BBRD galaxies could be experiencing the end stages of gas removal due to some external mechanism, as is usually the case in clusters. Perturbations to the gas could drive inflows into the centres of the galaxies, resulting in an increase in gas density and thus an increase in star formation rate. Unfortunately they don't have the detailed maps of the gas needed to say if this is the case or not, but here it would have been useful for them to say something more about environment.

2) Accretion

It could be that these galaxies were in fact already fully quenched and are experiencing something of a renaissance. Accretion is a somewhat controversial topic because it's hard to ever determine that it's really happening with any certainty, but clearly galaxies have got to get their gas from somewhere. One possibility is so-called cold accretion from the cosmic web, where the hot, diffuse extragalactic background cools and condenses, falling into the potential wells of galaxies along streams.

Reading between the lines I think this is their favoured scenario. There are several reasons to think the gas isn't doing what it's supposed to be doing. For a few galaxies they have well-resolved kinematics of the gas and stars, and here it tends to be either misaligned with and/or highly distorted compared to the stars. They've only got such data for the innermost regions, but the spectral profiles of the gas (measured over much larger scales) also suggest it's significantly asymmetric. And they're also systematically offset form the Tully-Fisher relation : that is, the gas kinematics has broader line widths that typical galaxies of the same mass. 

All this is what you'd expect if the gas had only recently arrived rather than evolving along with the galaxy throughout its history. What they can't say is whether this indicates a temporary revival of the galaxy or a full resuscitation. They might, potentially, be rejuvenating themselves back onto the star formation main sequence, or they may have just accumulated a little bit of gas and will soon burn out once again. My guess is that latter is more likely but this would mean galaxies would have extraordinarily complicated histories.

3) Mergers

The most dramatic way a galaxy can accumulate more gas is by gobbling up other galaxies whole. This is their least favoured scenario as there are no signs of the remnants of the encounter you'd expect, e.g. long stellar tails. They can't rule it out though, as it's possible the star formation is persisting long after the tails have dispersed, but resolved gas measurements could help verify this idea.

I've simplified the discussion here considerably but this is roughly what it boils down to; they themselves, in my opinion, are too focused on the details of the different processes rather than painting a clear picture of the main differences. Fortunately, they've got VLA time to resolve the gas in a pilot sample, so this looks like a solvable problem. As they say, "it is important to remember the results of McKay et al. (in preparation)"... well, asking to remember results that don't exist yet is a new one on me, but I'll try.