The Radial Acceleration Relation is just not a thing so let's all shut up about it

A long time ago the Radial Acceleration Relation was quite the thing for galactic dynamic studies. There's a tight correlation between the acceleration predicted by the observed material and the actual observed acceleration, which is unexpected because of the enormous amount of invisible dark matter. MOND supporters danced with joy at a successful prediction, only for standard model advocates to shoot them down days later, pointing out that the exact same thing was observed in their own simulations.

It took quite some time, though, before anyone gave a credible explanation as to why this effect was actually seen in dark matter-dominated models, where naively one wouldn't expect a connection between normal and dark matter. Plenty of people waved their hands with loud cries of, "SCALING RELATIONS !", but no-one really understood what they were on about.

Finally some clever chaps managed to explain these scaling relations in a reasonably convincing way. In brief, galaxies can't form in the largest (because of feedback and other effects) or smallest (because they don't have enough gravity to accrete much gas) dark matter halos. And galactic dark matter halos have a characteristic density profile in the standard model, giving rise to the observed relation.

But the others of the current paper aren't satisfied. They want to know just what it is that's happening physically to cause this. What exactly is it about the observed baryonic matter that could lead to this behaviour ? Why does it match the MOND predictions ?  "This connection", they say, "must be in some sense “universal.” " I won't say I fully understand it, but their proposed solution feels quite elegant to me.

Normally astronomers think of star formation as being related to the gas density. More gas => more self-gravity => more stars. We normally assume that the density of the dark matter is low so that we can neglect it. Here the authors implicitly say, "yes, that's usually true, but it's not technically accurate, now is it ?". After all, what drives the collapse of the gas is the total gravitational field, not only the mass of gas. We can't possibly expect to discover a dark matter-baryon connection without including the dark matter.

Their model goes like this. Star formation proceeds efficiently if the total acceleration is above some threshold, whereas if it's below this threshold then there won't be much star formation at all. In regions initially dominated by dark matter, and below the threshold, few baryons will ever be accreted : the feedback from the few stars which do form will easily suppress further star formation by blowing the gas away. So these regions remain dominated by dark matter, i.e. dwarf galaxies and galaxy outskirts. In the opposite case of regions initially dominated by baryons and above the threshold, star formation will proceed much more efficiently. Feedback will be less effective at disrupting the gas inflow due to the stronger gravitational field, so these regions will always be dominated by baryons. Thus the acceleration threshold marks the boundary between regions dominated by baryons or dark matter.

This means there's a physical reason to expect the observed change in the RAR at low masses. They go further, saying that this also predicts the characteristic scaling behaviour at the different extremes, and even why galaxies generally have flat rotation curves : it's not a "conspiracy", as MONDian advocates worry, but an entirely natural effect of stellar feedback in regions dominated by dark matter - a flat rotation curve (if I understand them correctly) is the natural form of a dark matter halo, and it's only the presence of baryons that disrupts this. They also say that this all in no way implies that they expect the relation to be perfect or perfectly universal : variations in the stellar mass function will alter the feedback, so there could certainly be deviations from galaxy to galaxy; they can't make detailed predictions with this hugely simplified model - the point is to predict basic scalings, not develop a new and exact model of galaxy formation.

Again, I need to think a lot more about this. At first glance it seems like a neat way to universally connect the microphysics of star formation with the more global parameter of acceleration. I wonder how far this can be pushed to explain star formation despite its limitations : what happens in the other cases, if dark matter dominates but the acceleration is above the threshold, or if baryons dominate but the acceleration is low ? What, if anything, does this suggest for ultra diffuse and dark galaxies ? Definitely interesting, and this is a case where I wholly support the authors putting the paper on arXiv and asking for feedback* before publications.

* "Feedback is welcome", they say, leaving it to the reader to decide if that's a pun or not.

Stellar feedback sets the universal acceleration scale in galaxies

It has been established for decades that rotation curves deviate from the Newtonian gravity expectation given baryons alone below a characteristic acceleration scale $g_\dagger \sim 10^{-8}\,\rm{cm\,s^{-2}}$, a scale promoted to a new fundamental constant in MOND-type theories.

