This is not (quite) the gas you're looking for

You can't study galaxies without studying star formation, and you can't understand star formation without understanding the gas. This is easy enough in the nearby Universe, but the radio emission from gas is very weak. You don't even have to go very far before it becomes impossible to detect at all. That's extremely annoying, because while we can see how the gas content of galaxies varies with environment, it's damn difficult to measure how it's changed over time.

This paper, like several others before it, attempts to overcome this through stacking. By observing hundreds of galaxies and combining their signals, it's possible to measure the average gas content of the whole sample. This has a lot of limitations : you won't know which galaxies have gas at all, or how much variation there is : some could have none and others lots, so long as the average is above your sensitivity limit. But it means you can detect the gas at much greater distances than is otherwise possible. The highest direct detection is of a galaxy at about 3 billion light years distance, whereas this stacked sample has an average distance of about 4 billion light years and with some as far as 5.

For this project they use India's Giant Metre-wave Radio Telescope. Previously this has had a reputation for giving rather hit-and-miss results, but its recent (under-reported) upgrade seems to have given significant improvements. While interferometers like this don't have the best sensitivity to low-density gas, they can still be perfectly good for detecting the dense, star-forming gas. And their large field of view means they can observe hundreds of distant galaxies at once.

It's an interesting question as to which is the more efficient observing strategy : observing individual galaxies one by one with giant telescopes like Arecibo or FAST, or doing a ginormous GMRT survey and stacking the galaxies instead. Hard to say. Anyway, they spent almost 120 hours on a field containing more than 400 galaxies suitable for stacking, and none of them were directly detected. But combine their measurements and a pretty clear detection emerges. With a bit of smoothing to improve the sensitivity even further, it looks solid - certainly better than previous comparable claims using similar methods. They're also able to show that the gas is likely only coming from their target galaxies, and isn't a result of unwittingly combining whole groups of galaxies.

What does this tell us ? The average gas mass of the sample is about 5 billion solar masses. That's pretty high, but not extraordinarily so. It seems that the gas consumption timescale hasn't evolved much between when those galaxies were around and the present day, since estimates from local galaxies give similar results. On the other hand, it does support a change on longer timescales, adding another data point to the existing (indirect) estimates, but it's not a major development in itself.

So no revolutions here. But it's an impressive technical achievement, which ought to get the GRMT some attention as a seriously capable instrument. For understanding how the gas changes over time, though, we're going to have to wait until we have the capability to look even further back.

Atomic hydrogen in star-forming galaxies at intermediate redshifts

We have used the upgraded Giant Metrewave Radio Telescope to carry out a deep (117 on-source hours) L-band observation of the Extended Groth Strip, to measure the average neutral hydrogen (HI) mass and median star formation rate (SFR) of star-forming galaxies, as well as the cosmic HI mass density, at $0.2 < z < 0.4$.