Wibbly-wobbly gassy-wassy

A nice, extremely careful paper on measuring galaxy gas asymmetries in spectral line profiles. The advantage of this is that it's much easier to get a single-dish measurement of the gas, which is usually much too low a resolution to examine the structures. All you get is a line profile, which shows how the gas brightness varies as a function of velocity.

Resolved observations show that most disc galaxies have flat-ish rotation curves, meaning that most of the gas is rotating at a single velocity. With one side coming towards and the other away from us, this generates the characteristic double-horn (a.k.a. "Batman") profile shape. But if there's more gas on one side than another, for whatever reason, or if the rotation is screwed up somehow, then Batman's horns go all wonky.

Measuring the wonkiness is straightforward enough : determine the central velocity and then measure the total flux on either side of this. By and large, the results tend to be quite subtle and it's by no means clear what the strongest driver of asymmetry really is. Here the authors do a tremendously careful job to measure everything as carefully as they can, carefully controlling for all the different possible errors in a really careful way. Carefully. A few times I even wanted to say, "Okay, okay, I believe you already !" but this is far better than the opposite case, if not as exciting.

They find that some of the asymmetries are just the result of the noise. In mock observations, they show how the distribution of the asymmetry parameter neatly broadens at lower signal-to-noise levels. It's not that they get systematically more asymmetrical, just that the measured asymmetry range increases. But, above a certain S/N threshold, asymmetry measures can be considered reliable. So to properly measure asymmetry, you need to measure it in a population of galaxies, not just in individuals, and you need a good comparison sample. Which they have, on account of being so bloody careful.

Their main result is that galaxies above this threshold tend to be less gas rich than galaxies of comparable S/N ratios with lower asymmetry levels. So asymmetry is an indication not just of gas displacement, but of actual gas loss. They also show that satellite galaxies tend to be more asymmetrical than their larger companions. Environment, it seems, drives gas loss more than other factors.

There's nothing much unexpected about that though. In fact it seems somewhat disappointing that all this work only results in a new way to show something we already knew about. More interestingly, they also show that there are significant asymmetries in isolated galaxies, which can't be explained by interactions with other galaxies. This could be a signature of ongoing gas accretion, but more work is needed to compare with simulations.

All in all, it's a really nice piece of work that explains everything very thoroughly without being outrageously dull. I'm a bit sorry for them that they didn't find anything more unexpected, but it presents a nice tool to use on further samples.

xGASS: Robust quantification of asymmetries in global HI spectra and their relationship to environmental processes

We present an analysis of asymmetries in global HI spectra from the extended GALEX Arecibo SDSS Survey (xGASS), a stellar mass-selected and gas fraction-limited survey which is representative of the HI properties of galaxies in the local Universe.

The early bird gets the one ring to rule them all

Was the early Universe much different from the modern one ? The answer is definitely yes : there are far more quasars, more merging galaxies, higher levels of star formation, and galaxies tended to look a lot messier. But there are some anomalies.


This first paper notes the discovery of a mostly-normal disc galaxy formed just 1.5 Gyr after the Big Bang. Using high resolution observations of ionised carbon with the ALMA radio telescope, the authors were even able to measure its rotation curve... and it looks perfectly normal. Overall, it looks like a galaxy that's a bit bigger than the Milky Way that's somehow travelled back in time to the early universe. The only major difference seems to be that it's forming stars an order of magnitude faster than contemporary galaxies, which is normal for galaxies in this era.

Interestingly, the molecular mass of the galaxy is very similar to its dynamical mass. Considering that its rotation curve is nicely flat, one wonders about the dark matter content - which, oddly, is barely mentioned. Especially since there was much-popularised result a few years ago showing that galaxies in the early Universe show declining rotation curves, which this one clearly doesn't, but they don't cite that. Though, that result probed the rotation out to much greater distances, with this new result only being able to examine the inner regions of the galaxy's disc. Still, it's a bit weird that it isn't even mentioned.


