Isosurfaces for fun and profit

The best way to look at at 3D volumetric data is, in my opinion, in its original 3D volumetric form. To this end I've spent several years years developing the 11,000 lines of Python code that is FRELLED (albeit all of which is just a script to load astronomical FITS files in Blender, which is what does the hard work). I've always been a bit skeptical of other ways of visualising the data... 2D slices are fine, and often necessary, but things like isosurfaces seem to me to be throwing away a lot of really pretty* information.

* I don't much care if it's meaningful or not.

To be fair, FRELLED does already include the ability to display renzograms, which are essentially contours of each slice of the data. While it's true that viewing the data at a fixed level, reducing it from a full volume to a thin surface, does remove a lot of information, I've been realising that by cutting away a lot of the noise it can actually become a lot easier to see interesting features. With the full volume, sometimes the noise just gets in the way. Using the renzogram facility of FRELLED, we've found a bunch of hydrogen streams in the Virgo cluster we'd just never noticed before (paper submitted).

So renzograms are super useful. But while 3D renzograms are sort-of isosurfaces, they're not proper 3D fits to the data. That's harder to do in Blender - I tried to get this a while back, and it sort of worked but it was very, very hacky. That method used someone's old Python script that skins a point cloud of vertices. It works well in some situations but not in others - a lot of manual cleaning is needed on complex data sets, and that's not much fun. The experience is a bit like using a half-broken toaster : you're never quite sure if you're going to have a nice breakfast or burn your house down.

But now I've found that the Python scikit-image module includes a "marching cubes" algorithm that generates proper isosurfaces. Fast, effective, and no mucking about with cleaning up artifacts at all. I've quickly hacked this into Blender, using another module to convert the vertex data generated into a Blender-readable format. The basic code is just a few lines long and it works without complaint.

So, time for some examples ! This first one is a bog-standard Virgo cluster galaxy with no interesting features whatsoever - it's just a long, cigar-like blob, with the long axis being velocity. Colours indicate brightness of the emisssion (purple, blue, green, yellow and red going from bright to faint).

For a second example, here's another Virgo galaxy which does seem to have a distinct protuberence on one side. It's probably losing gas as it moves through the cluster.

And then there are oddballs like this one, which seem to have a distinctly noisy appearance and a "tail" in velocity space :

Just to prove how incredibly easy this is, here's the whole data set of 102 galaxies. Even in this zoomed-out view, you can see that most galaxies are quite smooth and symmetrical, but some have pretty clear extensions and other weirdness (after a laborious statistical analysis we're highly confident these are real and not just due to variations in the noise).

You may be thinking that that's all very nice, but what about some nice resolved high resolution data ? No problem, here's one of my favourites - the M33 galaxy and its many associated clouds :

The M33 system is so complicated that I cheated a bit with the contours on that one. In all other cases, the same colour is used for identical brightness levels, but in the case of M33 I set the levels manually for each cloud - otherwise you start being totally dominated by noise in some cases, while not seeing anything at all in others.

Finally, here's an isosurface of Medusa a simulated galaxy undergoing strong ram pressure stripping. No noise to worry about at all for simulations.

All this is part of a larger effort to recode FRELLED in modern Blender. FRELLED currently relies on Blender 2.49, which is more than 10 years old. Blender 2.8 has a lot more features and comes with its own Python and Pip install, making it waaay easier to install the necessary modules. Perhaps that will help catapult FRELLED from obscurity to total domination of the astronomical community having more than a dozen users. That'd be nice.

Put a ring on it

The intergalactic environment is a messy place. When you've got masses of hundreds of billions of Suns hurtling past each other at hundreds of miles a second, you expect things to get ugly. What you might not expect is to find nice, neat rings.

To be fair, rings are pretty rare structures. Ring galaxies like the famous Cartwheel have been shown to (most likely) form during collisions of very specific encounter geometries. Others, like the implausibly neat Hoag's Object, are more mysterious.

