One thing that's never seemed terribly puzzling to me is where galaxies get their gas. Estimates of the star formation histories show that with their current gas content, galaxies should typically exhaust their gas within a gigayear or so. A lot of people find this suspiciously fast, that to maintain their currently constant levels of star formation is impossible unless the galaxies are being re-fuelled from somewhere.
Personally I've always found that one gigayear is not fast enough for a need to invoke refuelling (or accretion as it's usually known). It just means that galaxies had more gas in the past and will run out comparatively soon, but in a time equivalent to about one-tenth the age of the Universe isn't enough to set alarm bells ringing for me. Sure, if it was the next few or tens or even hundreds of millions years, then I might be concerned. Then I might say, "hang on, isn't it a weird coincidence that we've arrived on the scene just as galaxies are about to stop forming stars forever ? Isn't it more likely they're being replenished from somewhere ?"
On the other hand, the apparent constant rate of star formation does seem more legitimately odd. If galaxies are truly running out, naively you'd expect to see that reflected in their star formation histories. So people have postulated that galaxies are accreting gas from the field somehow, either from "hot mode" where gas cools very slowly and omnidirectionally, or in "cold mode" where it condenses into cooler, more distinct streams which funnel themselves into the galaxies. Claims to have detected the latter are always controversial because it's very hard to say that a stream of gas isn't just gas that the galaxy is losing by a host of much more well-understood processes.
Today's paper attempts to address this mild puzzlement. They use a sample of galaxies which is as homogenous as they can get and apply some reasonable scaling relation where necessary to calculate the change in gas. For instance, they assume that the bulk of the normal, "main sequence" galaxies today were already on the main sequence 4 Gyr ago. That is, they formed stars at a predictable rate given their total mass of visible matter. This is not at all an unreasonable assumption : 4 Gyr is enough to expect some evolutionary changes but nothing dramatic, and while of course plenty of individual galaxies might have experienced the odd burst or sudden cessation of star formation, there's no reason to think these would be statistically significant.
And kudos to the authors for clearly acknowledging their assumptions and how complex real star formation activity can be. In public talks I sometimes go on a breathless monologue describing the various process at work and how they relate to each other; they do much the same here, except that they clearly spell out how this is likely to affect star formation – rather than my own take-home message which is only that it's bloody complicated.
The really big assumption, the most difficult point to make reasonable inferences about, is how much the HI gas has changed. A handful of stacked observations have now managed to detect this atomic gas out to these vast distances, but these combine data from hundreds of galaxies. We currently have no real idea how it's changed in individual galaxies on these timescales; in contrast, the molecular gas can be observed directly and is much better understood.
Add to that that the relation between atomic gas and star formation appears to be subtle. It's popularly described as the galaxy's fuel tank, the reservoir from which star formation ultimately occurs. Which is quite appropriate, since understanding how much gas is in the tank and how fast a car is going is not easy ! The actual gas in the engine itself, the stuff that's exploding in the pistons, is thought to be molecular, but there are strong hints that HI is directly involved as well at least in some cases. Most HI-rich galaxies are blue and star-forming, but some have loads of gas but hardly any stars at all or only old red ones, so it's not at all a straightforward connection.
Using their various scaling relations as best they can, their conclusion is at least an interesting possibility. They say that what seems to be happening is that galaxies are both losing and gaining gas : overall, they're running out of the molecular gas (the stuff in the engine itself actually doing the business of star formation) but actually gaining atomic gas (the stuff in the tank). This is a small change for dwarf galaxies but really quite large (70 % !) for massive ones, even to the point where the galaxies have grown significantly in overall baryonic mass as a result of this. Galaxies then, are still assembling even today, not just from mergers but from the condensation of the thinnest gas in the intergalactic medium.
What this means is that something is changing star formation efficiency. Somehow galaxies in the early universe were able to efficiently convert all their gas into stars, whereas today something is preventing the HI from cooling into molecular gas which can form stars.
Why could this be ? Frustratingly they remain silent about this, which is annoying because this is such an obvious question you can't not ask it. Especially since modern galaxies are much more metal-rich, which enables very much faster cooling and condensation of the gas : if anything they should be more efficient at forming stars, not less. On the other hand there are far more stars around today (though less energetic), so perhaps stellar feedback is to blame.
Well, I dunno. Fair play to 'em for being clear about all their many assumptions, but where we go next with this is anyone's guess. It needs a lot more independent studies before anything else can be said about this.
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