Back in February I got very excited by the discovery of a really good, solid, dark galaxy candidate courtesy of China's FAST telescope. I haven't heard anything more about that since then so I remain excited about it.
Today's paper also comes via the FAST telescope and concerns another dark galaxy candidate. However this isn't by the same authors. It's actually based on this paper from June (which I've not read) by the FAST team, but the authors are unconnected. Basically what they're doing is saying, "hey, the FAST paper said that this might be a dark galaxy, and here's our more detailed analysis of the same data supporting that". There isn't any new data in this paper, just pure analysis. In fact I'm not sure if they even have access to the FAST data (which I would have thought was private) or just scraped it from the published images...
Look, dark galaxies are the thing that gets me most excited about astronomy, but I have to say the more I think about it, the less I'm a fan of this particular paper. It's not that there's necessarily anything wrong with it, it's just... odd, as a piece of work.
It's not just the lack of new data and their unclear access to the original data* that irk me about this paper. They also don't cite me, even though my work would be directly relevant here. And they insist on calling "dark galaxies" RELHICs ("RE-ionisation-Limited HI Clouds").... urrgh, that's an ugly term. Most irritating of all, they use the term "isocontour". The hell is that ? Are normal contours not plotting things at the same (iso) value anyway ?
* If they had it, surely they could have reprocessed it in a few other ways and at least reported what they did, even if it didn't show anything new ? Not to do this is very strange.
To be fair, RELHIC might be better as a term were it reserved for a very specific kind of dark galaxy : low HI mass, low rotation speed, small dark matter halos, as in this putative case. I'm not sure we really need another term for this besides the already-common "minihalo", but still.
Anyway what they do is to show that this particular object is consistent with being a RELHIC/minihalo/dark galaxy, in which the HI is in hydrostatic equilibrium. It would need a dark matter halo to be stable because the HI mass is undoubtedly far too low to keep it self-bound. And it can't be much further away or closer than the best-guess distance estimates, because that would make it an altogether stranger object.
And that's pretty much it. Even this depends quite a lot on the shape of the HI contours, and I'm a bit concerned that this circular shape - the distinguishing feature of a non-rotating system in equilibrium - is only due to the low resolution of FAST (though as they say, higher resolution would be better). All this is extrapolating quite a lot from very marginal data - I don't doubt the detection is real, only the confidence about the conclusions.
Nor do they consider other explanations either. True, it's quite well-separated from the nearest big spiral, M94, but 70 kpc is not that far. And as we and others have shown, objects with line widths this narrow (a mere 20 km/s, only twice the width of the line itself) can be stable on very long timescales even if they're completely unbound, because by definition they're dispersing slowly.
In short, the paper presents a perfectly valid summary of what the object could be, but it doesn't offer anything new and doesn't consider alternatives. I'd have been far happier about it if they'd at least applied this analysis to other objects and/or used at least some other data sets here. On the other hand, at least dark galaxies are getting more attention again, and that at least is something to be welcomed.
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