Time and tide wait for those granted sufficient observing hours on a big telescope

More on those galaxies which appear to lack dark matter. Hot on the heels of the recent simulations showing that dark matter can be tidally removed with little disruption to the stars, here's an observational paper claiming that's exactly what's happening.

This paper looks at one particular galaxy, NGC1052-DF4, which is not quite as famous as its neighbour DF2. However the distance controversy surrounding DF2 doesn't affect DF4 nearly as much, so even if it's quite a lot closer than the supposed 20 Mpc, it would still appear to be a weird object.

The "it's due to tides" theory has a lot going for it. There's no particular reason to expect any significant numbers of galaxies to form without dark matter all by themselves, so to find two in close proximity is suspicious. That they're in a group means tidal encounters are all but inevitable, and indeed, clear signatures of such interactions have already been detected elsewhere in the group. It's an elegant, simple way to explain what's otherwise a thoroughly perplexing observation.

But that doesn't mean the idea is automatically correct. Indeed, I find the results of this latest paper far from convincing.

They proceed in two ways. Using a combination of very deep ground-based and HST imaging, they search for both globular clusters and the diffuse light of the DF4 target galaxy. Globular clusters are handy because they're bright and compact - they should survive a tidal encounter but their overall distribution should give clues that something happened. The diffuse light, directly from the stars of the galaxy itself, is harder to detect but potentially a better diagnostic of tidal encounters.

First, the globular clusters. A few of these were already known and confirmed as being associated with the host galaxy by the gold standard of spectroscopic (velocity) measurements. This is hard to do, so they do the next best thing and do a lot of careful work to define sensible colour ranges to identify new candidates. After all this they find four new possible clusters. And faster than you can say, "small number statistics", here's their figure claiming to identify a signature of a tidal encounter :

The one nearest the bottom of the image is (they say) likely associated with the nearby blue smudge, not DF4. Okay, so that's really three new candidates. Two of them are close to the centre of the DF4 and don't change the distribution at all, so in fact we're down to a single new useful candidate. And the purple swathe ? That identifies the elongation of the "structure" they might have identified.

To be blunt, I often say to people, "I've seen worse claims", and this would be one of those semi-proverbial worse claims. It's highly dependent on two data points and has no 3D information. I'm glad they reported their results, but I don't believe for one second that this supports the claim of a tidal encounter.

On to the direct detection of the stellar light, which I find slightly better but still nowhere near as strong as the authors claim. They reach an impressive sensitivity level, again after a lot of careful work (this time to remove any contaminating light from the nearby brighter galaxies). The characteristic signature of a tidal encounter is a tail/counter-tail structure which they describe as S-shaped. Do they find anything ? Not really. Here's their figure :

Which they seem to think has a very clear S-shape. I looked at it and went, "huh ?", because it doesn't. It does show some hints of weak, broad, double-sided extensions, but it looks nothing like the classic tidal tails. It could be indicative of tidal disruption, but it's bloody faint.

Their surface brightness profile shows a distinct break, which I'll totally accept as real. They also identify a point in the isophotes which shows a "twisting", but this looks extremely weak to me and could be due to anything. The key thing they're missing is any kind of comparison sample : what would these same procedures reveal for isolated galaxies ? This is something we went to great lengths to explore when looking at gas tails, but there's no kind of equivalent analysis here. When an extension is this weak, I'd want to be damn sure it could only be the result of an interaction.

It looks even worse when they plot the isophotal contours :

Look, there might be something there. That it can be seen in different bands is somewhat more convincing, but the structure itself is so weak I'm not sure I'd even bother to report it. There is - maybe - some sort of extension. Is it compatible with a tidal encounter ? Sure. Is it evidence that this is what happened and not some other process ? Absolutely not. A tidal encounter is a great way to explain some very weird observations - I just don't believe that this data is good enough to support that conclusion yet. And there's no way to show that the small amount of stellar disruption (7% of the total stellar mass is in the diffuse extended light) really corresponds to significant levels of dark matter loss.

Finally, there are a lot of mutually-exclusive explanations floating around for these objects. As well as tidal debris and distance uncertainty, another idea is that the mass has been measured incorrectly. In one press release for the latter explanation, the authors were quoted with the Johnsonian phrase of "it's time to move on". But it can't be both of normal mass and the result of a tidal disruption. As long as there are conflicting explanations, "moving on" is exactly what we shouldn't do

The galaxy "missing dark matter" NGC1052-DF4 is undergoing tidal disruption

The existence of long-lived galaxies lacking dark matter represents a challenge to our understanding of how galaxies form. Here, we present evidence that explains the lack of dark matter in one of such galaxies: NGC1052-DF4. Deep optical imaging of the system has detected tidal tails in this object caused by its interaction with its neighbouring galaxy NGC1035.

