Sister blog of Physicists of the Caribbean. Shorter, more focused posts specialising in astronomy and data visualisation.

Wednesday 13 October 2021

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.

Wednesday 6 October 2021

Tantalisingly tidal

Here's a paper on that ever-popular topic : those two funky galaxies which apparently don't have any dark matter.

It seems that the great distance controversy has at last been settled; to my knowledge no challenges were raised to the latest measurements on that score, despite an abundance of dispute on similar previous claims. This means the galaxies are indeed very far away, so their small velocity dispersions do indeed imply a lack of dark matter. While if they were closer they would just be perfectly normal galaxies, which Occam's Razor could imply is the more likely scenario, the data really seems to rule this out.

Recall also that galaxies without dark matter do not, paradoxically, challenge the dark matter paradigm itself. If dark matter exists, then it can in principle exist independently of ordinary stars and gas. By contrast, if it's just an illusion and it's actually our theory of gravity that's at fault, then it shouldn't be possible for similar objects to have dissimilar velocity dispersions : similar objects, in similar environments, should always be kinematically similar. What they might potentially challenge instead is the nuances of how galaxies assemble, though conceivably they may just have had their dark matter removed in ordinary tidal encounters.

Searching for the effects of tidal encounters could give important clues to how these objects were formed. One such previous study clearly shows there are tidal streams of stars in the group in which our two little galaxies are likely located, but there's no sign of such features associated with the oddball galaxies themselves. Another claimed to have found characteristic S-shaped tails associated with one of them, but I was very skeptical about this because it looked marginal at best.

Today's paper uses the Dragonfly telescope to conduct a new search for tidal features associated with the weird galaxies. They give a very nice but probably unnecessarily thorough introduction, given that they don't actually find any tidal tails per se. Ideally, if they found nice classical extended tails, that would settle the argument pretty unambiguously, but alas such features aren't seen yet.

However, what they do find is evidence of tidal distortion. Looking at their images I was a bit skeptical - the galaxies look pretty normal, and the outermost measurements look like they're almost within the noise. But their measured surface brightness profiles (which show how the stellar density changes with radius) are rather more convincing, showing a clear, coherent change of gradient in both density and ellipticity over several data points. It certainly doesn't appear that they're really probing much into the noise, although of course even more sensitive measurements would always be better.

(The only real caveat I might raise to this is that there's no discussion of alternative explanations, especially how long some mild ellipticity might persist in a galaxy consisting purely of stars as these do. But that's probably not very important.)

Using this, they can estimate the radius within each galaxy at which the change of profile occurs. And from theories of tidal distortions, they can then estimate how close they must be to the larger galaxies present within the group. The neat finding is that if the galaxies do actually have dark matter, then for them to be this distorted, they'd have to be even closer to the bigger galaxies than is possible according to the observations. That is a pretty strong result that these galaxies really do lack dark matter. You can always say a galaxy might be at a different distance along the line of sight, but the distance across the sky is a very much harder limit. They also find no evidence of those S-shaped tails I was wary about, though this doesn't really change anything.

What does this mean ? To be honest, not that much as far as understanding their origins goes - though I got a bit bored towards the end because it is a rather long paper. We can now, I think, quite definitively say that these objects are strange - we're just going to have to wait for even deeper data to say more about what the bloody things actually are. Weird ? Yes. Challenge to theory ? Not so sure, for now.

Back from the grave ?

I'd thought that the controversy over NGC 1052-DF2 and DF4 was at least partly settled by now, but this paper would have you believe ot...