Darkest tales

Another paper ! But just a short one this time.

This one involves observations with China's giant "FAST" telescope, which I'm pleased to say I have ongoing observations for. So I'm naturally curious as to how well it works in practise. While it's collecting area isn't all that much larger than Arecibo's, its 19-beam receiver coupled with high-performance electronics looks like it will essentially overcome that. I still think their estimates of their major survey's expected performance are optimistic, but I also still think it'll be a fantastic survey regardless.

Here they point FAST at the giant spiral galaxy M101 and its companions, looking at our much-loved friend the 21cm HI (atomic hydrogen gas) line. While previous observations have detected a bridge to the nearby dwarf galaxy NGC 5474, with the improved resolution of FAST this is not detected. M101 does have a tail roughly in this direction though, so they suggest that the previous claims might just have been the effects of low resolution making the HI from both galaxies appear connected.

The tail could have originated from an interaction between the spiral and the dwarf. As well as the tail, they also claim to detect three distinct clouds associated with the end of the tail. But while the data they show appears to be of excellent quality (hooray !), they don't really show enough to determine the nature of these supposed clouds. To me they look more like small perturbations at the edge of the tail, rather than independent clouds in their own right. This might be different in the full 3D data though.

In one image, there is a suggestion of a somewhat more extended tail that might help explain the previous claims for a full-on "bridge". But what's missing is any hint of an extension in the much smaller NGC 5474. If the interaction was able to so disturb the gas in the giant M101, then how come the dwarf doesn't seem affected at all ? What also seems strange is that M101 has a strongly disturbed stellar disc on the opposite side to its disturbed HI content, which isn't remarked on.

But if it's not an interaction that produced the disturbance, then what was it ? Accretion of material from the intergalactic medium doesn't seem likely, as M101 doesn't appear to host the expected massive halo of hot gas expected in such a scenario. They also say that there's no sign of enhanced star formation activity near the disturbance, so the expulsion of gas from a violent starburst doesn't seem likely. Some discussion on the timing for this scenario would have been useful here though - would we still expect such an outburst to still be ongoing ? I don't know.

I think the whole thing would have been a lot more interesting if I was persuaded that those optically dark clouds really are separated from the main disc. As it stands, it's a set of minor oddities : it certainly looks like an interaction of some sort has happened, but whether this is really responsible for the disturbance in the gas disc isn't all that clear - it's a nice example of the difficulties of distinguishing external from internal processes. Still, the data from FAST appears to be meeting expectations, which is very encouraging : at least some of Arecibo's capability has been replaced, albeit in a limited fashion for the time being. This definitely has strong potential for becoming a powerful discovery machine in the near future.