Today's paper is unusual in that I hardly ever read anything about X-rays. Gas that's hot enough to emit X-rays tends to be incredibly hot, low density, smooth and featureless. It's useful for measuring the mass of galaxy clusters, but it's not interesting to look at and the physics sounds horrible.
The feature described here, however, is sufficiently weird for me to venture a comment. The bulk of the X-ray gas in the Abell 1367 cluster is, as you might expect, a big boring blob, filling the whole cluster and probably responsible for all the gas stripping of galaxies which fall into it. Perfectly normal. But just a little way outside the smooth main body is a so-called "orphan cloud", a distinct, crescent-shaped overdensity of gas. Now large individual galaxies do tend to have their own X-ray gas, but this one isn't clearly associated with any galaxy.
As well as the X-rays, the cloud also has a H-alpha component : much cooler than the X-ray material, but still hotter than the cooler gas normally found in galactic discs (atomic and molecular hydrogen). The H-alpha morphology is quite complex. Within the Orphan, it's found mainly in the tips of the crescent, but nearby it's seen in a stream extending from a galaxy roughly in the direction of the X-ray cloud.
The H-alpha material is interesting because its kinematics can be measured. There's a velocity gradient across the cloud of about 200 km/s, but there's no clear pattern of rotation. The velocity gradient and the size of the H-alpha region are not that different to some of the optically dark HI clouds seen in Virgo, but given that this one is embedded in hot gas of a mass a thousand times greater, they could well be completely different phenomena. On the other hand, since the Orphan has clouds of multiple components, perhaps the Virgo clouds also have hitherto undetected hotter gas.
The most likely explanation seems to be that the Orphan has lost its parent by some result of stripping, as most galaxies do when falling through the hot gas. But exactly how this happened, why it produced this one particular and quite unique feature, is unknown. Why don't we see more features like this ? The mass and kinematics of the cloud suggest a massive parent galaxy, which should be easy to spot, but they don't suggest any particular candidate. It's also unclear how it could have survived as such an intact, coherent feature for long enough to become well-separated from its parent, or why it's such a discrete feature and not part of a nice long stream.
I wonder if the nearby galaxy with the H-alpha might actually be not the source but a consequence of the cloud. Perhaps it passed through the overdensity of X-ray gas and thus had a much stronger amount of ram pressure than a galaxy normally would this far outside the cluster centre. That would take some detailed modelling to properly answer.
What else could the cloud be ? Being optically dark and with negligible or no star formation activity, it's unlikely to be a galaxy - it would have to be a really massive feature to hold this much hot gas, and it's very hard to see how it would prevent star formation. Nor is there enough star formation occurring to explain the cloud as an excitation of the general intracluster medium. So "stripped out of a galaxy" seems the most likely explanation, even if that hardly answers all the questions as yet.
An H α/X-ray orphan cloud as a signpost of intracluster medium clumping
We present the discovery of the first and still the only known isolated cloud (or orphan cloud [OC]) detected in both X-rays and H-alpha in the nearby cluster A1367.This example shows that stripped ISM, even long after the initial removal from the galaxy, can still induce ICM inhomogeneities. We suggest that the magnetic field can stabilize the OC by suppressing hydrodynamic instabilities and thermal conduction. This example also suggests that at least some ICM clumps are multiphase in nature and implies that the ICM clumps can also be traced in H α. Thus, future deep and wide-field H-alpha surveys can be used to probe the ICM clumping and turbulence
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