Batman's gas is all wonky

You don't need really high resolution observations to get a lot of interesting information about a galaxy's gas. An unresolved spectrum can still tell you quite a bit about the total content and rotation, just not anything about which bit of a galaxy is doing what. A typical example (taken from today's paper) looks like this :

This is just brightness on the vertical axis as a function of velocity. Due to the rotation of the galaxy, one half of the of the gas is at a lower velocity (as it comes towards us) and the other is at a higher velocity (as it moves away from us). The double-horn "batman" shape is because most gas tends to be moving at a constant rotational velocity. But why is it all wonky ? Did Batman order dodgy costumes again ?

I've often wondered if we would extract more information from the spectra than just a crude estimate of the rotation speed. If we could measure the wonkiness, could we at least get an idea of whether the gas is being disturbed by something ?

The answer from this paper seems to be basically "no". They note that as others have found, measuring the asymmetry - the ratio of the flux in both halves of the spectrum - is a surprisingly subtle process. There are so many uncertainties in the measurement (finding the exact centre, defining where the gas ends and the noise begins) that you can only get a reliable measurement of asymmetry for the brightest galaxies, with signal to noise above 30, and even then they've got to have really quite large ratios before you say they're at all interesting.

This paper uses the HIPASS survey to quantify asymmetry across the whole southern sky. HIPASS was the first ever all-sky survey, but it has some serious limitations in sensitivity and resolution. Not only could it only detect bright, relatively nearby galaxies, but its field of view is so large that there can often be multiple galaxies seen in the telescope beam. So they restrict their analysis to those galaxies which have only one plausible optical counterpart, so they know the asymmetry isn't just the result of measuring multiple galaxies at once. They also quantify how much gas the galaxy has lost (if any) compared to typical galaxies of similar size and morphology.

What they find is.... well, not much, to be honest.

They divide their sample into four "distance"* bins, and plot both their parameters (deficiency and asymmetry) as both maps and as functions of overall galaxy density. They only see significant gas loss within the Virgo cluster, which is present in the first and second bins (and a little bit in the third as well).

* To be more accurate they use velocity, which is a rough proxy for distance on very large scales.

In their closest bin, there's a clear trend with gas loss as a function of density. Which makes sense : the more galaxies smashing around, the more the gas is going to get torn out. And more importantly, high-density environments like Virgo experience other effects (like ram pressure stripping from hot external gas) that's far more effective at stripping gas than galaxy-galaxy encounters. But in the second bin the trend is MUCH weaker and far more scattered, while in the third and fourth bins it's gone completely. This is despite probing even more dense environments in the more distant bins (which groups or clusters these are they don't say).

Deficiency, like asymmetry, is a difficult parameter to measure because galaxies have strong intrinsic variations anyway. You can really only use it to quantify things in very broad terms : galaxies are either gas rich, normal, somewhat deficient, or strongly deficient. And almost all the strongly deficient galaxies are present in the first bin, with very few in any other bins. In part (as they say) this is a sensitivity effect of HIPASS, which will only be able to detect the most deficient galaxies at the lowest distances.

There's another problem which they comment on : measuring galaxy density. They used a 2D parameter, which is not perfect since this can include galaxies at very different distances. So that there don't appear to be any strongly deficient galaxies in the really dense environments could just be a combination of projection and selection effects.

What of asymmetry ? That's even worse. There's a tentative, weak trend in the closest galaxies for the most asymmetrical galaxies to become even more asymmetrical at higher densities, but this isn't visible at all in any of the other bins. There's no trend between asymmetry and deficiency either : asymmetrical galaxies aren't especially gas-deficient.

Basically the conclusion of the paper appears to be, "we did this so you don't have to - please don't try this again, the data just isn't good enough". Logically, the correlations should exist. But we'll have to wait for better surveys before we can really test this - for now, this looks like another example of Simpson's Paradox. It's not that the trends aren't there, necessarily, it's just that the data can't show them. Which begs the question as to why they didn't use the far more sensitive, higher resolution ALFALFA survey... follow-up paper, perhaps ?

HI Deficiencies and Asymmetries in HIPASS Galaxies

We present an analysis of the sky distribution of neutral hydrogen (HI) deficiency and spectral asymmetry for galaxies detected by the HI Parkes All-Sky Survey (HIPASS) as a function of projected environment density. Previous studies of galaxy HI deficiency using HIPASS were sensitive to galaxies that are extremely HI rich or poor.

