Dramatic instructions

I'm reviewing a paper for the first time. One of the instructions is a tad dramatic for my liking :

7. If qualified historians wish to use your report, we will ask you or your heirs. If you wish us to destroy your report, then please inform us.

Space Nazi Paper I

The author (my former housemate) is an expert on X-ray polarimetry and AGN, so why he's decided to do this I have no idea. It's quite fun though. I had a not inconsiderable hand in correcting the text prior to submission.

Basically he's written a code to try and estimate whether a population could survive in isolation given the effects of inbreeding and overpopulation, here applied to a multi-generational interstellar mission. The clever bit, as I understand it, is tracking the histories of each individual to calculate the degree of inbreeding. The deleterious effects of inbreeding are applied quite crudely, reducing lifespan by some value if above some threshold (neither of which are well-known, but can be adjusted). It can also apply an artificial breeding programme, in the sense of forbidding breeding between individuals who are too closely related.

Here he investigates three possibilities : 1) A small population (150) who are insanely horny and don't care a jot about food supplies, incest, or have any common sense - unsurprisingly, this doesn't work very well; 2) A small population where the number of births is tightly controlled but no other regulations are applied - this works much better but it's not great; 3) A large population (14,000) with the same simple population controls, which succeeds.

Oddly, he doesn't actually use the code's capability of limiting inbreeding. He mentioned something about that needing improvements, but I never found out what exactly. Also the populations of both the small and large (controlled) populations show an initial increase followed by a slow, prolonged decrease of a very similar pattern over time. I'm not sure why that should be, but it's worrying. I also never really understood why the populations are given such restricted initial age ranges (a crew entirely of twentysomethings running a starship ? hmmm...) - that just leaves it more vulnerable to disasters.

What I hope will be tackled with the second paper would be an attempt to find the smallest number that could survive the mission given sensible population controls and natural breeding restrictions (e.g. Jamie and Cersei aside, hardly anyone is going to fool around with their siblings). Of course in a real interstellar mission you'd just take along frozen genetic material and eliminate inbreeding with arbitrarily small populations, but it would be interesting to find the point at which that's not necessary.

I believe the intention is to make the code public (surely this could be applied to the Sims ?) but this isn't the case yet. My guess would be it's not quite in a sufficiently clean state for public distribution.
https://arxiv.org/abs/1708.08649

Astronomy is too big to fail

Alas the title, "A new astrophysical solution to the Too Big To Fail problem" does not refer to astronomers finding a way to support the banking sector, which would have been more unexpected...

The "Too Big To Fail Problem" is a variant of the "missing dwarf galaxy" problem, where there aren't as many dwarf galaxies detected as expected from simulations. Specifically, the most massive dwarf galaxies should be so large that there's no way - according to the simulations - that their dark matter "halos" can avoid attracting enough gas to start forming stars. It's not just that we're not finding many of the really small dwarfs (that's a problem too), it's that this is a problem even for the bigger guys. It only stops when you get to really massive galaxies.

Supernovae explosions and other processes might be able to explain why the littlest galaxies don't form any stars - a few big blasts could send the remaining gas off into the void, never to be seen again. So they form just a few stars, which eventually die off and thus we can't detect the smallest dwarfs any more. But this explanation shouldn't be possible for the largest dwarfs, which are so massive they ought to be able to hold on to their gas more strongly.

This latest paper uses a bunch of more advanced simulations with better resolution and (they think) treatment of the various processes at work. They make "synthetic observations" of their simulations so they can directly compare their results to real observations. They find that the observations would be underestimating the mass of the dark matter present. Although supernovae can't blast the gas out of the largest dwarfs, it can still cause disruption - the nice thin disc that we see in larger galaxies, and we assume is present in dwarves, gets "puffed up" and becomes substantially thicker. The gas is prevented from collapsing into stars both by its rotation and by this pressure support provided by the various feedback processes from the stars.

This means that the gas discs in the largest dwarves don't extend as far as we expect them to, so their rotation underestimates their true mass. So the dwarf galaxies we detect are actually quite a bit more massive than we've been estimating, if this scenario is correct. They haven't failed, they just aren't quite what we expected from a simpler assumption.

