A pleasant shock

Last year I made a rare foray into investigating the scary world of astro-chemistry. Anything other than atomic hydrogen is generally too complicated for the likes of me, but I was co-author on Robert Minchin's paper looking at how ram pressure can excite the CII (carbon two) line. Urrgh !

Why would I venture into such horribly difficult territory ? Well, it turned out that this was a really neat and wholly unexpected way to validate our models of ram pressure stripping. When a galaxy moves through an external medium like the hot, low-density gas found in galaxy clusters, the pressure on its own gas builds up. This injects energy, and it turns out that this excites it to radiate in the CII spectral line. So by looking for galaxies with more CII than expected, we can work out if they're experiencing ram pressure or not. 

What's really neat about this is that it's a totally different way to search for this than the traditional direct hunt for streams of stripped gas. It gives a handle on what galaxies are experiencing even if those effects aren't strong enough to actually strip the gas, and remarkably, it actually seems to work pretty well.

Today's paper features Robert Minchin as a co-author and it expands the idea to a detailed study of an individual galaxy. While better statistics - i.e. more galaxies – would be nice, having such a well-resolved, in-depth look at a single galaxy is also an important way to check the basic premise of the model. As a disclaimer, while Robert and I go way I wasn't involved in this one at all. 

Strictly speaking it's not the CII itself which is the key signature, but the ratio of CII to far infra-red emission. FIR is dominated by stars, whereas CII emission, the theory says, should be caused by any source of excitation. So if a galaxy has an excessively high CII/FIR ratio, then you know something else is happening to it besides star formation. You have to use other clues to piece together just what that might be.

In this case the target is Arp 25, a beautifully lop-sided spiral galaxy in a group of about eight other galaxies, about 28 Mpc from us. It doesn't have any know stellar or gaseous streams associated with it, or even any especially prominent spiral arms, but when you look at it... it clearly ain't right. And the group itself, weirdly, has its own intra-group X-ray gas, something normally only found in much more massive clusters of tens or usually hundreds of galaxies. Not eight. Understanding how that happened is, I suspect, a whole project in itself, and this isn't covered here at all.

The authors use a whole suite of data sources, from optical to UV and X-ray to HI and, crucially, the ill-fated SOFIA flying telescope, which provides the all-important CII data. Most of these maps are very nicely resolved. And what they find is convincingly simple : there's a clear shock front with an enhanced CII/FIR ratio. The intragroup gas may be low density, but the infall velocity of the galaxy they estimate at over 1,000 km/s : still not enough to cause strong enough ram pressure to strip its gas, but quite definitely enough to cause CII excitation.

And in scaling relations too, everything fits. The shocked regions are very clear outliers in terms of having excessive star formation given their gas density, but they also have more CII than expected given their UV emission (a key tracer of star formation). And whereas normal galaxies have a clear, tight relation between CII/FIR as a function of FIR density, the shocked regions have an unmistakable excess. Again, everything fits, including some other relations I'm not even going to go into.

So this basic model really does seem to work very well indeed, including outside of clusters which ram pressure is normally associated with. The one sting in the tail is that people have been using CII as a proxy for star formation in high-redshift galaxies. This shows that things aren't so straightforward, that it really does seem to trace all sources of injected energy. But once you know about this, it's an asset, allowing us to explore the environmental effects on galaxies without having to look for the relatively extreme cases where the poor things are being absolutely clobbered. Now we can study them when they're only being lightly molested. Yay science !