A lack of midgets

Ah, another paper on everyone's favourite problem with local cosmology : the chronic lack of dwarf galaxies. By rights, the Local Group should be more crowded with dwarfs than a Smurf convention or the mines of Moria in their heyday. There ought, say simulations, to be hundreds and hundreds of the little hairy sparkly buggers running around orbiting the Milky Way, but there aren't.

An ever-present question as to the significance of this is whether the Milky Way has been visited by the galactic equivalent of Gargamel or if all galaxies have this same problem. Are they all missing their expected dwarfs, or did something peculiar happen to the Milky Way ? Without knowing what things are like for galaxies in general, any purported explanation is largely speculative.

This paper extends this to the nearby spiral M101. At 6 Mpc, it's well within the Local Volume but well outside the Local Group. So if the dwarf-killer is peculiar to our most local environment, M101 shouldn't be affected. From a previous survey they identified potential candidate satellite galaxies of M101, and here they try and establish the distance to the four faintest objects using Hubble.

All four objects appear to be distant background galaxies. Known satellite galaxies at this distance are resolved into individual stars by Hubble observations, but these ones remain stubbornly diffuse. They even simulate what the colour-magnitude diagram for these objects should look like if they were as close as M101, and it's clearly different.

While they don't rule out that future surveys might change their results, since completeness at these very faint magnitudes is always a problem, for now it looks like M101 has an even worse problem than the Milky Way. If the Milky Way doesn't have enough dwarfs, then M101 is positively racist. They also show a comparison with other nearby galaxies where it's possible to measure dwarf abundances with some accuracy (the Milky Way, Andromeda, M94, M81, and Centaurus A). All paint much the same picture.

Wisely, they don't speculate or comment on the astrophysical significance of this. So I'll do their dirty work and point out that this does not constitute any kind of crisis or catastrophe for the standard model. First, we knew this was a problem for the Milky Way anyway, and second, virtually all so-called "problems" for the dark matter paradigm are nothing of the sort : they're problems for the baryonic physics. If Gargamel can't be everywhere at once, it's perfectly plausible that every galaxy has its own Gargamel - a universal property of galaxy formation that prevents star formation in the smallest dark matter halos. Yes, it could also be that the whole dang model is wrong, but much more likely we just don't understand the complicated lives of those pesky little dwarfs. We'll just have to wait and see.

The Satellite Luminosity Function of M101 into the Ultra-Faint Dwarf Galaxy Regime

We have obtained deep Hubble Space Telescope (HST) imaging of four faint and ultra-faint dwarf galaxy candidates in the vicinity of M101 - Dw21, Dw22, Dw23 and Dw35, originally discovered by Bennet et al. (2017).

Looks broken if you ask me

Late last year there was a paper which claimed that the fastest rotating "super spiral" galaxies deviate from the standard Tully-Fisher relation. They're not only enormously massive, but also rotating even more quickly than expected. Since MOND predicts a tight relation between baryonic mass and rotation speed, this is potentially a big problem.

It's taken a while, but Mordehai Milgrom, MOND's creator, has responded on arXiv, and he's very cross about it. He says the problem is all due to the way they measured the rotation speed. Rotation varies as a function of galacto-centric distance, so there are a number of different values one could use : the peak, the flat part, or the extrapolated value to infinity. These last two are normally the same, and this is what MOND predicts. But the previous authors used the peak value, which can be substantially different. He also notes that this sort of deviation has been seen many times before, and is well-understood to be due to using the wrong sort of rotation velocity. It's the flat rotation section, he says, which minimises the scatter in the TFR, not some other parameter.

He may well be right. But at first blush, it's difficult to believe the difference could be that strong in this case. The example rotation curve the previous authors show extends to a not inconsiderable 40 kpc, and it's still rising. It's difficult to believe that the flat part could be substantially lower than the rising part, and goodness only knows how far out we'd have to go to find it if it's still rising at 40 kpc.

Milgrom also raises a fair point in that the sample of the Ogle results is small, so what looks like a deviating relationship to them looks like a handful of a few outliers to him. That is, some of their sample lie near the standard TFR but a few are offset, so it might just be creating the illusion of a true change of slope. On the other hand, almost all of their sample are found to be rotating more quickly than the standard slope, so I think it can still be convincingly argued that there really is a change of slope here. At the very least, both interpretations are consistent with the data. And the rotation curve Ogle shows is a nice one, so outlier or not, it's far from obviously wrong. Just because a galaxy is unusual doesn't mean its data can or should be discarded; that it's an outlier from the general trend does not necessarily grant it immunity from causing problems for MOND.