It's a rather technical paper, and personally I think they could have explained their results in a simpler way. I don't think they explain their various scaling relations in a very clear way, making it hard to check if their interpretation of their simulations is sensible. Also (although this isn't crucial), their sample is currently rather small. It would be useful to include more galaxies at different masses to see how well the gas traces the true rotation for other objects.
http://adsabs.harvard.edu/abs/2017arXiv170303810V

I won a thing !!!

I have an email, so I guess it's official now...

Dear Rhys,
We have received an information from the Head Office of the Czech Academy of Sciences saying that you are selected for the prize for young researchers, which our Institute proposed some time ago.
There will be a ceremony in Villa Lanna on 4th October 2017. Please save the date!
An official letter will follow.
Congratulations,

It comes with the glamorous title : "The Award of the CAS for young scientific employees for outstanding results of scientific work, achieved with the financial support of the CAS before reaching the age of 35."
I should get that on a badge. This is a general CAS award, not even limited to astronomy...

As far as I can tell, I think I might be the first non-Czech/Slovak winner...
http://www.avcr.cz/en/about-us/awards/prizes-of-the-cas/

Villa Lana is also super-shiny :
http://www.vila-lanna.cz/

Excuse me, I'm off to drink enough tea to shrink my head back down to its usual size...
http://www.avcr.cz/en/about-us/awards/prizes-of-the-cas/

A lot of knowledge is a dangerous thing

Yesterday I removed from a certain community's spam folder a post consisting of 94 slides about some sterotypical pseudoscience theory. The usual stuff : "Let us begin with E = mc^2..."
No. Let's not do that, because it's obvious that you haven't got a clue what you're talking about. Lo and behold, it got rapidly worse, with some boiler-plate text praising God at the top of each slide. Aaaarrgggh. If you want to praise God, then fine, but do it in your own time, mate. Not in an internet-based slideshow you apparently expect people to read.

Total ignorance is indeed never damaging, providing one understands that one is totally ignorant and not put in a position where expertise might be required. A little knowledge is a dangerous thing as well. But a lot of knowledge without proper understanding is much, much worse.

The most useless law in nature ?

In which I summarise the current debate over whether this apparent "new law of nature" means anything or not, and conclude (spoiler !) that it probably doesn't.

It's been known for many years that there's a correlation between how fast matter in a galaxy rotates and how much gas and stars it contains. The problem is that there shouldn't be a nice relation, because galaxies seem to be dominated by dark matter. This strange relationship between normal and dark matter, which should be independent of each other, has been shown in different ways over the years. Recently it was claimed that all of these are just manifestations of a deeper underlying "law" : the Mass Discrepancy Acceleration Relation (or technically the Radial Acceleration Relation, but whatever).

The problem is that the expected acceleration of matter (based on its mass and standard Newtonian gravity) correlates very, very well with its actual acceleration. If there is, as we think for many other reasons, actually a huge amount of unseen "dark matter" present, then it shouldn't do that. Could it be that the dark matter theory is wrong ?

YES ! But it probably isn't. Although modified theories of gravity do predict this relation - indeed, did predict it 30 years ago - it seems that it also occurs in standard dark matter simulations without any difficulties. You can see a comparison of observations (left) and the simulations (right) in the example image below. Since both modifying gravity and using dark matter give the same result, this discovery is probably useless. Weird, but useless.

Read on for tales of academics behaving badly, pretty pictures of galaxies, lots of and lots of graphs, and a surprised dog...

Placeholder post intended to be replaced with a slightly better summary.

Ghostly giants versus huge dwarves - the battle continues

Ghostly giants versus huge dwarves - the battle continues

Ultra-diffuse galaxies are currently the trendiest thing in extragalactic astronomy. The same size as the Milky Way but a thousand times fainter, these things may be extremely boring to look at but they break all the rules of galaxy formation. Or do they ? Two years after first hitting the headlines, I review the current state of the art on the most important question : how massive are they ? Are they actually just huge dwarves or are they genuinely ghostly giants ? What have we really learned so far and where are we going ?