If boring normal galaxies that travel through time aren't exciting enough, how about one with a ring ? This second paper describes a ring galaxy found just a bit later at 3 Gyr after the Big Bang. It's a very small ring compared to modern galaxies, in the bottom 10% of the distribution. Either they spotted it just 40 Myr after it formed, if it was produced in a collision like many contemporary rings, or it was formed by a different mechanism. While most modern ring galaxies show higher star formation rates than their counterparts, this one has about the same activity as typical non-ring galaxies of its era.

The galaxy also has a large, extended stellar disc outside the ring. If this is a collisional object, they say, then the timescales for the formation of both structures (which I assume they get from the kinematics) are mutually exclusive : one must be more than 80 Myr and the other less than 50 Myr. They say this could be explained if the ring is actually the second formed from the collision, with the disc being the remnant of the first ring which has now been smoothed out (I didn't know multiple rings were much of a thing but apparently this is possible).


The strange thing about both of these galaxies is how they were able to form so quickly after the Big Bang. Back then the Universe was much smaller, so mergers were much more common and melodramatic, so even forming a nice stable disc galaxy ought to be a problem. And then to get one to form a ring as well implies things have really settled down quite quickly. One the other hand, rings in general should be more common due to all the merger, but the second paper says their estimates are that rings were no more common back then than they are now. Apparently the higher merger rate is neatly balanced out by the fraction of disc galaxies available for collision.

I'm pretty sure by time-travelling galaxy theory is the most sensible explanation for this, and I expect to be quoted on it. If I don't see headlines like, "MILKY WAY IN DANGER OF TRAVELLING THROUGH TIME, SAYS SCIENTIST", I'm going to be disappointed.

Time flies when you're commissioning a telescope

FAST, the Five hundred metre Aperture Spherical Telescope, a.k.a. the Chinese Arecibo, has already delivered a bunch of totally uninteresting pulsar results. At least I presume they're uninteresting, on the grounds that they're... well, pulsars. Because if it's one thing the world needs, it's pulsar-based racism. ALL STELLAR REMNANTS MATTER !

(I continue my ongoing quest to provoke outrage on Twitter without actually being on Twitter. One of these days I'll succeed.)

Anyway, this paper finally uses FAST to do something useful and look at HI in galaxies like a respectable radio telescope. Annoyingly, they cite just about every HI survey of any importance apart from AGES. This annoys me. Especially as they focus on the importance of sensitivity and looking to higher redshifts, both of which AGES does pretty well (though to be fair we haven't published any of the higher redshift detections yet).

It's a bit of an odd paper. Normally in a first-results paper there's tonnes of stuff about the instrumental capabilities and the technical specifications, but this is almost entirely absent. Basically they looked at four galaxies doing a totally standard observing mode and detected three of them. One of them is nicely consistent with a previous ALFALFA HI measurement, another shows a similar HI and CO profile, while the third shows a bit of a difference. Since the CO and HI line widths are similar, they infer that both components probe the flat part of the rotation curve. This is entirely reasonable but there's just not much more you can do with a sample of three. They also estimate the dynamical masses, although quite honestly I have absolutely no idea why.

There are two other oddities. One is the comment that you can use gravitational magnification to boost sensitivity, which is true but they then list surveys as examples which, as far as I know, do not make use of this. The other is where all their time went. They were allocated a total of 10 hours, or  or an average of 2.5 hours of observing time per source. The galaxy previously detected by ALFALFA required 48s of integration time in the earlier survey, while FAST used a 5 minute scan. They say the sensitivity level would have been about the same, correcting for the difference in observing time. All well and good, but what did they do with the rest of the observing time ?

In practise 5 minutes on-source typically means 15 minutes of actual observing time - you also need 5 minutes off-source to calibrate and a generous 5 minutes for slewing and whatnot. So a typical complete scan is 0.25 hours. Since they were given a total of 10 hours, so that amounts to 40 scans, or 10 scans per source. But they say they used 3-8 scans per source. If there were three scans for one source and eight for the others, then that's only 27 scans out of a possible 40 ! That's several hours of time unaccounted for. Do they have extremely slow slew times ? Does the data reduction take a long time ?

