Quite queerly quiescent

Why do some galaxies burn through their gas like a fire in an un-raked forest while some are content to sit back and relax ? In hand-wavy terms it's probably some combination of internal and environmental effects. In more massive galaxies the gas density can trigger more star formation throughout the disc, while in smaller ones the gas density tends to be lower. At the same time, when stars from in low-mass galaxies, their feedback effects can be more effective at suppressing further star formation by lowering the gas density still further, since there's less gravity to work again. Then again, high mass galaxies won't be much effected by encounters with other galaxies unless they're similarly massive, whereas low mass galaxies are much more vulnerable. So this simple question is not easy to answer properly.

What would be nice to have is so basic you'd think we'd already have it : some kind of census of the gas content of galaxies in different environments at different distances, masses, and star formation rate. Getting this is more difficult than you might think, because gas measurements are difficult except in the relatively nearby Universe. Getting a statistically significant sample, and understanding whether each galaxy is being affected by its own internal process of environment, is not an easy task.

The authors of this paper attempt to simplify things by looking at how the gas content varies in massive galaxies as a function of star formation rate. They have a sample of about 9,500 galaxies from the ALFALFA HI survey, matched with SDSS data to get star formation rates and stellar masses. They correct for their sample incompleteness and other statistical biases, so their final results should be an accurate representation of what's really going on.

Remarkably, they find that the atomic gas content of disc galaxies doesn't vary much at all even as star formation rate varies by a factor of a hundred. The proportion of disc galaxies with HI detections doesn't vary with star formation rate either (though it's not clear to me if this is the case for elliptical galaxies, which are anyway hugely biased towards low star formation activity). Nor does their gas fraction vary as a function of star formation rate either. And even the average HI spectrum of the star-forming and quiescent galaxies look incredibly similar.

It's a different story for the molecular gas. This shows a very clear, neat, strong trend, increasing in mass with star formation rate. That's not too surprising, as the consensus has been building for a quite a while that molecular gas correlates much more strongly with star formation activity than atomic, but it's nice to see. But why does this happen ? They say, "These galaxies are quenched because of their significantly reduced molecular gas and dust content and lower star formation efficiency", but this is a tautologous description and not an explanation. Galaxies which are quenched have low star formation activity by definition !

More interestingly, they note that the similar spectra imply that both quiescent and star-forming galaxies in their sample are likely rotating discs. They suggest that once the inner gas (which is denser) has been consumed by star formation, it takes a long time for the outer, less dense gas to either form stars or migrate inwards. So it just sits their, slowly rotating and generally doing sod all.

What would be nice to see next is a more detailed look at some of those individual galaxies. They make a testable prediction that the HI should be found in rings in quiescent galaxies, since the innermost gas will have been consumed. It would also be nice to describe their environments in a lot more detail : they say they work with "central" galaxies, but this doesn't help much. If they're central cluster galaxies then I'd be very surprised indeed if the quenching happened inside-out, since that's the exact opposite of how ram pressure normally removes gas in such galaxies. And I'd like to know a lot more about how they stacked the spectra - I'm very surprised that the velocity widths of the galaxies are apparently all so similar. It would also be nice to see what happens if they use specific (instead of global) star formation rates - that is, the star formation rate per unit mass.

The other thing I wonder about is this paper. Really massive disc galaxies have less baryonic mass than expected given how fast they rotate, which the authors there suggested might point to an upper limit to galaxy formation : above a certain mass, gas may be unable to cool and form stars. But this current paper says you can have really quite large gas reservoirs that disdain star formation, so perhaps things at the high mass end get more complicated. It would be interesting to see how the two samples compare, at least. It's nice to see such very clear evidence that it's molecular gas that matter most for star formation, but there's a lot more left to do to understand as to how atomic gas is converted into molecular.

Nearly all Massive Quiescent Disk Galaxies Have a Surprisingly Large Atomic Gas Reservoir

The massive galaxy population above the characteristic Schechter mass M * ≈ 10 10.6 {M} ☉ contributes to about half of the total stellar mass in the local universe. These massive galaxies usually reside in hot dark matter halos above the critical shock-heating mass ̃10 12 {M} ☉ , where the external cold gas supply to these galaxies is expected to be suppressed.