Ring galaxies typically have rings with highly active star formation and lots of gas. But there are a very few gas rings without any appreciable star formation activity at all. The most famous is the giant Leo Ring :

This is also thought to have been formed during an encounter. Of course there's also the much smaller but weirder Keenan's Ring :

Which is especially strange because the Ring is strongly offset from the main galaxy in this region and doesn't show much of a velocity gradient across it. That makes it very hard to explain by a collision.

Today's paper announces the discovery of another spectacular ring quite similar to the Leo giant. It's about half the size, but still much larger than Keenan's Ring. It's got about 3 billion solar masses of gas but not much in the way of associated stars. Actually, they point out a few stellar smudges within the structure that could be part of the Ring but it's by no means clear this is actually the case, so it might be completely optically dark. What's especially strange about this one is that the central galaxy is an elliptical, which don't usually have much gas at all.

Like Keenan's Ring, this feature is distinctly offset from the central galaxy - although not nearly as much as in the case of Keenan's. On the other hand, it has a much larger velocity gradient, making a collisional origin somewhat more likely. The lack of any stellar disturbances is a bit odd though.

What could be going on ? Well, in elliptical galaxies which do have gas, it's thought the gas density is much lower than in spiral galaxies. So the authors suggest a collision in those cases would have quite different effects to more spectacular features like the Cartwheel : the compression of the gas just isn't enough for it to reach the densities needed for star formation. Additionally, the shock heating from the compression may be more effective, making most of the gas ionised and unable to form stars. Alternatively, the ring could be the remains of a disrupted gas-rich galaxy that fell into an unpleasant orbit around the elliptical and never escaped.

There's not much more we can say at this stage other than, "wow, a giant ring, cool !". Observations of ionised gas could help trace other components and see if this really was formed by collision, while simulations could show if there's a plausible encounter that really could form a gas ring that leaves the stars so completely undisturbed. We'll just have to wait and see.

Discovery of a large HI ring around the quiescent galaxy AGC 203001 

Here we report the discovery with the Giant Metrewave Radio Telescope of an extremely large ($\sim$115 kpc in diameter) HI ring off-centered from a massive quenched galaxy, AGC 203001. This ring does not have any bright extended optical counterpart, unlike several other known ring galaxies.

Perhaps the cow is very large after all

The saga of galaxies without dark matter continues.

At first, I thought the thing sounded pretty cool. Lots of accusations were made that they'd got the distance wrong, which means they'd underestimated the total mass , but most of them didn't seem very credible to me, (granting that I'm no expert in this). But then another claim said that there were two groups in this part of the sky, one close and one more distant. That sounded a lot more believable and would give a good excuse for everyone getting the distances confused.

This latest measurement says nope, these galaxies are far away and don't have any dark matter. They use deeper Hubble data and show that prospect of the galaxies being much closer just don't fit. They say that previous data was too shallow, so that the red giant stars needed for distance measurements (red giants act as a sort of standard candle of known brightness) weren't visible, biasing the result in favour of a lower distance measurement. They don't really comment as to why different authors found different distances from the same data, except perhaps a hint that the calibration of the magnitudes may have been wrong.

Given the recent discovery of a whole population of gas-rich objects without dark matter, for which distance concerns aren't so important as they're quite a bit further away, these original claims should probably be given rather more credibility. There hasn't been much response to those latest detections yet, though it's still early days and the publication was only a letter, not a full article. Of course it's possible the original objects are actually normal objects but the new ones are indeed dark matter deficient, although that would be a bit weird.

The authors say that even deeper Hubble observations are on their way. Will this finally settle the matter ? I somehow doubt it. To my mind, the focus should switch to those gas-rich galaxies without dark matter, which don't have such distance or velocity ambiguities. Perhaps when there's a full paper published people will start to realise that there's a whole other bunch of really interesting objects to work on instead of these two usual suspects.