So just how different can the peak and flat velocities really be ? Milgrom is right to point out that it would have been far better for Ogle et al. to show all their rotation curves and not just one galaxy from their sample. For comparison, he references this paper*, which clearly shows a very different set of curves : generally, they rise steeply in the very centre, go a bit crazy for a short radius (a few kpc), and then quickly stabilise at levels a bit below their peak. The one Ogle et al. show is one is nothing like those. True, they're measuring the curve in the innermost regions (given the scale length of the disc**), but it hardly seems likely that a smoothly-rising curve over 40 kpc (!) is likely to plateau at a much lower value, given that it never even peaks at all in the measured radius.

* He also references this one, which plots the overall relationship between the different velocity measures. But I found the relevant figure hard to interpret, as it appears to show an almost perfectly linear relationship between maximum and flat velocities, with only miniscule deviations from a 1-1 correlation. Perhaps I'm reading it wrong.
** EDIT : Milgrom says that Ogle et al. measure the rotation at less than one scale length of the disc. This appears to be simply wrong : Ogle measure the rotation out to a radius of 40 kpc, whereas they say the scale length is 22 kpc. I also wonder how accurate this scale length is, as it seems exceptionally large while the galaxy doesn't look all that unusual.

Milgrom further points out an earlier paper of massive galaxies, in which the fastest rotator in that sample has a maximum speed twice as high as its flat value. But that peak, and also in the case of the few others which vaguely resemble it, is found in the innermost few kpc or so, and the curve is already almost flat by 20 kpc, never mind 40. And that particular galaxy is almost face-on, which makes rotation hard to measure (though I'm not going to dig so deeply as to check how accurate the values are). The next two fastest rotators in the sample have ratios that only differ by 20%.

So when Milgrom says that he expects the even faster rotators in Ogle to show even higher ratios, I think he's making a completely unjustified extrapolation. I don't see any evidence at all that faster rotators have different max/flat speed ratios; the various curves presented don't really resemble each other very much. Most galaxies show the curve going a bit wild in the innermost few kpc, not a few tens of kpc. His argument would be a lot more convincing if he gave an example of a galaxy with a peak velocity at several tens of kpc which then substantially declined, but as far as I know, no such galaxies are known to exist. So Ogle's data looks just fine to me, and it's Milgrom's comparison sample that doesn't stand up.

Fast-rotating galaxies do not depart from the MOND mass-asymptotic-speed relation

Ogle et al. have fallaciously argued recently that fast-rotating disc galaxies break with the predictions of MOND: the 6 fastest rotators of the 23 galaxies in their sample appear to have higher rotational speeds than is consistent with the MOND relation between the baryonic mass of a galaxy, $M$, and its `rotational speed', $V$.

No rest for the wicked

A few months ago there was a very interesting paper showing a striking trend : the HI content of galaxies does not vary as a function of star formation rate. Even at very low levels of activity, galaxies apparently remain gas-rich. So apparently there's a substantial population of really gassy galaxies that just don't do anything. What's stopping the gas from forming stars ?

"Nothing", say the authors of this latest paper, "it's because the gormless twits used a really daft way to estimate star formation activity, and they should be ashamed of themselves."

... okay, they don't actually say that. But they have two very nice figures where they compare the trend using two different ways to estimate star formation rate : using optical SDSS data as the previous paper did, and using a combination of UV and mid IR. It's clear that the optical data gives crappy results, with there being almost no correlation at all between SFR and and HI content, whereas using the other data gives a very clear correlation indeed - i.e. low star formation activity always means a low HI content; gas-rich passive galaxies are not a thing. They show a visual sample of the so-called passive galaxies identified previously, and it's clear that they're absolutely normal star-forming discs.

It's pretty damning stuff. There are two things I didn't quite understand though :
1) The previous authors showed that there was a clear trend between SFR and molecular gas. How does the "correct" SFR alter this ?
2) The previous authors did at least try using alternative methods to estimate SFR and found that this didn't make much difference, except to reduce the number of passive galaxies. It's not clear to me what they did wrong here.

Oh well, science marches on...

xGASS: passive disks do not host unexpectedly large reservoirs of cold atomic hydrogen

We use the extended GALEX Arecibo SDSS Survey (xGASS) to quantify the relationship between atomic hydrogen (HI) reservoir and current star formation rate (SFR) for central disk galaxies. This is primarily motivated by recent claims for the existence, in this sample, of a large population of passive disks harbouring HI reservoirs as large as those observed in main sequence galaxies.

MOND goes back to the future

A long-standing issue with MOdified Newtonian Dynamics, everyone's favourite alternative to dark matter, is that there are no good numerical simulations showing how galaxies could actually form and evolve in that framework. For example, MOND advocates are apt to cry that the missing satellite problem* is a serious difficulty for the standard model, but there's been very little indication of whether MOND would actually do any better. The same could be said for just about any cosmological problems.

* Standard models predict about ten times as many small galaxies as we actually detect.

