Check out my kinky curves

So here it is, my eighth paper as first author. It's very similar to the sixth, except that it's much better because it's half as long. I'll have a detailed blog post up in a few days, but for now here's the super-short version.

Most neutral hydrogen gas (HI, pronounced H-one) is associated with optically bright galaxies, but there are a few weird gas clouds that aren't. In particular, there are these six HI blobs in the Virgo cluster that look like they're rotating as fast as massive galaxies - but optically they're dark. They're miles and miles away from any other galaxies and there's no sign of any extended HI streams anywhere nearby. So the most obvious explanation - that they were just ripped out of ordinary galaxies as they fly past each other - has problems.

What's particularly weird about these clouds is their apparent rotation. We don't really measure rotation directly, just how fast the gas is moving along the line of sight. The difference between the fastest and slowest-moving gas gives us a line width, which for normal galaxies it's safe to assume represents rotation. If that's the case for these clouds, they need a fairly substantial dark matter halo to hold them together, because they're rotating so fast that their gas mass is nowhere near large enough to stop them flying apart. They would effectively be galaxies without any stars.

But they might not be rotating at all. It's possible the high line widths are really just due to streaming motions with the gas flowing at different velocities in the same direction. Other people have claimed from numerical simulations that this explanation works well for some other, similar features. And it does. The problem is this has been widely taken to mean that's definitely the best explanation for any and all HI clouds in any situation.

To test this, we numerically simulated a spiral galaxy falling into a cluster. The target spiral is based on a fairly typical known object and we varied its parameters quite a lot to see how much difference that made (and also its trajectory through the cluster). The spiral has gas, stars, and dark matter. The galaxy cluster is much simpler : 400 point masses all buzzing around just like in a real cluster. Those point masses just have gravity - they don't have their own gas and stars because that's computationally very demanding indeed. This is a big improvement on the earlier works though, because they just did one point mass flying past a target galaxy.

What we show here is that the tidal debris/streaming motion hypothesis categorically, decisively, does not work for objects like these clouds. I would even go so far as to say it fundamentally cannot work. It's easy enough to make large clouds with high velocity widths, but's damn near impossible for clouds as small as those we see in Virgo. We saw gas being torn off the galaxies into long streams, and those streams get "harassed" into small clouds, which is nice... but there's a problem. The greater the streaming motions within a stream, the harder they are to detect and the quicker they disperse. It's incredibly difficult to disperse the rest of the stream while preserving the features of the highest velocity widths.

We also examined the formation mechanism of the famous VIRGOHI21. This well-known dark galaxy candidate is a very sharp "kink" in the velocity of a long stream from a spiral galaxy, which again looks like it might be rotating. This is what earlier works were trying to reproduce. We show that actually they didn't really do this, even though they claimed to, but our simulations did. That might sound like petty bickering, and it is. But it's important because we can now very clearly say if a cloud is likely to be tidal debris or not. If it's in a stream, then that's probably a good explanation. If it's isolated, then that's only a sensible explanation if the cloud is rather large or has a low velocity width. If it's isolated but small and with a high velocity width, tidal debris is a lousy explanation.

As for VIRGOHI21 itself, I would say that after more than a decade, jury's still out. Much to my annoyance, and despite lengthy explanations in the paper, I wasn't able to convince the referee that we don't really understand it. It's true that we can explain the velocity kink... but that's all we can explain. We can't easily account for the rest of the features of the system, and we haven't really tested the dark galaxy hypothesis - so we don't know how well that explanation actually works. Despite the referee's abject protestations that "any scientist" should know that the success of one model not preclude the success of another, they fell for this very same fallacy when I added in a footnote about the alternative VIRGOHI21 hypothesis. And because I'm not insanely belligerent, I caved in and took the footnote out. Fortunately I don't have to do that on social media... :)

A much more detailed write-up can be found here.

https://arxiv.org/abs/1701.05361