It'a always in the last place you look

Just before Christmas last year, I had a paper accepted after more than a year of review. You can read about the horrors of the refereeing process here, or the extended public outreach post here. This post is the shorter description for the enthusiasts.


The Virgo Cluster is chock full of galaxies which are clearly deficient in HI (atomic hydrogen) compared to field objects. Hundreds of galaxies appear to have lost a lot of gas, yet there are only about a dozen with known hydrogen streams. Now, many of the galaxies could have lost their gas so long ago that it's dispersed and become undetectable, especially since the cluster contains its own (much hotter) gas that the streams could dissolve into. Most people, I expect, would be happy enough with that explanation. After all, the conventional explanation of how galaxies lose gas in the cluster - through the effects of ram pressure stripping, as the galaxies move at high speeds through the cluster gas - seems to do an excellent job of explaining everything else.

But there are a few of notable oddities that don't seem to fit. A small number of galaxies with streams don't seem to have lost much gas at all, being otherwise undisturbed. Galaxies with normal gas content and those which are deficient are found in close proximity to each other, suggesting that we should be seeing at least some which are currently in the process of actively losing gas. And of the streams which do exist, most are very short, faint little things, but a few are enormous, spectacular features containing literally tonnes and tonnes* of gas. If the stripped gas usually evaporates or disperses, why doesn't this happen in every case ? Is there something special about the largest surviving streams ?

* Okay, billions of solar masses.

We need to quantify what's going on. There are three aspects to the problem, all of which we tackle in this paper to various degrees :

1. How many galaxies are actually losing as right now ? If stripped gas is rapidly rendered undetectable, then we only expect to see streams from those which are actively stripping.
2. How many of the streams which exist should be detectable ? That is, what happens to the stripped gas once it's removed from its parent galaxy - how quickly does it disperse and reach such a low density (and/or change to a different phase) that we can't detect it ? 
3. Have we really found all the potentially detectable streams ? This has bugged me from the days of my PhD, ever since when I'd been adamantly declaring that there were no streams in our data sets - much to the surprise and annoyance of my PhD supervisor. Perhaps we were just not looking hard enough, although that seemed extremely unlikely to me.

For item 1, we already have a very nice analytic model of ram pressure stripping which can predict which galaxies are currently stripping. Unfortunately, the data we need to feed the model is only available for a limited fraction of the galaxies in our data, so we couldn't use this to make a prediction of the number of streams. We could still use it to say which galaxies should or should not be stripping, but not to give us the more important grand total. And, assuming we found a stream, we could also use it to estimate how long the stripping had occurred and thus how fast the gas was dispersing.

For item 2, we made a simple model of how streams should appear in our data. These weren't designed to be physically realistic, but to probe parameter space to see what we could detect. For a stream of any given mass and length, its detectability depends on viewing angle. If we're looking at its longest axis, so that its length on the sky is exactly equivalent to its true length, then its flux will be spread out to the maximum extent across the sky. This means the flux per spatial pixel is reduced. But if we're looking down the stream, such that it appears to have a much smaller length, its flux is distributed over more velocity channels. So the signal to noise is greatest within a range of viewing angles, depending on its exact characteristics. The lower the mass in the stream, the smaller the range of viewing angles over which it's visible.

Using this, and given the fraction of galaxies previously known to have streams, we estimated how many streams we ought to see in the archival data (assuming the previous cases were representative). The answer was 11, for the same data cubes I'd looked at for my PhD. And for the much larger ALFALFA survey (we don't have access to that data and it would take too long to search anyway), the number was 46. During our first analysis, we'd only reported 2 and ALFALFA only 5, so clearly there was a big discrepancy here.

The answer to item 3 turned out to be, "nope". For those first data cubes I'd looked at and so insistently proclaimed that there were no streams present, I was expecting any streams to be big, obvious features. You wouldn't need any special viewing technique, you'd just see them straight away. But actually, the method used does matter, a lot, - at least if you're looking for short, faint features. And lo and behold. contour plots and isosurfaces revealed that many of the galaxies showed distinct (if short) extensions that we'd never have seen otherwise.

Admittedly many of these are faint, and consequently we spent a lot of time proving their validity. The way we did this was to find empty regions in our data and inject them with fake galaxies with extensions. We took 100 blank, adorable little cubelets from our data and added galaxies into all of them, randomly varying the existence and characteristics (length, brightness, orientation) of the stream. Then we searched these, not knowing any of the properties, or even if a stream was present at all, until after we'd finished. This meant we could quantify how many false positives we'd expect due to the noise in the data. The answer ? None of the false positives we found were as strong or as extended as the features we'd found around real galaxies.

At least, that holds for the ten strongest streams we'd detected. We found 16 other streams we're much less confident about, and a significant fraction of those may well turn out to be spurious. But we largely ignored these in our analysis, so they don't really matter very much.

Taken all together, it now seems that we can explain the "missing stream" problem pretty well. Plugging the new numbers back into our model, we could calculate how fast the streams we see are evaporating. It's quite a narrow range (between 1 and 10 solar masses per year), but enough to explain why we see lots of short, faint streams but also a few long, massive ones. Think of a very tall waterfall. If just a trickle flows over the edge, the water will disperse and evaporate before it even hits the ground. But if there's a raging torrent, then plenty of water will make it all the way, even though some gets lost in the process - sheer mass plays an important role.

(More speculatively, I'd also suggest that the longest streams are probably more the result of tidal encounters than ram pressure stripping. Close interactions can remove gas extremely rapidly, whereas this should only be possible for ram pressure near the very centre of the cluster.)

Interestingly, one referee seemed to think we were making an astonishing, even outlandish claim. Ten streams ?!? Surely not - there are only a dozen known in the cluster. Another seemed to view it as mediocre. Only ten streams ? It's not worth this much effort, chaps.

The truth is somewhere in the middle. Ten streams in an area of about 10% of the cluster means we can expect of order 100 streams in total. When we eventually have that data, we might be able to say something really interesting about how galaxies in the cluster evolve. For now, having ten streams is very nice, but they're pretty much exactly what we expect to find, so it shouldn't come as a revelation. We can also say that the isolated dark clouds (that look like galaxies and are usually claimed to be tidal debris) probably don't originate from any of the streams we've detected, which is interesting. Mainly we've solved a problem that hardly anyone else was bothered by but I personally found quite annoying, so I for one am satisfied for a while. Happy Rhysy FTW !

Faint and fading tails : the fate of stripped HI gas in Virgo cluster galaxies

Although many galaxies in the Virgo cluster are known to have lost significant amounts of HI gas, only about a dozen features are known where the HI extends significantly outside its parent galaxy. Previous numerical simulations have predicted that HI removed by ram pressure stripping should have column densities far in excess of the sensitivity limits of observational surveys.