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.