A Tip of the Red Giant Branch Distance to the Dark Matter Deficient Galaxy NGC 1052-DF4 from Deep Hubble Space Telescope Data

Previous studies have shown that the large, diffuse galaxies NGC1052-DF2 and NGC1052-DF4 both have populations of unusually luminous globular clusters as well as a very low dark matter content. Here we present newly-obtained deep Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging of one of these galaxies, NGC1052-DF4.

Pretty things are pretty

Time for some more pretty pictures of hydrogen....

It's proving surprisingly difficult to convince people that some streams I've found in the Virgo cluster radio data are real. So to settle the matter once and for all, I've resorted to creating synthetic galaxies and adding fake streams and noise extracted from real data. Then I blindly search the data, labelling what I think looks like a stream and what doesn't. Since I don't know in advance which galaxies have streams or not, this should be a good way to quantify very rigorously how many false positives can occur just due to the noise, as well as measuring how many of the known fake streams would actually be detected by the search technique.

Each of these 100 pillars is a synthetic galaxy, with the vertical axis being velocity. Each of the "segments" is a contour at a different velocity channel, extended into 3D. Real galaxies would look a bit more complicated than this, but these are good enough to search for features as extended as the ones in the real data. Arranging them into a neat grid makes it easy to search the whole data set very quickly and isn't just for the sake of making something minimalist.

You can't really see the fake extensions from this angle - they're more visible from underneath. This particular data set has a stream in every galaxy, always pointing in the same direction. For the real search I vary the length, angle, brightness, presence, and number of velocity channels of the streams.

These pretty pictures aren't going in the paper - for that, I'm showing a boring but more easily comprehensible 2D plot. They look nice though.

The tangled web we weave

Large-scale simulations of the Universe show a characteristic network of filaments and voids, which is in spectacular agreement with real observations of the distribution of galaxies. Even within the voids, what few galaxies are present are distributed in thin tendrils. Galaxies are like flies caught in this cosmic web of dark matter, except fortunately there aren't any giant spiders coming along to eat them, which is a shame because that would inspire some pretty bad-ass mythology.

But what's in between the galaxies, in the filaments themselves ? They appear, as predicted, to have their own dark matter, acting as a sort of scaffold onto which other material can accrete. Detecting this infalling gas would be pretty neat as this could give clues to galaxy formation and survival. For example, the Milky Way is currently forming stars at such a rate that it ought to run out of gas pretty quickly, so unless we happen to be witnessing it in its final phase of star formation, it's likely that it's being re-supplied from somewhere.

Claims for direct detection of the gas in filaments are many and various, the difficulty being that it's hard to distinguish between material present in the primordial filament and stuff that's been chucked out of galaxies during interactions (not to mention that the gas is especially thin and hard to detect at all). There have been a few credible possibilities though, such as this single giant filament seen by Planck, and a convincing statistical detection from stacking observations of many pairs of galaxies. But information is very, very scant.

This paper takes things to a new level with a direct detection of faint UV emission in a complex of very bright, very distant galaxies. Like the earlier Planck observations, the emission is too extended to likely result from galaxy interactions (it's about 1 Mpc across), but unlike Planck it shows several different structures, and looks a lot more web-like.

The detection here was possible only due to an exceptional circumstance. At a redshift of 3, the Universe was only about 2 billion years old, and therefore a lot smaller and denser than it is today. Star formation activity and supermassive black holes were also churning out energy like nobody's business, but this particular target region is exceptional even by the standards of the time : it has a density of active galaxies about 1,000 times greater than the average of the day. Which is slightly insane and terrifying, but does explain how the web can be detected here when it's normally so faint.

