Conferences are exhausting things, and I'm tempted to give Wheel of Star Formation (a.k.a. JanFest) its own post. The unofficial conference after-dinner party kept going until 2:30am, as it should, which meant surviving the final morning session was no small challenge. The end, when it eventually came, was a blessed mercy. I ran home as fast as my little legs would carry me and collapsed into a four-hour nap. And when I awoke, I was further rewarded to discover my paper was accepted for publication.
Coincidence ? I think not. That's okay, it means my talk couldn't have been that bad...
Now normally I have to go on a rant about how unpleasant the review process was. Not so here. This time, just for once, the reviewer was lovely. When they said they were "somewhat skeptical", they actually meant this literally, instead of it being code for, "I think you're a total idiot and I'm going to reject your paper because I don't understand it", which seems to have been my experience of the process one too many times. So today I shall go straight on with the science. And for that, I'll start by using the conference poster.
(But... beware. Even with the best will in the world, the result of this can't be said to be anything other than a big confusing mess. I'll try and wrap things up as cleanly as I can, but don't expect a nice neat narrative today, because there really isn't one.)
|I like this version of the poster best because all the dates are wrong. We started planning it in 2019, so the original 2020 date had to be postponed, and then the second date for 2021 also had to be delayed. Thanks, covid.
Where does my research fit in ? Regular readers already know of my obsession with gas clouds that don't do anything. After all, star formation is rubbish and a waste of precious neutral hydrogen gas, or, as I phrased it in the conference :
|Basically the only connection of my talk to the wheel was that it was about the lack of a wheel.
Actually, starless gas clouds are interesting because they're difficult to explain. The most mundane idea is that they're just torn off from regular, bright galaxies. A much more controversial prospect is that they're places where the wheel never started turning, galaxies in their own right where star formation never even got started. And no-one is sure if that's even possible. Some clouds have plenty of stars whereas others, which are otherwise indistinguishable, have none - and we really don't know why.
Normally I do a protracted introduction to the scientific background. Today I'm going to try something different and dive straight into the latest research, so let's see how that works. I'll say only in advance that the main survey I work on is called AGES, the Arecibo Galaxy Environment survey, which looks for atomic hydrogen gas with a great big radio telescope. Everything else I'll fill in as we go.
Everyone's Favourite Gassy Lion
Although my main interest lies in small gas clouds (here meaning "quite a bit smaller than the Milky Way"), I have to admit that the giant features are basically the porn of the radio astronomy world. You can't really not be impressed by features like the Magellanic Stream, spanning more than half the sky yet not even vaguely suspected to exist until the twentieth century.
|Here shown as it would appear from Cardiff, were it visible to the eye.
Well, I suppose you can be unimpressed, but then I'll look at you as though you have the same towering intellectual and spiritual capacity of a monkey that enjoys hurling its own faeces at other monkeys.
Anyway, the Leo Group harbours one such nerd-bonder-inducing feature : the Leo Ring. It's 200 kpc (650,000 light years) across and has a mass of gas of well over a billion Suns.
|The Ring itself (here shown from an earlier survey) isn't quite a gas cloud that isn't doing anything. Some evidence has been found that there is in fact star formation happening within it, at least in localised areas - though most of it remains dark.
When you're planning a survey with extreme sensitivity, a region like this is a natural target field. I mean, can you think of any reasons not to do a survey of a giant gas ring with better sensitivity than any previous observations ? Of course you can't, because there aren't any.
Or somewhat more prosaically, the deeper the search, the more likely we are to reveal clues to its formation. Gas is never totally stable, and even once removed from a galaxy it can become dispersed or dissolved by a variety of processes. It might collapse under its own gravity to form stars (forming so-called "tidal dwarf galaxies"). Or it might be of such low density and high velocity dispersion that it simply flies apart, fading into undetectability. Or, it might be subject to ionising radiation and so change from nice, easily detectable neutral atomic hydrogen (which we can see with a radio telescope) into ionised gas (which we can't - or more accurately, requires totally different observing techniques).
But the Ring is not the subject of today's paper, not because it isn't awesome, but because we didn't find anything new that leaps out and says, "hey, look at this !". This isn't to say we found no new features in the Ring at all - we did, and some of them might be important. We just didn't find anything that adds anything of immediate value. So our focus on the Ring is now very much on numerical simulations, a project which is going to take a long while to bear fruit. We did, however, find something else that offers a much faster route to publication.
Six Little Clouds
|You can see an interactive 3D version of this here.
|Cloud 7 is a little blob that appeared after smoothing. It may or may not be real, we don't know.
|Especially appropriate because dispersion is exactly what we'd expect.
|I've simplified this just a tad, removing a few known and explicable outliers and combining a couple of different data sets. The axes are in logarithmic units, with the vertical one being mass and the horizontal one being rotation speed. Dotted and dashed lines show the scatter at different significance levels.
|Opinion is divided as to whether this represents something inexplicable and impressive or monumentally dull, but either way, it's pretty neat.
|Disclaimer : spinning yourself or your pets won't help you live longer.
- Cloud 1 is between the two big spirals. It has much the largest velocity width, but smoothing didn't reveal it had any kind of extended component. Its location and line width suggest a tidal formation mechanism, but its lack of extension doesn't fit this view.
- Cloud 2 is between the two big spirals. It has a typical velocity width and smoothing reveals it has an extended "tendril" connecting it to one of the spirals. Its location and extension imply a tidal origin, but its agreement with the BTFR is unexpected, and its extension doesn't really fit the general expectation for tidal debris.
- Cloud 3 is between the two big spirals. It has a typical velocity width and smoothing reveals it is significantly extended, though not connected to either galaxy. It is marginally deviant from the BTFR, and generally just ambiguous all-round.
- Cloud 4 is between the two big spirals. It has a typical velocity width and is at a similar location to clouds 1-3, but uniquely has a clear optical counterpart. Its mass is only marginally greater than the others and smoothing reveals it may be slightly extended, so its density doesn't seem likely to explain why it alone has formed stars. It follows the BTFR of normal galaxies.
- Cloud 5 is close to the Ring, and it's hard to tell if this is really a discrete feature or not. Smoothing revealed no signs of anything extended.
- Cloud 6 is as isolated as anything can be in this region. It lies on the the BTFR for normal galaxies but, like the other clouds, has no optical counterpart. The galaxies nearest to it on the sky are actually likely to be at quite different distances, and none of them show signs of extensions anyway.
|Humanity has a love-hate relationship with rings which is so powerful that it's literally mythical.