To be fair, the nature of MOND makes running simulations difficult. The strength of gravitational acceleration in MOND varies in a more complex way than in standard Newtonian gravity, being much more dependent on the distribution of matter. Work on this problem has been ongoing for some time, and now at last the first MONDian simulations of galaxy formation have been unleashed. And... they're eerily familiar.

I have to say I found this paper really excessively long at 58 pages so I had to skip over large parts of it, which mainly seem to consist on tedious descriptions that could be better expressed with figures alone. But the gist of it is this : they simulate a big blob of gas, let it evolve, find that it goes phwhooop and out pops a healthy spiral galaxy that spins around nicely.

To be fair, in some ways this is no mean feat. You may remember my own efforts to construct a spiral galaxy from scratch (using standard models). Without dark matter, this is extremely difficult to set up. Get anything slightly wrong and the whole thing can become horribly unstable, blasting itself apart in a whole variety of interesting ways. Dark matter is great at stabilising everything, but without it, specifying the parameters of a disc and having it remain stable is feckin' hard.

And dark matter makes it pretty easy to start off with something that looks nothing much like a galaxy and get a very convincing spiral with minimal effort. Start with a big rotating blob of gas and let it do its thing, and bam! out comes a spiral : I should know, because I've run such simulations myself. Pretty much all the major properties of a typical disc galaxy emerge quite naturally. No need to fine-tune anything very much - it just works.

Twenty years ago, this "monolithic collapse" approach was interesting, but even by then, the rival theory of hierarchical merging and more-or-less replaced it. Monolithic collapse is elegant, but nobody could see how such giant monolithic clouds could ever form. Far more natural to suppose a series of smaller objects could gradually merge, an approach which has by and large been very successful (albeit not without plenty of hiccups and occasional bouts of serious illness).

So MOND's revist of this idea looks decidedly odd. It's reasonable of them to say that the different MONDian gravity should mean we expect fundamentally different initial conditions than the standard model, though this does open a pandora's box of free parameters : not only is gravity different, but so are all the initial conditions. It would be nice if they at least postulated what the new initial conditions should be, but they don't speculate on that here.

But it's not at all reasonable to claim that these results - their major one being the formation of an exponential stellar disc - constitutes much of a success for MOND. We did this twenty years ago using dark matter and got the same thing. True, this won't work with purely Newtonian gravity without dark matter, but no-one is claiming such a scenario. Instead, since MOND mimics the effects of dark matter quite precisely, in effect all these simulations have done is recreate the dark matter's gravity by another method. Their claims that the results don't depend on the precise baryonic physics is true, but misleading : that violent relaxation phase is going to wipe out the initial conditions, and it's that which is going to set the final outcome. That realistic-looking galaxies are a "generic outcome of collapsing gas clouds" is as true for the standard model as it is for MOND. Nothing new under the Sun...

The formation of exponential disk galaxies in MOND

The formation and evolution of galaxies is highly dependent on the dynamics of stars and gas, which is governed by the underlying law of gravity. To investigate how the formation and evolution of galaxies takes place in Milgromian gravity (MOND), we present full hydrodynamical simulations with the Phantom of Ramses (POR) code.

Accidental optical illusions

I have a fun little side-project to make a 3D model of the Milky Way using all-sky HI data. By measuring how fast the gas is moving and doing some trigonometry, it's possible to convert velocity into distance. The equations are a bit awkward, and if you get things a bit off, the end result looks very strange. They also have a limitation that they give a meaningless double solution for any point closer to the centre of the Galaxy than the Sun.

This meant there was quite a bit of trial and error involved until I got the correct result (more on that in a future post). To check I where things were going wrong, I had the code output the calculated galactic coordinates (latitude and longitude across the sky, measured from the galactic centre, as well as velocity along the line of sight), with the data set to zero inside the solar circle where the solutions would be garbage. Actually I'm pretty sure I got the position of the solar circle wrong, so this is just a complete mistake.

But it did produce a couple of fun little optical illusions :

Raw image here.

Both are quite similar. The colour in the grey circle looks like its varies, but it doesn't : it's completely uniform.

The effect is strongest with the left figure, which is velocity. The right side of the circle appears significantly brighter and the left significantly darker, a bit like looking at a crater in partial shadow (or, if you take the inverse perspective, a dome). Cover everything except the circle with your hands and you'll see this is entirely the result of your brain inventing stuff.

The figure on the right (galactic longitude) can be subtle at first, but once you see it, it's very hard indeed to make it stop. This time the right side of the circle appears darker and the left brighter, especially when you focus on the edges. I find that I can more-or-less control how strong this appears by concentrating on different parts of the circle, but sometimes it becomes so strong that I can barely make it stop even by covering the edges.

If we apply an animated mask to the regions outside the circles then things get even more fun (apologies for the small radial artifacts caused by gif compression) :

Raw image here.

It really is quite hard to graphically prove that the circles are always of constant colour. The only way to show it for sure is to download the images and an examine them in extreme close-up for yourself.