Could there be a bunch of teeny-tiny little galaxies orbiting the Milky Way that are just too damn faint to see ? Maybe. We certainly know about ultra faint galaxies and ultra diffuse galaxies. It seems reasonable that if there are low surface brightness galaxies, where their gas hasn't managed to form many stars, there could also be no surface brightness galaxies - where the gas failed to do anything much at all except sit there like a great big lemon.
This isn't entirely idle speculation either. Not so long ago, it was very difficult to map the sky at the 21 cm wavelengths. It was hoped and thought that such surveys might uncover a whole new population of objects that we hadn't seen before. And since cosmological models predict a much larger number of dark matter halos than detected nearby dwarf galaxies, it seemed entirely reasonable that 21 cm surveys might find a bunch of dark or nearly dark galaxies.
They didn't.
Why not ? One reason might be re-ionisation. Back in the early Universe, the first stars are thought to have been much more massive and more luminous than the pathetic weaklings we have today. Their high-energy photons could have ionised much of the gas, heating it up and driving it out of the smallest dark matter halos. So most of the smallest halos could still be around, but they never accumulated much gas (and hence never formed (m)any stars) before that nasty reionisation front swept in like a great big Nope train of death.
Although getting the details of this to work is damn tricky, there's some pretty good observational support for this : the nearest dwarf satellites of the Milky Way are, quite literally, absolutely devoid of gas. Except, that is, for a galaxy called Leo T, which is nearby, faint, shows signs of fairly recent star formation and has around 100,000 solar masses worth of atomic hydrogen milling around like nothing's the matter. What's going on there ? Could there be any more Leo T's around to help shed some light on this ?
That's what the authors investigate here, using an HI (atomic hydrogen) survey from Arecibo. While extragalactic surveys like AGES and ALFALFA do observe the low redshifts of the Milky Way, they're not designed to deal with really bright extended sources like that. Consequently they're not much good at very low redshifts. They also have a velocity resolution that's fine for more massive galaxies, but poor enough that they might miss the smallest ones. The GALFA-HI survey, on the other hand, doesn't search for more distant galaxies, but is much better for searching nearby in greater detail.
So the authors search the GALFA-HI data using an automatic HI-finding algorithm plus some manual inspection, and come up with a catalogue of 690 candidate detections. Most of these a very high signal to noise so are likely real (and they deliberately use a high S/N cut so their catalogue needs only a minimum of human oversight). They also cross-correlate their results with the optical Pan-STARRS survey. But they didn't find anything much like Leo T. They found one candidate galaxy in the Local Volume, but not actually in the Local Group, that's quite a lot more massive than Leo T - it's probably just an ordinary dwarf galaxy and doesn't tell us much about re-ionisation from the young Milky Way.
Does this lack of Leo T's confirm the re-ionisation theory ? Partially. If you get very close indeed to the Milky Way, other forces like ram pressure stripping come into play, and they're careful to note that their results are only at least consistent with the re-ionisation interpretation.
But surely much more interesting are those 689 detections ! They do not seem to be detected in Pan-STARRS at all, although surely some of them are blocked by the Milky Way (they don't say how many). Only 47 of these were found in the ALFALFA survey, which is surprising because these are all pretty bright objects (the coverage overlap is 50%). True, ALFALF's velocity resolution isn't as good as GALFA-HI, but it's not that bad : and they limit their search to objects with velocity widths > 10 km/s anyway, so it shouldn't make much difference.
So what are all those detections ? They say they're likely to be a mixture of high velocity clouds and faint or dark galaxies. Personally I would have made a much bigger deal out of this. Sod off Leo T, I wanna know about the dark galaxies ! And there are quite likely many more candidates hidden in the data. It's understandable that they used a high S/N cut, but the really interesting detections have a very strong tendency to be faint. With any luck there'll be more papers about this in the not-too-distant future. As it stands, this is a classic case of, "we found something really interesting, but it has nothing much to do with what we were looking for."
A Correlated Search for Local Dwarf Galaxies in GALFA-HI and Pan-STARRS
In recent years, ultrafaint dwarf (UFD) galaxies have been found through systematic searches of large optical surveys. However, the existence of Leo T, a nearby gas-rich dwarf, suggests that there could be other nearby UFDs that are optically obscured but have gas detectable at nonoptical wavelengths.
No comments:
Post a Comment