Full stack

One of the major difficulties with observing atomic hydrogen is that the emission is very weak. In the very nearby Universe, say within the Local Group, this isn't a big limitation. But just a few tens of millions of light years away and it starts to demand gigantic telescopes and/or obscene amounts of integration time. Beyond about a billion light years it's nigh-on impossible.

This is a problem. In optical wavelengths we can see how star formation evolves throughout the ~14 billion year history of the Universe, and it evolves strongly. Galaxies today are but dim embers compared to the blazing fires they were a few billion years ago. But because of the weakness of the HI line, how that star formation's fuel supply has changed has remained largely a mystery. So we're missing a key part in the story of how galaxies grow up.

There have been a handful of attempts to go deeper. Short of building a gargantuan, all-crushing telescope, one solution is to stack observations of lots of galaxies together, getting you the equivalent of hundreds of hours of integration time. Thus far, none of these efforts have ever really looked terribly convincing in my opinion. You look at the spectra and go, "eeehhh, I mean, I've seen worse, but.... really ?".

This paper changes that. Actually the authors already have a similar paper, which I overlooked because I've fallen for a boy-who-cried-wolf fallacy and stopped reading such claims. So it was a pleasant surprise to see that the detection they present here is pretty unambiguously convincing, as indeed was that in their previous paper.

Using about 400 hours of integration on the GMRT in India, they average together almost 3,000 galaxies. Because it's averaging, all they retrieve is information on the average galaxy in the sample, which is the major downside of stacking (the alternative would have been to do a 400 hour integration on a single galaxy, but this would arguably be worse and certainly riskier, there being no guarantee that any individual galaxy contains a detectable amount of hydrogen). Even so, the results are much more interesting than I expected.

Most of the paper is understandably given to the observational details, but their main result is an average hydrogen mass of about 30 billion solar masses. There are a few galaxies known in the local Universe with masses this large, but not many. More interestingly, the gas to stellar mass ratio at this distance (about 8 billion years ago) is markedly different to what we see today. Nearby bright galaxies tend to have substantially less mass in gas than in stars, but these objects have ratios well above one. So there has, as expected, been a definite evolution in the gas fraction over time. That's not surprising, but it's important that we have concrete evidence now rather than mere hypothesising. The big, bright galaxies we see today were not the same earlier in their evolution.

(Though, as an interesting caveat, this might be a slightly misleading selection effect. We wouldn't expect today's big bright galaxies to have been so big and bright back in the past, because they need that time to build up their stellar content. But this should only be a modest effect, as the main result that bright galaxies today are different to bright galaxies in the distant past is still interesting.)

One of the other puzzles is where the gas is coming from. It's easy to see how it gets consumed in star formation and lost via stripping processes. My understanding was that while star formation has remained fairly constant in the recent past, the gas consumption timescale is only 1-2 Gyr, suggesting it's being replenished somehow (in particular, there was a lot of discussion about whether clouds seen around the Milky Way could be fuelling this, with the conclusion being quite clear that they could not). I dunno if I just missed some big development here, but they say the depletion timescale in the local Universe is more like 5-10 Gyr, meaning there's no mystery since the gas is being used very slowly, whereas at the greater distances it's only ~2 Gyr - so they're witnessing them at the peak of consumption. As they start to run out of gas, star formation activity should naturally drop as the gas density decreases, so it's no mystery that there's still gas around today for slower consumption. This is all nicely consistent, so I'm a bit puzzled why earlier results seem to have given such a different estimate for the local consumption rate.

Anyway, it's a very nice piece of work. An obvious follow-up would be to try a very deep integration on a single galaxy. The problem with stacking is that you wash out a lot of valuable information, so confirming the results on a single galaxy - which would also let you measure the kinematics - could be an interesting compliment. Though just try getting "let's stare at this one galaxy for 400 hours and hope we get an interesting wiggly line as a result" past the proposal committee...

Giant Metrewave Radio Telescope Detection of HI 21 cm Emission from Star-forming Galaxies at z=1.3

We report a 400-hour Giant Metrewave Radio Telescope (GMRT) search for HI 21 cm emission from star-forming galaxies at z=1.18−1.39 in seven fields of the DEEP2 Galaxy Survey. Including data from an earlier 60-hour GMRT observing run, we co-added the HI 21 cm emission signals from 2,841 blue star-forming galaxies that lie within the full-width at half-maximum of the GMRT primary beam. This yielded a 5.0σ detection of the average HI 21 cm signal from the 2,841 galaxies at an average redshift ⟨z⟩≈1.3, only the second detection of HI 21 cm emission at z≥1