FAST begins to CRAFT its HI surveys

You may remember that I wrote a short piece on the sensitivity of China's giant FAST telescope back in 2016 as its construction ended. The 500 m size of FAST is a bit misleading because they only use 300 m to collect radio waves, while at Arecibo they use about 225-250 m. So FAST won't be massively more sensitive than Arecibo, nor have much of an improvement in resolution. Its main improvement in terms of telescope design is that it will be able to cover about twice as much sky as Arecibo, so it can detect more galaxies overall but not many more per unit area.

Since then FAST has been undergoing commissioning to prepare for actual science, and today on astro-ph we get some substantial reports as to how well this is going. The first paper is a detailed report on the telescope characteristics and performance, with a basic overview of the instruments, while this second one is specifically about the prospects of extragalactic HI (atomic hydrogen) surveys : i.e. what I'm interested in.


Size matters

There's more to a telescope than just the size of its dish. The dish acts as the ultimate constraint on sensitivity, a theoretical limit you can't exceed. But receivers matter too. The more pixels you have (we call them "beams" in radio astronomy), the wider the field of view and the faster you can reach any given sensitivity level.

While at optical instruments you can have millions or even billions of pixels, at HI wavelengths it's difficult to get more than a handful simply because the wavelength of the radiation (21 cm) is so large. So for example the Australian Parkes telescope has a 13 beam receiver, Britain's Jodrell bank has 4, while America's Arecibo has 7 (with plans to upgrade to the equivalent of 40). FAST will have an impressive 19.

FAST has already begun basic HI observations (single pointings, i.e. measuring the HI at individual points on the sky). Here they reveal their plans for the Commensal Radio Astronomy FasT Survey, a project to survey most or all of the sky visible to FAST. So just how much better will this mighty instrument be ?

The authors of this paper are bullish. When the ALFA receiver was first installed on Arecibo, three surveys were designed to take advantage of its capabilities. The largest and most important of these was ALFALFA, which covered about 7,000 square degrees - half the sky visible to the telescope. It took more than a decade to go from first observations to its final catalogue of over 31,000 galaxies. The other surveys were both smaller and deeper : AGES, which is smaller but more sensitive, will probably have 2-3,000 detections when all is said and done; AUDS is very much smaller again with only around 100 detections.

CRAFTS will follow a similar strategy to ALFALFA and they're expecting a somewhat improved sensitivity level (as much as a factor two). In this paper the authors set out the main characteristics of the survey (resolution, sensitivity, etc.) and predict how many detections they expect. Their expectation is around 600,000 detections : almost twenty times more than ALFALFA !


Will this make everything else obsolete ?

Or in other words : is this figure credible ? I'm skeptical. Their coverage area will be a bit over 20,000 square degrees : almost three times that of ALFALFA, so we should expect around 90,000 galaxies at the same sensitivity level. But of course their sensitivity level will be about twice as good as ALFALFA (or probably a bit less). If detection numbers scaled linearly with sensitivity, we might therefore expect around 180,000. That's a huge number, but far short of the 600,000 claimed.

The question is whether the number of detections does scale linearly with sensitivity. In my experience, it's not as good as that. AGES is about four times as sensitive as ALFALFA but our detection rate per square degree is about three times the ALFALFA level. That's based on real-world examination of actual data, though it's limited because AGES is small - and our detection rates do vary just because galaxy numbers vary across the sky.

The CRAFTS estimate is based on ALFALFA's determination of the HI mass function : basically a measure of how many galaxies there are of different HI masses. In essence there are relatively few really massive galaxies but quite a lot more small ones. Knowing how many galaxies of which mass there are per unit volume, you can make a reasonable extrapolation as to how many you'll find in a new volume with a different sensitivity level.

This isn't bad, but it's a bit crude. Sensitivity is a highly complex parameter and it can't be defined in terms of HI mass alone. Galaxies of the same mass have different detectabilities depending on their distance, rotation speed, and orientation to the observer; what's detectable in principle may not actually be detected in practise. CRAFTS assume that all galaxies have the same rotation speed, which is just not the case at all. So I'm definitely suspicious about this calculation : a more realistic estimate would account for the rotation velocity distribution of galaxies as well as masses.