Could it be that this is just unusually extended gas from tidal interactions, and not related to the primordial material in the web ? Probably not. Although a few extensions of similar length are known, the gas here doesn't show much variation in velocity, unlike tidal cases. And its velocity dispersion is lower in the filaments than close to the galaxies, which is what you'd expect if it was cool gas accreting onto the dark matter skeleton. So while there are various other previous claims for detecting parts of the web directly, and a couple of good cases of directing it statistically, this seems like a pretty solid claim on being the first direct detection of an actual proper web and not just a mere bridge between a couple of galaxies.

Gas filaments of the cosmic web located around active galaxies in a proto-cluster

Cosmological simulations predict the Universe contains a network of intergalactic gas filaments, within which galaxies form and evolve. However, the faintness of any emission from these filaments has limited tests of this prediction. We report the detection of rest-frame ultraviolet Lyman-alpha radiation from multiple filaments extending more than one megaparsec between galaxies within the SSA 22 proto-cluster at a redshift of 3.1.

It's not a feature, it's a bug

A couple of weeks ago I mentioned a paper that tries to quantify how weird galaxies without dark matter are in the latest, cutting-edge simulations. The properties of galaxies which look like this in reality depend strongly on their distance, so the authors took the nice approach of quantifying how rare such objects are depending on their true distance. If they're close, then their peculiar velocity would be unusual but their dark matter content would be normal; if they're far away, then their velocities would be normal but their dark matter content would be unusually low.

What annoyed me was that they found objects in the simulations matching those criteria, but didn't describe how such objects (lacking dark matter or moving at weirdly high velocities) form in the simulations. If their formation strongly depends on environment, then the global numbers for how rare they are might be woefully misleading. At least one of those authors has a nasty habit of doing that.

The authors in the current paper, however, have indeed gone and found dark matter deficient galaxies (they do not consider the possibility of strong peculiar velocities here) in simulations and looked at them very, very carefully. And it turns out that there is a strong environmental dependence - but it's a bug, not a feature.

You can't simulate the entire visible Universe, and even if you could, you'd still have to make assumptions about its edge. The standard approach is that the boundary conditions are periodic, so that a galaxy which happens to exit* on one side simply re-appears with the same velocity on the other side. And all of the identified "oddball" galaxies without dark matter are found very close to the edge of the simulation.

* Pursued by a bear ?

Given that the near-edge volume in question is a tiny fraction (0.1%) of the whole simulation, that's already a massive red flag. But it's not quite enough to say for sure if this is the cause. For these weirdos share another common feature of experiencing recent mergers. Although galaxy mergers are common events, it's possible that the galaxy-finding algorithms could occasionally get very confused and start misidentifying which particles belong to which galaxy. So the galaxy could only appear to lose mass because its associated dark matter has been wrongly identified.

But that's not the case. When they looked at the oddballs in detail, they found that as they approached the edge of the simulation they slowed to a crawl, and they do indeed lose much of their dark matter. They're still not sure what causes the bug, as plenty of galaxies cross the boundaries without incident, but it appears to be related to the galaxies having unusually dense inner cores where acceleration is high. It's highly unlikely that this bug has caused any other problems - it's only affected a handful of galaxies out of many thousands - but a bug it very much is.

What does this mean for the earlier paper ? Well, if all simulated galaxies without dark matter are actually due to numerical artifacts, then it strengthens their claim that they're incompatible with the standard model. But this is by no means clear : selection criteria may mean the two teams have identified completely different objects. Since the current team's criteria are quite strict, there could still be dark matter deficient galaxies in the simulations which are a feature, not a bug. And it should also be a cautionary note that the simulations are not yet the be-all and end-all of the standard model of cosmology.

Isolated dark matter deprived galaxies in hydrodynamical simulations: real objects or artefacts?

We searched for isolated dark matter deprived galaxies within several state-of-the-art hydrodynamical simulations: Illustris, IllustrisTNG, EAGLE, and Horizon-AGN and found a handful of promising objects in all except Horizon-AGN. While our initial goal was to study their properties and evolution, we quickly noticed that all of them were located at the edge of their respective simulation boxes.