One of our arms is still missing and it's not getting better

Remember that galaxy with the great big twirling loops around it that recently disappeared ? Well, there's this normal-looking spiral galaxy that in really deep images has these spectacular double-loops of stellar streams. Except recently there was a claim that one of those loops might not exist, and might be due to the data reduction by some unspecified mechanism.

I find it difficult to believe that any data reduction procedure could give rise to an artifact that nice and coherent, but here the independent authors do their own observations and find that there is indeed only one loop. Things are getting strange.

An argument raised in flavor of the existence of a double loop is the fact that many amateur astronomers have repeatedly detected it. However, the data reduction procedures adopted for the amateur images are not always transparent. The fact that now two completely independent professional teams could not confirm the double loop puts some doubts on at least some of the previously found very low-surface brightness features... It seems that professionally handled data always yields a single stream, while data processed by amateur astronomers uncovers more features.

It's surely fair to say that amateur observations do not have such clearly-described data reduction procedures as the professionals, but does that mean they're wrong ? It's a bit strong to say that professionals "always" detect only one stream if only two professional teams have looked at it, but reasonable to raise doubts. Surely this points not to pitting amateurs against professionals, in a bloodthirsty battle royale in which everyone is armed with spiked clubs and wears only loincloths, but... sorry, what was I saying ?

Ahem. Right, no, the point is this should encourage the need for more dialogue and co-operation between amateurs and professionals. Perhaps the amateurs have been over-zealous in searching for fainter structures, in which case it's important for us to understand what went wrong. But equally, maybe they've found some clever way to reveal faint structures that professionals have missed. After all, it would hardly be the first time that amateurs have made important contributions to astronomy, and I daresay it wouldn't be the last.

Hunting ghosts: the iconic stellar stream(s) around NG5907 under scrutiny

Stellar streams are regarded as crucial objects to test galaxy formation models, with their morphology tracing the underlying potentials and their occurrence tracking the assembly history of the galaxies. The existence of one of the most iconic stellar streams, the double loop around NGC5907, has recently been questioned by new observations with the Dragonfly telescope.

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.