Wibbly-wobbly gassy-wassy

A nice, extremely careful paper on measuring galaxy gas asymmetries in spectral line profiles. The advantage of this is that it's much easier to get a single-dish measurement of the gas, which is usually much too low a resolution to examine the structures. All you get is a line profile, which shows how the gas brightness varies as a function of velocity.

Resolved observations show that most disc galaxies have flat-ish rotation curves, meaning that most of the gas is rotating at a single velocity. With one side coming towards and the other away from us, this generates the characteristic double-horn (a.k.a. "Batman") profile shape. But if there's more gas on one side than another, for whatever reason, or if the rotation is screwed up somehow, then Batman's horns go all wonky.

Measuring the wonkiness is straightforward enough : determine the central velocity and then measure the total flux on either side of this. By and large, the results tend to be quite subtle and it's by no means clear what the strongest driver of asymmetry really is. Here the authors do a tremendously careful job to measure everything as carefully as they can, carefully controlling for all the different possible errors in a really careful way. Carefully. A few times I even wanted to say, "Okay, okay, I believe you already !" but this is far better than the opposite case, if not as exciting.

They find that some of the asymmetries are just the result of the noise. In mock observations, they show how the distribution of the asymmetry parameter neatly broadens at lower signal-to-noise levels. It's not that they get systematically more asymmetrical, just that the measured asymmetry range increases. But, above a certain S/N threshold, asymmetry measures can be considered reliable. So to properly measure asymmetry, you need to measure it in a population of galaxies, not just in individuals, and you need a good comparison sample. Which they have, on account of being so bloody careful.

Their main result is that galaxies above this threshold tend to be less gas rich than galaxies of comparable S/N ratios with lower asymmetry levels. So asymmetry is an indication not just of gas displacement, but of actual gas loss. They also show that satellite galaxies tend to be more asymmetrical than their larger companions. Environment, it seems, drives gas loss more than other factors.

There's nothing much unexpected about that though. In fact it seems somewhat disappointing that all this work only results in a new way to show something we already knew about. More interestingly, they also show that there are significant asymmetries in isolated galaxies, which can't be explained by interactions with other galaxies. This could be a signature of ongoing gas accretion, but more work is needed to compare with simulations.

All in all, it's a really nice piece of work that explains everything very thoroughly without being outrageously dull. I'm a bit sorry for them that they didn't find anything more unexpected, but it presents a nice tool to use on further samples.

xGASS: Robust quantification of asymmetries in global HI spectra and their relationship to environmental processes

We present an analysis of asymmetries in global HI spectra from the extended GALEX Arecibo SDSS Survey (xGASS), a stellar mass-selected and gas fraction-limited survey which is representative of the HI properties of galaxies in the local Universe.