Where do galaxies lose their gas ? Galaxies in clusters have been shown to have much less gas than in the general field. In Virgo, for instance, on average they have ~50% or less than comparable field galaxies of the same morphology and brightness.
Now we know there are lots of processes at work inside clusters themselves than can do this, especially ram pressure stripping. That's almost certainly responsible for the majority of the gas lost. But there are many hints that galaxies elsewhere can lose gas too, not usually as much as in clusters, but enough to be significant. This is important because most galaxies don't live in clusters, so if we really want to understand galaxy evolution, we should probably stop spending so much time on the sexiest 1%* of the population and look at all the others from time to time.
* Well, it might be a few percent, but not more than this.
This is another paper I wouldn't normally read because reprocessing existing data tends not to accomplish much. But in this case I think they're on to something... ironically by looking at clusters. Though I have to say, their sample definition seems convoluted in the extreme and they gave such a detailed breakdown of how they divided everything I wanted to slap them and shout JUST TELL ME THE DAMN NUMBERS ! Which they eventually do, and it's a few thousand per object type.
Key to this is that they can distinguish between different sorts of galaxies in clusters. I was racking my brains because this seemed curiously familiar. In fact this idea was mentioned a couple of years ago in a conference, but disappointingly the presenter isn't on the author list or cited. It may, of course, be a completely independent discovery.
Anyway, most galaxies in the field are in small groups, often very small : two or three members, though sometimes more. So this means that clusters tend to assemble by absorbing whole groups of galaxies rather than individual ones. In some cases it's possible to identify subgroups within a cluster, which are therefore likely recent arrivals. Individual galaxies within clusters, not part of any subgroup, could be individual field galaxies falling in for the first time, but they're more likely to be older arrivals whose original constituent group has been broken apart by the chaos of the cluster.
By analogy, nightclubs. It takes a while for groups of people to get broken up by the general throng, so if you see a group of people all together, chances are they just arrived. And equally, not many people go to nightclubs by themselves, so solitary people looking lost and confused have probably become unfortunately separated from the group that originally dragged them in there.
But... galaxies. What the authors do here is very simple. Using samples that identify these different galaxy types, they plot (see their figure 1) the fraction of star-forming galaxies as a function of cluster-centric distance. They find that for solitary galaxies in clusters, the star-forming fraction steadily increases as you go further from the cluster... right up to the level of the general field. There's no distinct break in star formation activity for individual galaxies as they enter the cluster, it's just a smooth curve.
In contrast, galaxies which are still in sub-groups within clusters also increase their fraction of star-forming members, but they reach a plateau. At high enough distances, the star-forming fraction never increases for these galaxies, whereas for the solitary ones it just keeps rising.
In other words, the gas-rich, late-type (spirals and irregulars) individual galaxies just continue losing gas as they get ever-closer to the cluster centre. But exactly the same type of galaxies which are in groups never had as much gas to begin with. They've already lost some of their gas. Not as much as they eventually lose in the cluster, but still a detectable difference compared to individual galaxies. Ergo, pre-processing definitely happens.
I think this is a very nice confirmation of something already strongly suspected. The main accomplishment here is using a substantial sample to increase the cluster-centric distance well beyond what was previously attempted, which explains why it wasn't seen before. You really need to go to very large distances indeed to see this, but when you do, it's astonishingly clear. Honestly it's rare and gratifying to see a result in astronomy which is so clear-cut as this. Hooray !
One small caveat : you might be wondering, well, shouldn't the individual galaxies still show a plateau if they too tended to be in groups to begin with ? Probably not. Deep within the cluster all galaxies will be dominated by cluster-specific processes. But at the distances where there's a difference between individual and group galaxies, it's probable that the solitary objects were never in groups : they're just too far from the general melee of the cluster to have been much affected by it. So there is a change in the nature of individual galaxies as you descend into the cluster, from being dominated by the habitually-solitary to those who were indeed once in a group. But both of these end up being equally affected by the cluster's ram pressure, hence the smooth, continuous change in star-formation properties.
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