Twinkle, twinkle, little Starlink...

Well this is worrying.

Last night, they were taking about 40 exposures of the night sky, looking towards the small and large magellanic clouds, two dwarf galaxies that neighbor the Milky Way. But during one set of those observations, 90 minutes before sunrise, the train of SpaceX’s Starlink satellites moved into view, glinting in the early morning sunlight and taking five minutes to pass across the telescope's line of sight. 

"This happened just before astronomical twilight," says Johnson. "By almost any observing standard this was still the heart of the night, exactly when you want to be taking data. And especially when you want to use every minute of observing time you get on these telescopes and these facilities."

There are several factors that could affect how much worse this will get :

• The number of satellites : "So far, SpaceX has launched just 0.14 percent of its total planned Starlink constellation.... Bassa has calculated that up to 140 satellites could be visible at any one time if all the planned satellites launch.". Doesn't specify what's meant by "visible" here, i.e. in what field of view.
• The altitude of the satellites : "The satellites were deployed in a long train at an altitude of 280 kilometers... but are in the process of being raised to their operational altitude of 550 kilometers". The higher they are, the longer they'll reflect sunlight, though they will also be fainter.
• The albedo of the satellites : "...painting future Starlink satellites black to reduce their reflectivity, although it’s not believed this was done for this latest batch – while the glinting of the large solar panels on each satellite still poses a problem."

On the second point, it would be nice to know exactly how long before twilight this was. There are detailed, credible-sounding articles saying that the satellites impact on astronomy will be minimal, but that they're visible even in full night somewhat contradicts that. So just how much astronomical night will be lost ? If it's five minutes then that's not awful (although I wonder how much this will affect flat fields also), but if it's longer then that could be a big deal. Especially for targets of opportunity, which can't be neatly scheduled to avoid inconvenient hours. Maybe painting them black will be enough, but wouldn't it be nice to test this before going full throttle ?

[Walks away angrily singing the Firefly theme tune...]

'This Is Not Cool!' - Astronomers Despair As SpaceX Starlink Train Ruins Observation Of Nearby Galaxies

In the early hours of the morning today, November 18, two astronomers checked in on their remotely operated telescope in Chile, expecting to see images of distant stars and galaxies. Instead, they saw a train of SpaceX satellites crossing the night sky, a worrying sign of what might be to come for astronomy.

Quite queerly quiescent

Why do some galaxies burn through their gas like a fire in an un-raked forest while some are content to sit back and relax ? In hand-wavy terms it's probably some combination of internal and environmental effects. In more massive galaxies the gas density can trigger more star formation throughout the disc, while in smaller ones the gas density tends to be lower. At the same time, when stars from in low-mass galaxies, their feedback effects can be more effective at suppressing further star formation by lowering the gas density still further, since there's less gravity to work again. Then again, high mass galaxies won't be much effected by encounters with other galaxies unless they're similarly massive, whereas low mass galaxies are much more vulnerable. So this simple question is not easy to answer properly.

What would be nice to have is so basic you'd think we'd already have it : some kind of census of the gas content of galaxies in different environments at different distances, masses, and star formation rate. Getting this is more difficult than you might think, because gas measurements are difficult except in the relatively nearby Universe. Getting a statistically significant sample, and understanding whether each galaxy is being affected by its own internal process of environment, is not an easy task.

The authors of this paper attempt to simplify things by looking at how the gas content varies in massive galaxies as a function of star formation rate. They have a sample of about 9,500 galaxies from the ALFALFA HI survey, matched with SDSS data to get star formation rates and stellar masses. They correct for their sample incompleteness and other statistical biases, so their final results should be an accurate representation of what's really going on.