I dunno. It's all just a bit strange. And given that they go for sensitivity, I would have expected some discussion on what their results indicate for future surveys, but there isn't any - they discuss science instead, which there's bugger all you can do with four galaxies. Surely it would have been better to accept a modest drop in sensitivity and go for 40 targets instead of four !

Anyway, good news that FAST is finally starting to do proper science. Even with all the oddities and strange fixation on pulsars, I'm sure it will be a valuable addition to the arsenal of telescopes pointed at the HI sky.

The atomic gas of star-forming galaxies at z$\sim$0.05 as revealed by the Five-hundred-meter Aperture Spherical Radio Telescope

We report new HI observations of four z$\sim$0.05 star-forming galaxies undertaken during the commissioning phase of the Five-hundred-meter Aperture Spherical Radio Telescope (FAST). FAST is the largest single-dish telescope with a 500 meter aperture and a 19-Beam receiver.

These dynamic dimensions are too dynamic

Do Ultra Diffuse Galaxies rotate too slowly or is it all just a measurement error ? Last time I said, "I think this paper reasonably settles any concerns about the inclination angle measurements, though I won't say it's unquestionable."

This paper swings me back the other way. Their sample looks much more likely to be consistent with miniscule measurement errors that shift the velocity widths into an apparent realm of weirdy weirdness.

For those who haven't a clue what's going on, there were claims that certain galaxies are rotating much, much more slowly than expected given their mass. There are a variety of galaxies that do this. Most are so-called Ultra Diffuse Galaxies, meaning that their stars are unusually spread out, but there are a few brighter objects too. This would be extremely strange because the rotation-mass relation is normally quite tight. These oddballs are way off, in some cases as though they had no dark matter at all. And many of them are too isolated to explain by interactions with any other galaxies.

This latest paper is pilot for a big follow-up study of a sample of UDGs, getting gas measurements with the Green Bank Telescope to estimate their rotation. The GBT doesn't have the spatial resolution to get proper rotation curves - for galaxies this far away, it can only do line widths. So they use the optical images to estimate the inclination angle of the discs, and use that to correct the line widths into true rotation speed. (Gas measurements tell us about motion along the line of sight, so if we're viewing a galaxy face-one, it's line width is zero. If it's edge-on, we measure its rotation directly and no correction is needed.)

They observed 70 galaxies and detected 18. Half of these were confirmed to be UDGs while the other half were found to be foreground dwarfs, i.e. not as extended as previously thought. Interestingly, there's basically no morphological difference at all between a distant UDG and a much closer dwarf. The old "these cows are far away" problem is a tricky one indeed when it comes to galaxies.

Most of the rest of the results are not at all surprising, e.g. the gas rich UDGs tend to be bluer and more irregular than ones without detected gas. They have some low levels of star formation detectable via UV emission, but there's nothing much unexpected about that.

The dynamics are the interesting bit. Seven of their nine UDGs lie off the normal baryonic Tully-Fisher relation while the other two are well within the scatter of typical galaxies. For the outliers, they calculate how badly they'd have to have got the inclination angle wrong in order to bring them back to the standard TFR, and it's pretty substantial, ranging from 20 to 42 degrees. Large errors certainly aren't impossible, given that these are all very faint objects and the gas disc might be oriented differently to the observer than the stellar disc. But it sounds at first glance just too big to explain all of them.

Helpfully they also show what these inclination angles would look like in comparison to the ellipses they fitted to the actual data. What's really surprising is that these "corrected" ellipses are often very close indeed to the measured fit, sometimes almost perfectly overlapping despite an error of 20 degrees or more. I even had to manually plot the circles myself to convince myself that this is correct. It is. Should I ever give another course, I'll be sure to add a plot of what inclination angles look like in practise. Here's a simple version that uses thin discs - the thick disc formula is more complicated and I can't be bothered to show that right now.

