The Incredible Shrinking Galaxies

Ultra Diffuse Galaxies are a dead sexy topic in extragalactic astronomy, and took the community by storm as soon as they were found. The one-line description of why they're so cool is that they're the same size as the Milky Way but a thousand times fainter.

But are they really the same size as the Milky Way ? Maybe not, say the authors of this paper. I was fortunate enough to have a sneak preview of the results at a conference last year in Tenerife, but the authors asked everyone very nicely to keep things under wraps. And quite properly so, as this is the kind of "I wish I'd thought of that !" result that could be extremely important for the lead author as a PhD student (or early postdoc, I forget which).

It's often awkward to define the size of a galaxy. Their shapes vary tremendously, as do their brightnesses, and while some do seem to have distinct edges, others fade more gradually. The two main ways around this are to use the effective radius (R_e - the radius enclosing half the total light) or the Holmberg radius (R_H - the radius measured at a fixed brightness level). Both have their uses, but both are pretty arbitrary definitions and don't really have any physical motivation.

That's where this first paper comes in. The authors clearly had a flash of enviable inspiration, because their new proposed definition is simple, intuitive, and just makes absolutely everything better. They noted that there seems to be a well-known threshold for star formation in terms of gas density, about 1 M_⨀ pc^-2. This is hardly exact, mind you, and there's a lot of complicated physics at work. But it's pretty good, and all things considered, the threshold is remarkably robust. Above 10 M_⨀ pc^-2 the atomic gas seems to saturate and become molecular, which inevitably leads to star formation, but star formation really only seems to be actively suppressed below the lower value.

Somebody in the team realised that if this quite neat threshold applied so well to the gas, it might be important for the stars to. So they define their new proposed radius R₁ as corresponding to the distance at which the stellar density drops to 1 M_⨀ pc^-2. Very simple, and very effective. Using this new definition, the scatter in the size-brightness relation drops substantially. Visually, R₁ looks much closer to the true edge of both dwarf and giant galaxies - R_e tends to be far too small, since it can be dominated by small, bright knots of intense star formation. And at very high masses, where galaxies transition from rotation-dominated discs to dispersion-dominated spheroidal systems, the scatter in the size-brightness doesn't change but the slope does*, clearly showing that the galaxies are dynamically different. Hooray !

* This reminded me of a similar-ish recent finding on the most massive spiral galaxies, though I'm not sure the results are really comparable.

Of course, galaxies are messy things, so there are a few caveats. Interacting galaxies can show very complex outer morphologies indeed; dwarf irregulars can have highly complex internal structures; a few galaxies have averaged stellar densities below this new threshold everywhere, so R₁ would not give a good value in those cases. But for the most part it should be helpful, clearly demarking where the galaxy disc ends and any extensions begin. It's not perfect, but it does better than existing parameters.

A more practical matter is that R₁ is harder to measure. Other values can be measured using a single wavelength, but R₁ needs at least two independent wavebands in order to properly calculate the stellar mass. And that means making multiple surface brightness profiles per galaxy. On the other hand, R_H looks like a reasonable approximation in most cases, so perhaps a prescription can be developed to convert between the two.

A few small caveats on the paper itself. First, they keep saying that they measure R₁ with an isophotal contour, but all their figures show ellipses, not contours. So some averaging has obviously been done. It would be interesting to see a true contour, especially for interacting galaxies. Second, it's not clear to me how many slope changes their are in their size-brightness relation plots - sometimes it looks like there's just the one, in others like there might be two. And on a related point, they say that the slope change using R_e is not monotonic, but I can't see any evidence of that.

To summarise the different measurement parameters :

• R_e is the radius enclosing half the light, often much smaller than the "true" size of the galaxy.
• R_H is the radius at a fixed brightness value, which usually seems a much better estimate of the true size but lacks a physical motivation.
• R₁ is the new parameter, measured at the point the stellar density drops to the equivalent of 10 M_⨀ pc^-2 (the threshold gas density for star formation).

What does all this mean for the Ultra Diffuse Galaxies ? In their second, companion paper (linked below), they essentially say this means they're just ordinary dwarfs.  Their R₁ sizes are perfectly typical for dwarf galaxies and not at all unusual. Using the new R₁ parameter, they follow exactly the same size-brightness relation as for previously known dwarfs. Even their R_e values, though on the high size, aren't that extraordinary compared to other dwarf galaxies.

But of course these new galaxies are not totally identical to the old. UDGs clearly have much lower R₁/R_e ratios. They allude to this many times, noting that the reason R_e is so small in many dwarfs is because of bright central knots of high star formation activity. But they never quantify it explicitly (and unfortunately we'll have to wait for the final published paper for the full table). So we might not want to chuck out the whole UDG bandwagon just yet - these guys clearly are different from typical dwarfs, but it's a bit more subtle than the standard take-home message. Something must be going on to suppress star formation in the central regions of these guys that doesn't happen in regular dwarf galaxies.

While the innermost regions of UDGs are clearly different, their outer regions look to be identical to other dwarfs. They plot the surface brightness profiles of normal dwarfs and UDGs, and the slope of the outer regions looks the same. So any suggestion that UDGs are giant galaxies looks highly unlikely. Should we even still call them ultra diffuse galaxies, or use ultra flat instead ? I think we can probably stick with UDGs, not need to go around confusing everyone... and since their stars are less concentrated, it's still okay to call them diffuse.

There are two major caveats to all this. First, there are UDGs with gas, sometimes a great deal of gas. It remains to be seen if the overall star formation activity in UDGs is abnormally low compared to other dwarfs, but I'd be very surprised if it isn't. Second, on a related point, those peculiar rotation velocities of the gas aren't going away - at least some UDGs look to be rotating so slowly that they don't have dark matter at all. That marks them out as spectacularly different from ordinary galaxies.

So the day of the UDGs is by no means over yet. This paper is also a very small sample of UDGs, so who knows, some of the others may turn out to be different again (the first claim for a giant UDG seems to have been completely forgotten, perhaps unfairly*). Even though it's now getting on for five years since UDGs first hit the headlines, we still don't have enough data to really say what the hell is going on. Is their gas density unusually high or low ? No idea, we have just six objects with well-resolved gas measurements. Are their kinematics truly weird or have we got their inclination angles wrong ? Again, dunno. The research continues.

* If Dragonfly 44 really is representative of a new class of "failed giants", then while it may be superficially similar to UDGs, most other UDGs are clearly different beasts.

Are ultra-diffuse galaxies Milky Way-sized?

Now almost 70 years since its introduction, the effective or half-light radius has become a very popular choice for characterising galaxy size. However, the effective radius measures the concentration of light within galaxies and thus does not capture the intuitive definition of size which is related to the edge or boundary of objects.