Remarkably, they find that the atomic gas content of disc galaxies doesn't vary much at all even as star formation rate varies by a factor of a hundred. The proportion of disc galaxies with HI detections doesn't vary with star formation rate either (though it's not clear to me if this is the case for elliptical galaxies, which are anyway hugely biased towards low star formation activity). Nor does their gas fraction vary as a function of star formation rate either. And even the average HI spectrum of the star-forming and quiescent galaxies look incredibly similar.

It's a different story for the molecular gas. This shows a very clear, neat, strong trend, increasing in mass with star formation rate. That's not too surprising, as the consensus has been building for a quite a while that molecular gas correlates much more strongly with star formation activity than atomic, but it's nice to see. But why does this happen ? They say, "These galaxies are quenched because of their significantly reduced molecular gas and dust content and lower star formation efficiency", but this is a tautologous description and not an explanation. Galaxies which are quenched have low star formation activity by definition !

More interestingly, they note that the similar spectra imply that both quiescent and star-forming galaxies in their sample are likely rotating discs. They suggest that once the inner gas (which is denser) has been consumed by star formation, it takes a long time for the outer, less dense gas to either form stars or migrate inwards. So it just sits their, slowly rotating and generally doing sod all.

What would be nice to see next is a more detailed look at some of those individual galaxies. They make a testable prediction that the HI should be found in rings in quiescent galaxies, since the innermost gas will have been consumed. It would also be nice to describe their environments in a lot more detail : they say they work with "central" galaxies, but this doesn't help much. If they're central cluster galaxies then I'd be very surprised indeed if the quenching happened inside-out, since that's the exact opposite of how ram pressure normally removes gas in such galaxies. And I'd like to know a lot more about how they stacked the spectra - I'm very surprised that the velocity widths of the galaxies are apparently all so similar. It would also be nice to see what happens if they use specific (instead of global) star formation rates - that is, the star formation rate per unit mass.

The other thing I wonder about is this paper. Really massive disc galaxies have less baryonic mass than expected given how fast they rotate, which the authors there suggested might point to an upper limit to galaxy formation : above a certain mass, gas may be unable to cool and form stars. But this current paper says you can have really quite large gas reservoirs that disdain star formation, so perhaps things at the high mass end get more complicated. It would be interesting to see how the two samples compare, at least. It's nice to see such very clear evidence that it's molecular gas that matter most for star formation, but there's a lot more left to do to understand as to how atomic gas is converted into molecular.

Nearly all Massive Quiescent Disk Galaxies Have a Surprisingly Large Atomic Gas Reservoir

The massive galaxy population above the characteristic Schechter mass M * ≈ 10 10.6 {M} ☉ contributes to about half of the total stellar mass in the local universe. These massive galaxies usually reside in hot dark matter halos above the critical shock-heating mass ̃10 12 {M} ☉ , where the external cold gas supply to these galaxies is expected to be suppressed.

Stripping with jelly

You might remember a paper back in September that found tails of gas from a couple of galaxies around 6 billion light years (3 Gpc) away. Here's another, similar paper looking at a galaxy quite a bit closer (1.2 Gpc). Which makes it a bit odd that the first line declares this to be the first such study at intermediate redshifts. Either :

• Their paper completed the submission process before publication of the other one
• They submitted later but both the authors and referee were unaware of the other paper
• They're doing a Donald Trump, like when he declared that impeachment required both high crimes and misdemeanours. Perhaps they deem the other galaxies to be at high redshift, so it's okay that this one is only at intermediate redshift.

I also found their introduction to oddly imply that mergers between galaxies are not an effect of environment, but that's nick-picking. Otherwise it's nice. 

They note that the effects of ram pressure stripping, the primary way of forming long one-sided tails, are still unclear. Given long enough and it will strip all the gas in a galaxy, quenching its star formation. But what happens during that process is potentially much more complicated : it might immediately reduce star formation by lowering the gas density, but it might also trigger star formation by compressing the gas (at least at the point of collision with the external gas). So a galaxy could potentially have its star formation reduced in some areas but temporarily increased in others. 

