A jellyfish running out of gas

 A "jellyfish" galaxy is one which has long tails of ionised gas. The term is a bit liberal, but mainly of them have very thin, tendril-like structures, sometimes much larger than the main stellar disc, so they do often look quite a lot like jellyfish.

But ionised gas is rubbish and everyone hates it, so what about the much more sexy neutral gas ? This nice little paper presents some VLA observations of a jellyfish galaxy in a nearby-ish cluster, which they then compare with an arbitrary other galaxy in some other cluster. To be honest, I didn't find that aspect of the paper at all useful. It's a bit like picking one jellyfish out of the Atlantic ocean and comparing it with another picked at random out of the Pacific : you can't really learn anything by such a comparison. It's not the author's fault, mind - it's hard to get neutral gas observations at this distance, so the current sample size is very small.

The other bit I don't like is the claim that this jellyfish has only a short (40 kpc) tail of HI, compared to the ionised gas which extends ~100 kpc. Given the poor sensitivity of the VLA in C-configuration, I don't think this means anything much except that the density of the gas at the larger distances must be lower.

What is quite interesting is that the star formation activity seems to be somewhat enhanced compared to non-jellyfish galaxies. This particular object is almost certainly the result of ram pressure stripping as the galaxy ploughs through the hot, thin intracluster medium. While this ultimately drains all the galaxy's gas and prevents star formation, in the initial stages it can compress the gas and so give the galaxy a final burst before its untimely death.

But that's not terribly surprising. The most interesting thing to me is that while the HI doesn't seem to be at all spatially displaced from its parent galaxy, or marginally so (its "tail" being a slightly lopsided and ragged edge on one side), its kinematics are quite different. The systemic velocity of the gas and stars differs by ~100-200 km/s. This isn't seen in similar cases in the  much closer Virgo cluster : stripping galaxies there have clear tails but with gas discs that are relatively unperturbed. Perhaps this is a case of particularly strong stripping, or maybe most of the motion is along the line of sight. It'd be nice to get much deeper observations of objects like this, but that will probably have to wait for future instruments.


EDIT : Hot on the heels of this gassy-tailed jellyfish comes another paper about a similar object with higher-resolution data. This is an especially fun object : it has strong stellar tails indicating a tidal interaction with a nearby galaxy, with most of its gas being displaced in both space (only slightly, but very clearly) and velocity (by ~340 km/s). Since the gas morphology does not well-match the stellar morphology, the authors attribute this to ram pressure occuring along the line of sight. Unlike the other object, there's no evidence that this has increased the star formation rate, even though the surface density of the gas is higher outside the stellar disc. They say it might be stretched, so its volume density remains low despite the higher surface density. A very nice object indeed !