Another paper about so-called "almost dark" galaxies, a term I intensely dislike. Just call them dim ! "Almost dark" sounds lame, like being an "almost professional" snooker player or something. Although I suppose being called a "dim" snooker player would be even worse...
With a stellar mass of about 400 million times the mass of the Sun, it would be a bit of a stretch to call this one especially faint anyway. But what it is is quite dramatically extended to compared even to other, otherwise similar Ultra Diffuse Galaxies. Its stellar mass profile is much more extended than the famous DF44, emblematic of these especially fluffy systems, and this one clearly deserves some attention.
But I'm getting ahead of things. From the outset, this paper is about how this galaxy can help inform us about the nature of dark matter rather than merely saying, "look, here's a system almost entirely dominated by dark matter, isn't that neat" rather than addressing any actual science. Kudos to them for that, this is much needed.
They seem to have found this object accidentally and then gone and got some seriously impressive follow-up data : with an optical surface brightness magnitude of 31 magnitudes per square arcsecond, this is a sensitivity I don't think I've heard quoted outside of Hubble papers before. Their 12 hours of integration on the GBT, however, has only given at best a marginal HI detection (even though it's detected in both polarisations), and it's somewhat annoying that they don't seem to quote its linewidth clearly anywhere. With single-object papers I think it's always a good idea to put the major parameters in a nice clear table, but never mind. I wouldn't have too much confidence in the measurement anyway given how weak the signal is. This doesn't affect their main analysis in any case.
[See edits below for major corrections about these points !]
From their optical data, they find that the colour profile is flat, not varying at all from the innermost regions to the outskirts. So just one single stellar population everywhere with no regions being particularly susceptible or resistant to star formation, which would seem to fit with UDGs in general. The stellar density profile does show a steady decline, indicating that they've likely reached the edge of the object and even deeper observations probably wouldn't reveal anything else.
There seem to be multiple possible ways to form faint, extended galaxies in clusters, but in isolation nobody (so far as I know) has come up with a convincing explanation. Here they ask the obvious question as to whether it could be the result of tidal interactions but the nearest other galaxy is bloody miles away so that seems unlikely; it also has a chemical composition much more typical of larger galaxies whereas tidal dwarfs tend to be even more metal-rich. Diligently, they concede that it could be a very old tidal dwarf that's had billions of years to enrich its metals and could probably have lasted this long quite happily given its isolation, but its extremely high dark-to-light ratio (several tens) and rather low gas content all but rule this out.
What they do next is a pretty neat thing to dry and I'm glad they did it, but personally I wouldn't have had the audacity. They speculate that because such objects don't appear to be compatible with the Standard Model, they maybe the point to something different about the nature of dark matter. They say that something called ultralight axions might be responsible. Now here I want to say YES ! Thank you for trying this and using these objects to probe fundamental physics, that's what's interesting about them ! And then I immediately want to say NO ! You can't provide any meaningful constraints from just one object !
I'm exaggerating somewhat. By no means do they claim to have overturned the Standard Model or anything else, they just venture an intriguing idea. Good for them. But a couple of paragraphs about how incompatible this object is with simulations seems to me to be a bit of a stretch to then go immediately for exotic physics as a viable explanation. I think what's needed here is much more about context. What are the other galaxies in the vicinity like ? Is there deep optical data for any of them ? Can we really be confident about its isolation ?
I don't think there's anything wrong with posing more radical alternatives, but I'd need a lot more persuading that the other possibilities can be so firmly dismissed. To my mind there's still enough uncertainties in baryonic physics that we shouldn't be overly-concerned that simulations aren't predicting objects like this. Regardless, it's good to see someone firmly pointing out the continuing weirdness of isolated UDGs.
EDIT : After a group meeting it's clear that I've given the authors too much credit here. True, they did include everything in a nice clear table, at which my eyes glazed over so I didn't notice their velocity width measurement. Fair enough, they do state all the major parameters then – my mistake ! But their line width estimate is just 34 km/s, with an error bar of 11 km/s... after smoothing their native velocity resolution from an exquisite 0.7 km/s to a ghastly 25 km/s ! Lots of numbers, but the bottom line is that their velocity width measurement isn't terribly meaningful. And the line width dictates total dynamical mass, so what they can really say about the dark matter mass here is... not much.
Let me break that down a bit more. Almost certainly, they smoothed their initially very precise resolution to increase sensitivity. Nothing wrong with that, and in any case, the HI line itself is seldom less than 10 km/s in width due to the temperature of the gas anyway. So you don't really need 0.7 km/s resolution. But 25 km/s is getting pretty bad, and to try and claim any sort of accuracy that the true width is 34 km/s which is barely higher than the resolution, especially with a signal which is anyway marginal... nah. This is really stretching credibility. Any claims that this galaxy is dark matter dominated a la DF44 need to be viewed very suspiciously until somebody obtains better data.
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