On the other hand, CRAFTS also significantly increases the distance range of the survey. Whereas ALFALFA is limited to a redshift of 0.06 (a distance of roughly 250 Mpc, or 800 million light years), CRAFTS will go up to 0.35 (1.4 Gpc or 4.4 billion light years). So a much, much larger volume than ALFALFA. In fact, they say the median distance of their detections will be slightly above the maximum distance ALFALFA can even observe at all !

But this too is problematic. Since sensitivity decreases with distance, and those frequencies are unfortunately more contaminated with interference, it's hard to know exactly how much this will really increase the number of detections. The Malmquist bias means that more sensitive surveys tend to detect most of their sources at greater distances. Yet whereas the median distance of ALFALFA sources is about 110 Mpc, the median distance of AGES (which is four times more sensitive) is only about 141 Mpc - not a huge increase.

Mass of the detections from ALFALFA (red) and AGES (blue) as a function of their velocity (i.e. distance).

Now it's true that AGES has mostly been limited to the same distance range as ALFALFA, so we can't be sure if our median distance might not increase further with greater distance. We do have one data set which includes data out to much greater distances though - not as far as for CRAFTS, but far enough that the comparison should be valid (since there is a cutoff due to interference and sensitivity anyway). In that case our median distance increased from 145 Mpc (470 million light years) only to 160 Mpc (520 million light years). This is nowhere near ALFALFA's maximum distance. In short, it looks unlikely that CRAFTS is going to find a huge number of sources at higher distances : there just aren't enough massive HI sources out there that it can detect.

I say "HI sources" rather than "galaxies" very deliberately. The authors comment on the prospect of confusion, i.e. not being able to determine which galaxy the HI source comes from. This is particularly tricky at high distances, where the resolution becomes so low that there can be multiple contributing galaxies within the telescope beam. They state very briefly that they do not expect confusion to be a problem... yet at the highest distances, it most certainly will. The detections at the greatest distances in AGES are likely confused sources, where the combination of multiple galaxies within the beam gives an HI mass much greater than for a single galaxy, making it possible to detect whole groups of galaxies that would otherwise not be out of reach.

All in all, while FAST will certainly detect tens of thousands of galaxies at the very least, I'm not convinced that it's likely to discover as many as they're hoping for, and I doubt very much that confusion won't be problem at the highest distances.


Sooo... it's useless, then ?

Whut ? No ! It'll be a fantastic data set, but in my opinion it just won't be as awesome as these predictions make out. Perhaps I'm wrong though. Fortunately we shouldn't have to wait too long to test it. One huge advantage to having 19 beams is that the survey speed is going to be a lot faster than ALFALFA : they predict it should be done in less than two years (once commissioning ends). That's assuming it's the only survey running and the telescope doesn't do anything else, but I get the distinct impression that's the plan. Which makes sense, because this survey has a killer advantage of simultaneously searching for gas in our own Galaxy, pulsars, and the still-mysterious Fast Radio Bursts. Make no mistake : this will be a very important project and well worth doing.

Of course, what they do with the data is just as important as how good it is. So far we've only seen spectra, not images, so the data quality remains to been seen. But assuming it works, they should learn from ALFALFA and not keep it to themselves. While ALFALFA was initially slated to make its data public, this never happened - we got catalogues and spectra but not the full data products. True, the data volume will be extremely large, but in this day and age that ought not to be a serious issue. FAST's data policy is as yet unclear, so we'll have to wait and see what they choose to do with it. If they're sensible, this will be a huge legacy to the community - even if things don't work out as well as they're predicting.

Status and Perspectives of the CRAFTS Extra-Galactic HI Survey

The Five-hundred-meter Aperture Spherical radio Telescope(FAST) is expected to complete its commissioning in 2019. FAST will soon begin the Commensal Radio Astronomy FasT Survey(CRAFTS), a novel and unprecedented commensal drift scan survey of the entire sky visible from FAST. The goal of CRAFTS is to cover more than 20000 $deg^{2}$ and reach redshift up to about 0.35.