Today's paper is a strong contender for "least exciting discovery I've ever blogged". It's still interesting though.
A typical spiral or irregular galaxy has three basic aspects to its gas distribution. There's the huge, hot, very thin outer halo that emits at X-rays (though rarely at levels strong enough to detect, except in the most massive galaxies). This million-Kelvin cloud extends well beyond the stellar disc. Then there's the much smaller, cooler (a mere 10,000 K) disc of atomic hydrogen, which extends to about twice the stellar radius. Finally, within the stellar disc there's more atomic hydrogen but at considerably higher densities than in the outskirts, along with a heavy smattering of massively denser, more compact, much colder (few hundred K) molecular hydrogen clouds where star formation happens.
Galaxy clusters posses their own hot X-ray gas. So galaxies falling through them build up a ram pressure as they move through the intracluster medium. This can easily remove their own outermost gas, which does nothing much except starve them of gas for future star formation on very long timescales. Stronger ram pressure can strip the atomic hydrogen directly, and in extreme cases can remove some of the molecular hydrogen itself.
All this is well-known in galaxy clusters, and generally reckoned to be one of the main drivers of galaxy evolution in that environment. But galaxies well outside clusters also frequently show signs that their star formation has been affected too. Could this "pre-processing" be the result of ram pressure stripping in seemingly more passive environments than clusters ?
To figure this out, ideally you need a direct signature of ram pressure, rather than just looking for the side-effects like a change in stellar colour or whatever. A one-sided gas tail is a pretty darn good indication of this, since gravitational encounters usually produce two-sided structures. The problem is that such tails tend to be quite short, so you need good resolution to detect them. Since ram pressure is expected to be weaker in groups (they're less massive than clusters, so they have less gas and the galaxies in them move more slowly) you also need a large sample to stand a chance of detecting a significant number of candidates.
That's where this paper comes in. They use the shiny new LOFAR telescope, which has both excellent resolution and area coverage. Instead of detecting the gas directly, it's sensitive to emission from cosmic rays - which, like the gas, can also be stripped into long one-sided tails by ram pressure.
They have an enormous sample of both groups, clusters, and isolated field galaxies. They use a combination of automated plus visual inspection of the radio images to search for one-sided features, with their field sample acting as a control. And the result is, for once, as clear as day. Only about 2% of field galaxies have tails, whereas it's about 10% in groups and 20% in clusters.
There's also strong evidence that these are truly ram-pressure tails and not the result of some other mechanism. The orientation of the tails tends to be only away from the cluster centre, whereas for groups it could be either away from or towards the group centre. This is exactly what you'd expect. Simplifying slightly, a tail pointing away indicates the galaxy is still on its first infall, whereas if it points towards the centre then the galaxy is now moving back out. Since ram pressure is so much stronger in clusters, the whole gas content can be stripped on first infall - hence most few cluster galaxies with tails pointing outwards. But being much weaker in groups, it can take a lot longer to strip the gas, hence a bimodality in the tail directions.
This also fits perfectly with the position of the tailed galaxies relative to their groups of clusters. Galaxies close to the physical centre but offset in velocity from the cluster centre are expected to be dominated by recent arrivals, and indeed, this is exactly where most of the tails in clusters are found. That's not the case for groups, with tailed galaxies having no particular preferred location in this "phase space".
What of the 2% with tails in the field ? Those could be false positives, since some galaxies do have weird asymmetries from internal processes anyway (strong star formation, active galactic nuclei) or past interactions with other galaxies. That the fraction is so much lower here is a good indication that their sample in groups and clusters doesn't suffer too much from this and that they're genuinely examining the effects of environment. A more interesting possibility is that there could be some weak ram pressure happening even in large-scale filaments of galaxies, with even isolated galaxies sometimes being stripped in this way. That requires further research.
Finally, the only point they've left unexamined is the structure of the tails themselves. That could potentially give more clues to the differences in ram pressure between different environments, e.g. the different length of the tails, morphological features, etc. But for now, this is a very satisfying result, like assembling a piece of IKEA furniture and finding that you don't have any screws left over. It's not exciting. It's not unexpected. It shouldn't be controversial in any way. It's just damned neat.
Ram pressure stripping in groups versus clusters
We compare the group jellyfish galaxies identified in this work with the LoTSS jellyfish galaxies in clusters presented in Roberts et al. (2021), allowing us to compare the effects of ram pressure stripping across three decades in group/cluster mass. We find that jellyfish galaxies are most commonly found in clusters, with the frequency decreasing towards the lowest mass groups. Both the orientation of observed radio continuum tails, and the positions of group jellyfish galaxies in phase space, suggest that galaxies are stripped more slowly in groups relative to clusters. Finally, we find that the star formation rates of jellyfish galaxies in groups are consistent with `normal' star-forming group galaxies, which is in contrast to cluster jellyfish galaxies that have clearly enhanced star formation rates.
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