A stream or not a stream, that is the question

Have our two largest nearest neighbours interacted in the past ? It's widely believed so, although some people think that M33 (Triangulum) and M31 (Andromeda) are experiencing their first encounter. There's a well-known HI stream linking the two, which has sometimes been claimed to be part of the "cosmic web", though I'd bet money most people think it's just a signature of an interaction.

This new study is mainly about modelling the orbital history of the two galaxies.  To be honest my eyes glazed over for most of this; long story short, they think the two galaxies have previously interacted (you can watch their simulations here). More interesting is that they make a distinct, testable prediction : as well as the stream linking it to M31, M33 should have a counter-stream of much more diffuse material extending in the opposite direction.

"But Rhys !", I hear you cry, "Weren't you involved in a super-sensitive survey of the M33 region, which was like, totally the shizzle ? Didn't this take you bloody years to complete ? Wouldn't that be something to check ?"

Well, yes, indeed. It took us five years and we got the highest sensitivity ever reached in this region. We found a bunch of new clouds never before seen, including a giant ring-shaped feature that we still can't explain (none of which are seen in these simulations). It was, in short, very cool. But we didn't find anything terribly odd about M33 itself - at least nothing that wasn't known before. True, it has a weird warp in its disc and a funny gas distribution to the north-east, but people knew that already. And true, the gas disc is much more extended than the optical, but we didn't find gas out to significantly higher distances than other surveys.

That said, we couldn't properly identify the edge of the gas disc. Its density profile continuously decreases right until it hits the noise level, so it doesn't look at though we reached the real edge - for that, we'd expect to see a sudden drop. Yet the density the authors predict for the stream is high enough that it should be readily detectable with AGES, and though sensitivity calculations are not always straightforward, I'd be a bit surprised if AGES couldn't see something if it was there.

Still, while they speculate that such a feature might eventually be detectable with FAST, they neglect the existing deep observations. That's a bit odd. I emailed the first author and sent him a link to our paper and data, but I didn't get any response yet.

The Andromeda System: A new orbital history and its implications

We revisit the orbital history of the Triangulum galaxy (M33) around the Andromeda galaxy (M31) in view of the recent Gaia Data Release 2 proper motion measurements for both Local Group galaxies. Earlier studies consider highly idealized dynamical friction, but neglect the effects of dynamical mass loss.