Gas off a plane

Galaxies are pretty disgusting things, and understandably so. They whirl around at hundreds of kilometres per second, with their inner parts moving at different speeds to their outer regions, occasionally bumping into each other and even swallowing other galaxies accidentally. It's no wonder they feel a bit queasy.

If two galaxies have a close encounter, they can rip off parts of each other's gas. Just as it's easier to launch a rocket into orbit from the equator than the pole, so it's easier for galaxies to disturb each other if the interaction is aligned with their planes of rotation. Their gas is already rotating rapidly, so it only needs a slight shove to remove it completely - whereas to remove it vertically requires a powerful kick, since the gas is barely moving vertically at all.

But some galaxies show gas which is quite clearly well above their main rotating plane, so how does it get there ? There are several ways. A sufficiently strong interaction can do it, but would usually be heavily disruptive to the stars as well as the gas. In certain environments, galaxies can move through external gas which can build up a sufficient "ram pressure" to push the gas out of the disc. If a galaxy gobbles up a much smaller companion, the unlucky dwarf can be ripped apart without much affecting the giant. And even galaxies that live in isolation can belch out enormous amounts of gas through stellar winds and supernovae explosions, producing "galactic fountains" of vomited gas that eventually rain back down elsewhere in the galaxy's disc.

All this is an important part of the baryon cycle of how matter is arranged in the Universe. If we want to understand how galaxies form stars, then we have to study their gas. And if we want to estimate for how long they're likely to continue to form stars, we need to look at this extraplanar gas. It's often difficult to work out if we're seeing gas that's being expelled or accreted onto a galaxy's disc.

Actually, it's often hard to even decide where the galaxy's disc begins and the extraplanar gas begins. This paper attempts to model that using high resolution gas measurements taken with the Westerbork radio telescope. Most galaxies are not seen directly face-on or edge-on to us but at an angle. The radio data gives us information not only about the position on the sky but also how fast the gas is moving along our line of sight. By modelling how the main gas disc is expected to move, the authors believe they can disentangle the disc and extraplanar material even when the two are similarly aligned on the sky.

Since the data is of very high resolution, at any pixel the spectral profile displays a simple Gaussian shape in the disc (that is, the brightness peaks at a certain velocity and falls off smoothly at higher and lower velocities). Extraplanar gas is found in distinct "wings" at higher and/or lower velocities. So by fitting the Gaussian to the brightest parts of the profile, this can be removed to leave behind only the gas outside of the disc, which is moving quite differently to the disc material. This is pretty ingenious because otherwise it would be very difficult to remove only the disc material without also taking a big chunk of the extraplanar gas along with it.

One would imagine that at this point the disc can simply be masked, leaving behind a nice image of only the extraplanar stuff. But that doesn't seem to be the case, as they go on to model the shape and rotation of the extraplanar material. I cannot say I fully understand how or why they do this (it wasn't clear to me if this is somehow a necessary part of the process or just a nice bonus to extract more information), and there are clearly a lot of uncertainties in how they do this, but they claim that this means they can even determine if the gas is flowing towards or away from the disc.

They find that the typical extraplanar gas in this sample accounts for about 15% of the parent galaxy's gas disc. That's a fair old chunk, and it seems that this material is a common feature of galaxies - so it's unlikely to be due to some external, environmental effect. Galactic fountains therefore appear to be normal. More unexpectedly, material tends to be found only one one side of the galaxy and is always flowing into it. They say that this is because they're looking at neutral gas, while gas ejected by supernovae explosions should be ionised. But this isn't convincing, as there are plenty of large neutral bubbles known. As to why the material should be lopsided, I don't know.

This is definitely a very clever paper with an interesting technique and results. I do wish they'd shown nice simple maps of the extraplanar gas though; they show it in rotation curves but a simple map would have been better (unless I've misunderstood something fundamental about their method). I imagine that this can also be applied to many more galaxies which have suitable observations, so it'll be fun to see this applied more widely.

HALOGAS: the properties of extraplanar HI in disc galaxies

We present a systematic study of the extraplanar gas (EPG) in a sample of 15 nearby late-type galaxies at intermediate inclinations using publicly available, deep interferometric HI data from the HALOGAS survey. For each system we mask the HI emission coming from the regularly rotating disc and use synthetic datacubes to model the leftover "anomalous" HI flux.