The tide of darkness ebbs away

That galaxy without dark matter is back in the news again. Avoiding the hoo-hah of the distance measurements, despite the author's obvious skepticism, this paper presumes the original measurements are correct and tries to simulate if such an object can ever form in standard cosmology. At face value, it does seem extremely weird : while tidal dwarfs are well-known to have little or no dark matter, their origins are usually pretty obvious because they're ugly, messy little things. This galaxy, however, is smooth and symmetrical, and shows no signs of any disturbance.

Now I'm always keen that papers should be lively and the need to be strictly accurate should not infringe on the need for readability. But here the skepticism is a tad... abundant. It's pretty obvious that the authors think the original discovers did just about everything wrong : "erroneously reported", "a result incompatible with", "a result orthogonal to", "once again, this value is incompatible"... so expect a robust response from the discoverers.

Anyway, what they do is try and simulate if such an object can be formed by tidal encounters. We already know that strong gravitational disturbances can strip stars and distort them into all kinds of fantastic shapes, and that's true for the gas as well. Simulations also show that the dark matter is easier to remove since it extends considerably further than either of the baryonic components. What they do here is to try and quantify this, using high-resolution "zoom in" simulations taken from larger projects.

At first glance things don't seem promising. None of their simulated galaxies, they say, show velocity dispersions (which is what we use for measuring total mass) anywhere near as low as those for the real objects claimed to be deficient in dark matter. Somewhat confusingly, they then go on to say that this is an effect of environment, and that actually yes, these things can form. It's not at all clear to me if they mean their initial search was only for isolated galaxies (and not satellites), or due to the low resolution of the large-scale simulations, or if something else is going on.

Their main result is that objects with low velocity dispersion can form as a result of tidal forces preferentially stripping the dark matter while leaving the stellar component more-or-less intact. This can happen to satellite galaxies in a "dense environment", whatever that means. They find several objects in their zoom-in simulations which are in good quantitative agreement with the real objects.

Importantly, they say the original findings overestimated the expected dark matter content for such galaxies, meaning that reproducing the observations requires removing an order of magnitude less dark matter than previously believed - it still means getting rid of a lot, but not that much. The other important factor is the nature of the dark matter. Dark matter particles are on much more extended orbits than the stars, and a particle found at one moment in the centre isn't likely to remain there (unlike the stars). And when it moves to the outer regions, it can be easily stripped by tidal forces. So dark matter can be removed essentially from anywhere in the galaxy without disturbing the stars very much.

Personally I think this sounds like a very serious challenge to these objects as being the weirdos they were claimed to be. The major remaining issue is how common these things are therefore expected to be. Large parts of this paper are very nicely explained, but other bits are a lot less polished. How does the environment in the simulations compare to the real Universe ? How close to the satellites come to their host galaxies ? Do objects like this require very special circumstances, or would we expect to see them everywhere ? I expect an interesting and hopefully angry response from the discovers.

Creating a galaxy lacking dark matter in a dark matter dominated universe

We use hydrodynamical cosmological simulations to show that it is possible to create, via tidal interactions, galaxies lacking dark matter in a dark matter dominated universe. We select dwarf galaxies from the NIHAO project, obtained in the standard Cold Dark Matter model and use them as initial conditions for simulations of satellite-central interactions.

Coming soon : FRELLED version 5

I had two lockdown projects. One was to develop an interactive model of Arecibo, which I more-or-less have working but just have to find the time and inclination to get into a useable format (which is tedious and boring). The second was to recode FRELLED, my Python script that imports 3D FITS files into Blender. This was originally written for Blender 2.49, released back in 2009 (!), but Blender 2.5 has a completely different Python syntax - more like using another language than making minor modifications. So it took a global pandemic to force me to re-write the bloody thing in a modern version of Blender.

After several months, I'm pleased to announce that this is done. Well, sort of.

The new version uses Blender 2.79. This isn't the very latest version, but for a very good reason. Blender 2.8+ doesn't support the OpenGL realtime shaders that FRELLED relies on, and unfortunately neither Cycles nor Eevee are suitable replacements. Apparently it will get a modern OpenGL equivalent at some point though, and the Python syntax is almost identical to that used in 2.79. This means the next update won't be anything like burdensome as recoding the entire thing again.