Circles inclined from 0 (outer) to 80 degrees in steps of 10 degrees.

When you're dealing with faint fuzzy blobs, I can easily believe that errors of tens of degrees are possible. The authors stop short of saying that the previous results suffered from this problem, as they did have gas maps as well as the optical, but it's hard to see the obvious implication. So maybe UDGs don't have anything much unusual about their dynamics at all, which re-opens the whole controversy about galaxies devoid of dark matter. We'll see.

Systematically Measuring Ultra Diffuse Galaxies in HI: Results from the Pilot Survey

We present neutral hydrogen (HI) observations using the Robert C. Byrd Green Bank Telescope (GBT) of 70 optically-detected UDG candidates in the Coma region from the Systematically Measuring Ultra-Diffuse Galaxies survey (SMUDGes). We detect HI in 18 targets, confirming 9 to be gas-rich UDGs and the remainder to be foreground dwarfs.

Ghostly gas or just a ghost ?

Once upon a time, there was a lovely little gas cloud that turned into a star. That much we know. What we know a lot less about is exactly how gas clouds assemble into nice happy star-forming galaxies.

We know for certain that galaxies can lose gas through mutal interactions - we can directly see spectacular tails and streams that closely match predictions as to what such features should look like. But how gas gets into galaxies is much more controversial. Several streams have been proposed as signatures of accretion over the years, but it's never been very clear to me why any of them are more likely accretion features rather than tidal tails. If accretion happens, why don't we see it everywhere ?

This paper describes some exceptionally deep observations of two nearby edge-on galaxies, reaching density levels about ten times (or more) lower than more typical values. They do this is the old-fashioned way, by sheer observing time. That means they're limited to a few different "pointings" per galaxy - they don't produce any shiny maps, just spectra.

What they find, though, is in my opinion extremely suspicious. They detect neutral gas well above and below the plane of the discs, but it very closely matches the velocity profile of the disc - to within 10% or so of the velocity width. That just doesn't make any sense to me. Whatever the extraplanar gas is doing, there's no obvious reason why it should match the velocity width of the disc : tidal tails or infalling clouds should be all over the place. In some cases, they even see hints of a double-horn structure, a classic signature of a rotating disc. But you just shouldn't see that outside the disc itself : it couldn't be stable.

To be fair, this isn't seen everywhere. Some emission closer to the disc is a markedly different shape, but still with a very clear cutoff at the same velocity width of the disc. Going further out, in one case the emission appears to drop and then get stronger again. That sounds a lot like a sidelobe detection to me - detecting the galaxy's disc again because the sensitivity profile of the dish varies non-linearly with distance from the target.

EDIT : Although they don't explicitly discuss sidelobes, they do something much better and show a figure of the beam strength. While they claim that the emission they detect is well above the beam strength, I find it even more suspicious that their detected emission peaks at locations very close indeed to the sidelobes.

Perhaps I'm wrong. At least a few of the authors are way more experienced than me, but they only mention sidelobes once in passing (to say that they're weak). They discuss several possible origins for the gas, but rarely address that suspicious similarity in velocity widths except to note that it's "close to the velocity range expected for random motions in the halos of the galaxies". I don't find that terribly likely, so I'm far from convinced. I should get back to my cloud-mapping project...

Detection of the diffuse HI emission in the Circumgalactic Medium of NGC 891 and NGC 4565

We present detections of 21-cm emission from neutral hydrogen (HI) in the circumgalactic medium (CGM) of the local edge-on galaxies NGC 891 and NGC 4565 using the Robert C. Byrd Green Bank Telescope (GBT). With our 5$σ$ sensitivity of $8.2 \times 10^{16}$ cm$^{-2}$ calculated over a 20 km s$^{-1}$ channel, we achieve $>5σ$ detections out to $90-120$ kpc along the minor axes.