Ghostly gas or just a ghost ?

Once upon a time, there was a lovely little gas cloud that turned into a star. That much we know. What we know a lot less about is exactly how gas clouds assemble into nice happy star-forming galaxies.

We know for certain that galaxies can lose gas through mutal interactions - we can directly see spectacular tails and streams that closely match predictions as to what such features should look like. But how gas gets into galaxies is much more controversial. Several streams have been proposed as signatures of accretion over the years, but it's never been very clear to me why any of them are more likely accretion features rather than tidal tails. If accretion happens, why don't we see it everywhere ?

This paper describes some exceptionally deep observations of two nearby edge-on galaxies, reaching density levels about ten times (or more) lower than more typical values. They do this is the old-fashioned way, by sheer observing time. That means they're limited to a few different "pointings" per galaxy - they don't produce any shiny maps, just spectra.

What they find, though, is in my opinion extremely suspicious. They detect neutral gas well above and below the plane of the discs, but it very closely matches the velocity profile of the disc - to within 10% or so of the velocity width. That just doesn't make any sense to me. Whatever the extraplanar gas is doing, there's no obvious reason why it should match the velocity width of the disc : tidal tails or infalling clouds should be all over the place. In some cases, they even see hints of a double-horn structure, a classic signature of a rotating disc. But you just shouldn't see that outside the disc itself : it couldn't be stable.

To be fair, this isn't seen everywhere. Some emission closer to the disc is a markedly different shape, but still with a very clear cutoff at the same velocity width of the disc. Going further out, in one case the emission appears to drop and then get stronger again. That sounds a lot like a sidelobe detection to me - detecting the galaxy's disc again because the sensitivity profile of the dish varies non-linearly with distance from the target.

EDIT : Although they don't explicitly discuss sidelobes, they do something much better and show a figure of the beam strength. While they claim that the emission they detect is well above the beam strength, I find it even more suspicious that their detected emission peaks at locations very close indeed to the sidelobes.

Perhaps I'm wrong. At least a few of the authors are way more experienced than me, but they only mention sidelobes once in passing (to say that they're weak). They discuss several possible origins for the gas, but rarely address that suspicious similarity in velocity widths except to note that it's "close to the velocity range expected for random motions in the halos of the galaxies". I don't find that terribly likely, so I'm far from convinced. I should get back to my cloud-mapping project...

Detection of the diffuse HI emission in the Circumgalactic Medium of NGC 891 and NGC 4565

We present detections of 21-cm emission from neutral hydrogen (HI) in the circumgalactic medium (CGM) of the local edge-on galaxies NGC 891 and NGC 4565 using the Robert C. Byrd Green Bank Telescope (GBT). With our 5$σ$ sensitivity of $8.2 \times 10^{16}$ cm$^{-2}$ calculated over a 20 km s$^{-1}$ channel, we achieve $>5σ$ detections out to $90-120$ kpc along the minor axes.