Even more complex and controversial is what happens to the gas that gets stripped. Molecular gas is so dense that it probably doesn't get stripped directly, though some people think it might be possible in extreme circumstances. A few galaxies - not many mind - seem to have stellar tails as well as gaseous ones. It's unclear if these star-forming wakes result from direct stripping of molecular gas or if the molecular gas forms in the tail from the stripped atomic gas. And goodness only knows what happens to the stars formed in the tails after all the hullabaloo is over.

The galaxy they study here is a particularly dramatic case, forming stars as much as five times faster than other galaxies this massive. It's also a very massive galaxy, making material harder to strip. Yet it shows very clear tails and tentacles extending quite neatly and continuously from its disc - it is, as they say, a textbook jellyfish galaxy. What seems quite clear in this case is that the star-forming regions they see in the ultra-violet (which traces hot young stars) aren't just found in the tentacles, suggesting that ram pressure can indeed trigger star formation within the disc as well as in the stripped wake. Of course it's hard to be sure, since we don't know what the galaxy was like before stripping began, but the visual is awfully convincing.

Some of their other points are less clear. I don't say they're wrong, only that it's not obvious to me how they reach certain conclusions. They say, for example, that the stripped gas lags behind in velocity compared to the disc gas, but it doesn't look like that to me in the figure. They also say that if they account for this lag then the galaxy is rotating exactly as expected, but again I don't see how they actually do this. They also try a simple simulation to work out the galaxy's trajectory, finding that it's consistent with infall along a filament from another nearby cluster, but their description seems unnecessarily geared towards experts in orbital dynamics.

It's still a nice paper. Perhaps in a few years we'll have statistically significant samples of such objects, and then things will get a lot more interesting. 

Jellyfish: Resolving the kinematics of extreme ram-pressure stripping at $z\sim0.3$

We present and discuss results from the first spatially resolved kinematic study of ram-pressure stripping of a massive late-type galaxy at intermediate redshifts. Our target, the spectacular "jellyfish" galaxy A1758N\_JFG1, was previously identified as a fast-moving member of the equal-mass merger A1758N ($z=0.28$) with a star-formation rate of 48 M$_\odot$ yr$^{-1}$, far above the galaxy main sequence.

Wibbly-wobbly spacey-wacey

A few weeks ago I posted some first tests of displaying isosurfaces in Blender. This got interrupted by the receipt of a referee report, so now back to the wibbly-wobbly spacey-wacey stuff.

I used to poo-pooh isosurfaces as being inherently inferior to volumetric renders because they lose information. They also feel somewhat like cheating, because rendering a surface is easier and uses a lot less memory. While all this is true, I'm somewhat reconciled to their uses : isosurfaces depend far less strongly on viewing angle than volumetric renders. That makes them much easier to highlight features in an objective way, and contrary to my intuition, this can make them better for finding faint structures rather than worse. The eye tends to get very confused when you have really bright and faint sources together, but slap on a fixed level surface and BAM you can see if what you're looking at is significant or not. You might still miss the very faintest stuff, but there can be a surprising amount to see at relatively bright levels.

The other nice thing about isosurfaces is that because they're very fast to generate (typically seconds per surface in these examples, if not less), they're easy to animate. Last time I showed some fixed-level turntable animations. Here's M33 from the AGES HI survey shown at a variable flux level, starting with the brightest gas and ending with the faintest. Each frame decreases the flux to 97% of the previous value.

There's a whole bunch of clouds around M33, most of which show up quite well in the animation. It's not as good as manually tweaking the level of each region, but it basically works. I probably should have frozen the final level and rendered a full rotation, but never mind.

Here's the same sort of animation but using NGC 4361 from the WSRT HALOGAS survey, which also has a bunch of weird stuff going on around it :

And finally, here's a test of a true time series from a simulation. This is an old one from when I was trying to learn the FLASH hydrocode and my galaxy got all unstable because of a bug in the boundary conditions. Four fixed levels all animated, showing how the nice stable galaxy gradually goes fully wibbly-wobbly and eventually gives up. Does it help analyse what went wrong ? Not really, but it looks nice.