Detecting the gas at large distances is difficult, but detecting stars is relatively easy. This gives us a bit of a confusing picture. We know a lot about the stars in very distant galaxies, but not so much about the gas they form from. We also don't know too much about how their environment changes. In particular, the major reason galaxies lose their gas in clusters is thought to be ram pressure stripping, where galaxies plough through the hot, thin intracluster gas (ICM - M for "medium") fast enough for it to push their own gas away.
The strength of ram pressure is directly correlated with the density of the ICM and the speed galaxies move through it. Those two parameters, however, have complex dependencies on the structure of the cluster. Clusters assemble through the accretion of galaxies, so in the distant past, clusters would have been less massive (meaning galaxies would fall into them more slowly) and have less ICM. So it's thought that initially gravitational interactions would have driven galaxy evolution, which have stronger effects at lower speeds. On the other hand, it's thought that galaxies were entering clusters in larger numbers in the past.
Directly detecting signatures of ram pressure stripping at large distances would be extremely helpful. These authors make a very solid case, using the MUSE instrument on the VLT. That lets them detect the ionised gas with high precision measurements of its motions. Ionised gas is easier to detect and seems to be a better tracer of ram pressure stripping than neutral gas anyway. The downside is that because of the distance (redshift) they have to look at oxygen, which is much less common than hydrogen, but this should still give a nice view of what's happening.
And indeed it does. As part of their survey, they found two galaxies in a cluster with neatly parallel tails. They're pretty long features, one 30 kpc and the other 100 kpc - not astonishingly long, but spectacular nonetheless. By themselves, these results can't really say much about the overall importance of ram pressure in earlier epochs, but they do demonstrate that it's now possible with more data. They at least show that it was definitely happening, and that there hasn't been that much fundamental change in clusters in the ~6 billion years or so since the light left these galaxies.
The objects are perhaps more interesting in their own right. They're both moving at extremely high speeds relative to the cluster, ~800 and 1900 km/s. That means the cluster is massive and that they should indeed be experiencing extremely strong ram pressure. And ionised gas, left to its own devices, will recombine to form neutral gas very quickly. They estimate that in these conditions the recombination timescale is a mere 10,000 years, very much shorter than the ~100 million years or so needed to form the tails (given their lengths and the galaxy's velocities). But there aren't any stars in the tails, so what's keeping the gas ionised ? They suggest - in very hand-waving terms, "thermal conduction, magneto hydrodynamics waves, and shocks". Alternatively it could be that much of the gas is neutral (or even so hot it's not emitting at these frequencies), so the ionised gas we're seeing is just the small fraction which is in the process of cooling.
It's also interesting to note that the tails have very high random motions, ~80 km/s. The higher the velocity dispersion, the harder it is to detect neutral gas. That may be why atomic gas isn't such a good way to trace ram pressure in the nearby Universe - combined with the rapid ionisation timescales and long neutral tails should be rare things. I'll have more to say on that one I find a referee who's prepared to actually listen, but I digress...
Perhaps the oddest thing is that the tails are neatly parallel despite the high velocity difference between the galaxies. The different velocities suggest the galaxies are entering the cluster from different sides (at these redshifts, distance measurements are none too precise). So it's a bit of a surprise that the galaxies seem to be on such similar trajectories, but that could just be a neat coincidence. We won't be able to understand that one without many more observations. As they conclude, somewhat dramatically, MUSE is, "opening a new era in the study of the role of the environment on galaxy evolution."
Evidence for ram pressure stripping in a cluster of galaxies at z=0.7
MUSE observations of the cluster of galaxies CGr32 ($M_{200}$ $\simeq$ 2 $\times$ 10$^{14}$ M$_{\odot}$) at $z$ = 0.73 reveal the presence of two massive star forming galaxies with extended tails of diffuse gas detected in the [OII]$λλ$3727-3729 A emission-line doublet.
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