FRELLED version 5 looks like this :

This is the main display section with an example cube loaded. Blender's GUI now enables adjustable panels, so the user won't be overwhelmed with information. Presets are now such that loading a cube should be a matter of about five mouse clicks. This, I hope, will be easy enough to persuade people that it's worth installing and using.

Incidentally, installation should now be MUCH simpler. Blender 2.79 comes with its own internal Python and PIP kept completely separate from system Python (you can download it in a zip file, no other installation needed). So installing the modules FRELLED needs is now trivial... at least it was for me on Windows. It should even work on Linux networks.

Loading cubes is now much faster, hence there's generally not so much need to worry about which projections are being imported, so this is all hidden by default. But all that is still there for enthusiasts and those using very large or weird data sets. In particular, the "sparse sampling" option now lets you import only every nth slice of the data, adaptive to the size of the cube in different directions, so in principle even arbitrarily large data sets should be no problem. Contrary to expectations, loading in less of the data often makes the appearance better rather than worse.

Not every feature in the GUI is currently functional - the major one being multi-component rendering, but also the quick import and preview buttons. Both of these are actually fairly simple -  they just requires me to work out the most efficient way to do it (for multi-compment/volume rendering, the GUI buttons will greatly simplify what used to be a rather ugly, hacky process that worked but was unpleasant to use). 2D mode, though, is fully functional.

The Analysis menu does look a bit scary, but you can hide any panels you're not using and most of them should be simple enough. A big advance is that you can change the spectral axis units and it's no longer hardcoded to assume the data is HI, so the velocity of any molecular line just needs the rest frequency (a drop-down menu provides a few preset values and also access to Spatalogue). It's also possible to hide the axes with a single button, which used to be a much sillier process.

Region analysis tools remain much the same as in the original FRELLED but with improvements. Contours are now much faster and true isosurfaces are supported (and are fast enough that you could even show these for an entire cube, as an alternative to volume renders). You can also show velocity maps as well as much nicer-looking flux maps that use the requested colour scheme rather than only greyscale. There's a simple toggle for using a geometrical progression for contours (or logarithmic display for maps), with built-in safeguards to stop the user trying to display unfeasibly large and complex contours. And SDSS maps are now opaque, making them very much easier to see.

Isosurfaces are functional, though currently with only limited display capabilities. The mbspect section also has limited (but significantly improved) capabilities : it can only produce the input files and not run interactively, but it does allow all the options to be set directly in the GUI. The interactive version will be restored once I have access to a Linux system to test it on. Finally, users can also set some options for NED queries instead of just having it return absolutely everything.

I've tried as much as possible to test everything and test again. But as you can imagine, it's just not possible to test everything to destruction. So before making an official release, it's time for some beta testing. Volunteers are welcome ! Preferably those who aren't scared of working with FITS files. I can provide instructions and example cubes to try, but I'm especially keen to see what happens with data sets I've never tried, and with using it in anger : doing things in odd sequences and using features in unexpected ways. So if anyone out there wants to help, do get in touch. Leave a comment on social media or this blog, or contact me directly at feedback @ rhysy . net, and I'll add you to a beta-testing email list for next week.

Stop judging the gassy dwarfs

Last year there was an interesting attempt to constrain the mass of those pesky Ultra Diffuse Galaxies by measuring their X-ray gas content. This was clever : UDGs are normally so faint that determining their rotation to get a direct mass measurement is very difficult. Measuring the X-ray content, or at least placing an upper limit on it, is relatively easy, and since there's a tight relation between X-ray luminosity and total mass, this should give a decent constraint on the mass of the galaxies. Basically the more hot, tenuous gas a galaxy can cling on to, the stronger its gravitational field must be.

Since they didn't detect any hot X-ray gas at all, the authors concluded that they're more consistent with being dwarfs (which don't have much hot gas) than giants (which do). Which is a shame because if they were giants they'd be much more interesting, pointing to some unsuspected problem in star/galaxy formation at the high-mass end, where models were thought to be reasonably okay.

Here one of the authors splits from the band to start a solo career. While still controversial, it seems to me that most people would accept that most UDGs are probably dwarfs. But what about the most famous candidate giant objects ? Here the author takes a good hard look at two of the most prominent examples using very deep data.

Again, he doesn't find anything, so concludes that even these are just weirdly large dwarfs. But the same objections I raised last time still apply. If UDGs formed in a somewhat different way to brighter giant galaxies, why should we assume that'd have the same sort of X-ray content ? The whole reason they're interesting is that they have far fewer stars than other galaxies of their size, so I don't see why we should expect them to have the same amount of hot gas. Likewise, if you assume that conventional scaling relations are always correct - even if they have very low scatter - you'll never find anything that deviates. So this is circular reasoning : if you assume they follow the standard relations, you automatically exclude them from being weird.

