Galaxies are pretty complicated things that come in all shapes and sizes. But at least one thing has tended to be quite simple : their colours. By and large, they divide themselves into two distinct sequences when you plot their colour against total stellar mass - a narrow red sequence, and a fuzzy blue cloud. In between lies the "green valley", or transition region, where there are relatively few galaxies. Most galaxies in the blue cloud tend to be spirals and irregulars, whereas most on the red sequence tend to be "red and dead" ellipticals. The blue galaxies tend to be gas rich and actively forming stars, whereas the red ones basically don't.
These are not hard-and-fast rules by any means : there are plenty of exceptions, and pretty much all of parameter space is populated to some degree. But these guidelines are pretty good and do seem to be true most of the time. And various other studies have claimed different evidence of a "bimodality" in the galaxy population, with active galactic nuclei being far more common above a certain mass threshold.
A popular view of galaxy evolution is that when galaxies run out of gas, they stop forming stars. Since young stellar populations are dominated by short-lived, bright blue stars, galaxies with ongoing star formation tend to be bluer. And the collisional nature of the gas helps it to form complex structures like spiral arms. So when the galaxy runs out of gas, the bright blue stars quickly die off and only the less massive red stars survive. The transient spirals that were sustained by the gas quickly dissipate, and the galaxy becomes smooth, red, and dead. It ought to move from the blue cloud through the green valley and end up on the red sequence.
One particularly nice paper from 2009 found that galaxies in the green valley are dominated by galaxies which have less gas than expected. Since there aren't very many of these galaxies, this suggests that galaxy evolution is a rapid process that's dominated by a sudden event. This is supported by analyses of environmental processes like ram pressure stripping, which can quickly strip a galaxy of its entire gas content in certain circumstances.
This paper disputes this interpretation, calling (in a later paper in the sequence I've not yet read) for nothing less than a new paradigm for galaxy evolution. A bold statement, but potentially justified.
Here the authors have used the Herschel Reference Survey, which claims to be volume-limited (that is, it's found every bright galaxy within its survey volume). They've also got a whole slew of different estimators for the star formation activity, so their data is high quality stuff. They then plot how the star formation rate (normalised to the total stellar mass of each galaxy) varies with stellar mass. This is much more physics-based than the traditional plots, which use colour as a crude proxy for star formation activity. And they don't see any evidence of bimodality, they see a continuous sequence. Even the galaxy structures apparently vary very smoothly from the galaxies with highest to lowest star formation rates.
One issue that's often raised for galaxy morphology evolution is that the stellar density profiles of spirals and ellipticals is so different that it doesn't seem that removing the gas and quenching star formation would be enough to cause this change. The spirals structures could disperse, sure, but why would the end of star formation lead to the presence of a very dense central bulge ? Here the authors note that the galaxies may not be as different as they appear, with recent studies finding evidence of residual rotating discs in elliptical galaxies. That makes the smooth evolution of disc to elliptical at least somewhat more plausible, as do the discoveries of gas in elliptical and lenticular galaxies.
This is definitely very interesting, but I'd be a bit cautious (I may change my tune when I eventually read the other papers in this sequence). First, while the correlation is obvious the authors claim the slope of this trend is curved, but their best fit is patently lousy. I'll show their plot without their best-fit curve in the comments below - I could probably agree that there's a slight curve, but definitely not the one the authors fit. That doesn't really change the conclusions, but it does raise a flag about the statistical analysis.
Second, there does seem to be some hint of bimodality, even if it's only weak. It would be nice to see a 2D density histogram of their main plot to see if galaxy density really does vary or if this is just an illusion.
More concerning is that the galaxy sample includes the Virgo cluster, which has a much higher galaxy density than the general field so here we expect environmental processes to be different. When they do the same plot without the cluster members, they claim to see the same trends, but I'm not so sure (see their figure 3). I would say it's at least arguable, though I'd fully accept that it's not certain, that two populations are evident : there appears to be a group of actively star-forming galaxies and a smattering of others. It would have been nice to also plot only cluster members as well - I suppose we'd see the reverse, a population dominated by dead galaxies with a smattering of live ones. Unfortunately they don't do this. I for one would be very surprised indeed if galaxy evolution was a largely continuous process inside clusters - I would almost say that doesn't make any sense.
So I don't know. It's intriguing, but I'm not so sure their sample is as good or uniform as they claim. I'm not at all sure about this business of specific star formation rate (s.f.r per unit mass). While s.f.r. will vary in galaxies simply because of their mass anyway (bigger galaxies have more gas available for star formation), galaxy size will affect their environmental susceptibility as well. So I'm a bit worried that this might be misleading in some way, though I can't quite put my finger on it... what we're missing is how individual galaxies evolve, of course, we only have this statistical picture of what they're doing now. Also, Virgo cluster galaxies do seem to either have lots of HI (warm gas) or none at all, with not much middle ground. It would be interesting to see if this is also true for the colder molecular gas, which is thought to be less vulnerable to stripping.
Interesting stuff, but I reserve the right to remain unconvinced for no good reason...
http://adsabs.harvard.edu/abs/2017MNRAS.465.3125E