The author does make the good point that X-rays originate from sources than just hot diffuse gas, however. They also originate from high-energy binary stellar systems (white dwarfs orbiting black holes and suchlike). But it's not obvious that this makes any difference. If UDGs form far less stars than similar sized galaxies, why expect them to have a similar fraction of such binary systems ? I remain unconvinced. It's still an interesting observation, but I'd change the title and emphasise that these observations are only consistent with dwarf galaxies, not proof that UDGs aren't giants.

The Archetypal Ultra-Diffuse Galaxy, Dragonfly 44, is not a Dark Milky Way

Due to the peculiar properties of ultra-diffuse galaxies (UDGs), understanding their origin presents a major challenge. Previous X-ray studies demonstrated that the bulk of UDGs lack substantial X-ray emission, implying that they reside in low-mass dark matter halos. This result, in concert with other observational and theoretical studies, pointed out that most UDGs belong to the class of dwarf galaxies.

It's as dark as dark can be, probably

This year has been downright weird in just about every way, and writing papers is no exception. Before the pandemic I was working on a nice little paper about Ultra Diffuse Galaxies, which I got carried away with and made everything much more difficult for myself, but then lockdown hit quite suddenly and I resorted to happily recoding FRELLED. That's nearing completion, at least in the core aspects.

As working from home transitioned from weird to normal to actually desirable, observations with Arecibo, once defying the general trend by continuing to operate relatively normally, came to a literal crashing halt. But other telescopes are still managing. Michal Bilek, a colleague from Prague who's now based in Strasbourg, was fortunate enough to get some extra time on a 1.4m telescope in Serbia, and he had the idea of doing some really deep imaging of those optically dark Virgo clouds I love being the cheerleader for so much*.

*Only without any pom-poms, which is frankly disappointing.

Now you might be wondering what a 1.4m optical telescope can possibly contribute to the research of hydrogen clouds discovered with a 305m radio telescope. Well, for one thing it's a lot easier to get multiple hours of integration time on a 1.4m optical telescope for a single source than a 305m dish, for which we only had the equivalent for 5 minutes on-source. This means we can get really high sensitivity. And while for various reasons I strongly suspect there's no optical counterpart to be found, the only way to verify that is to go and look.

We chose one particular target since it was among the brightest of the Virgo clouds and has a nice high velocity width, although it's not outstandingly different from the others. During the discussion I learned just how difficult it is to get really deep optical data, which suffice to say is a lot more of a complex process than just doing a really long exposure. There are all kinds of post-observation procedures that need to be applied, most of which I don't understand (although one, injecting fake sources to estimate the sensitivity, is pleasingly similar to measuring radio tails).

The bottom line is that we can indeed get extremely high sensitivity, constraining the the object to have significantly less mass in stars than gas (by at least a factor three). We also didn't find any sign of optical disturbances in the nearby galaxies which might have suggested that this object is just another boring old bit of tidal debris. Although it's unlikely that a galaxy could have a nice stellar stream without an accompanying hydrogen tail, it's by no means impossible in an environment as chaotic as the Virgo cluster. So this adds, tenatatively, to the idea that it's a genuine dark galaxy, a scenario which has no problem explaining the observations.

I say "tenatatively" since this does rely a bit on statistics. Our previous simulations show that forming such objects is almost, but not quite, impossible. This makes it incredibly improbable - I would say even ruling it out pretty decisively - that all six such clouds are tidal debris, but we certainly can't make such a strong statement for each individual cloud. And there are a few short hydrogen tails in the surrounding region, but none are a good match, and the cloud would still be radically different from any other known tidal debris feature. So yes, tidal debris is a possibility, but in my view it's not terribly likely. Of course whether it's more likely that the cloud is actually a starless galaxy... well, that's another matter.

Basically, as usual, I'll be satisfied if people simply acknowledge that these clouds are weird. I don't much mind what they think they actually are - that is left to future research. For the time being, I'll continue cheerfully waving the metaphorical pom-poms for gas clouds that don't do anything until people give in and admit that they're interesting or everyone gets bored and gives up.

Deep optical imaging of the dark galaxy candidate AGESVC1 282

The blind HI survey Arecibo Galaxy Environment Survey (AGES) detected several unresolved sources in the Virgo cluster, which do not have optical counterparts in the Sloan Digital Sky Survey. The origin of these dark clouds is unknown.