With star-studded cast of a giant squid, Father Ted, Gimli, Ghostbusters and for some reason James Corden...

Placeholder post intended to be replaced with a better summary.

Yet another Dark Matter versus MOND paper

Well... partially. Mainly this paper is about whether the standard dark matter model can explain some of the recently discovered "ultra faint" dwarf galaxies in the Local Group. Cold Dark Matter (CDM) models predict that really small dark matter halos, which have low rotational speeds, shouldn't be able to form any stars at all. They shouldn't have enough mass to pull in enough gas to form stars... but that's just what these new galaxies seem to have done.

In this paper, the authors use new simulations with much higher resolution and more complex physics than previous efforts. They show that similar objects can form through tidal stripping (i.e. gravitational effects), which can remove large fractions of the dark matter and stars but, interestingly, doesn't change the size of the galaxy. So apparently weird new objects (like the "giant" Crater II) may make sense after all.

They also show more quantitatively that the main objections to this idea aren't as fatal as might be thought. First, tidal stripping is thought to be very disruptive, with stars and gas and dark matter flying all over the place. And it is... but not for very long. The damage is done only when the dwarf galaxy flies closest to its bigger, scarier neighbour. The rest of the time the tidal effects don't really do a lot, and afterwards the galaxy can settle down into a nice round shape, like those that are observed. In essence it's like that bit in Jaws where the decapitated head suddenly appears - everyone jumps, but you don't go running off down the street screaming your head off for ten minutes afterwards. Usually.

The second objection is that these tidally-disrupted galaxies should only be found near their larger neighbours. But because the scary moment of disruption is short-lived, the galaxy can continue on its merry way back out to the hinterlands. So it's no surprise that the observed galaxies are found quite far away from the Milky Way.

All well and good, but now we reach the controversial stuff....

First, there were several recent papers claiming the discovery of a "new law of nature", where the gravity from dark matter appeared to be closely correlated with the gravity from normal matter. Which seemed a bit odd, but then there were several other papers which said, "yeah, but we see this is dark matter simulations anyway, shut your ugly face, it's not very interesting." Or words to that effect.

This paper shows that the scatter in this "mass discrepancy acceleration relation" is much higher for faint galaxies than was previously reported. They also claim that their simulations explain this. I think this is true, but the agreement is not that great. It's OK, but not particularly impressive. More interesting is simply the claim about the size of the scatter, which I find fairly convincing. Our resident tame MOND expert says that the original papers already discussed this, but I don't think this is true. However, they do mention that they only use the highest-quality data available. A better objection might be whether these new galaxies are rotating at all or if the stars are just moving on random orbits - this might not change the conclusions, but it should be discussed.

The second controversial point was the claim that they have discovered something which poses a "possibly insurmountable challenge" to Modified Newtonian Dynamics, the main alternative to dark matter. Strong words. Strong, angry words - which have been used before against MOND... but with admirable if cockroach-like tenacity the bloody thing keeps coming back.

MOND has this funny thing called the "external field effect". If there's another galaxy nearby, acceleration in a smaller galaxy reverts back to standard Newtonian behaviour. When you account for this, it seems that the velocities in the faint galaxies are in strong disagreement with MOND's predictions (they're also in strong disagreement even if you ignore it).

The tame MOND expert agrees with the method used, but disputes the conclusion. He's probably right that the conclusion is too strong, although personally I find claims of "a new law of nature" to be far worse on that front. His objection is that the fraction of binary stars in a galaxy can change the overall measured velocity dispersion, or it could be that even those the galaxies aren't in the process of being tidally disrupted right now, they're still out of equilibrium. It's possible, but I'm not convinced this can really explain the strong systematic offset seen in the data. It needs to be quantified though.

The paper concludes by worrying that this model requires some pretty dramatic tidal disruption to explain the galaxies - they've have to have lost 99% of their original mass (whether this is common in their simulations is not clear). But with measurements of the 3D velocities of the galaxies it would be possible to trace their orbits back in time and work out whether they really have experienced close encounters with the giant galaxies of the Local Group. So, as usual, watch this space.
https://arxiv.org/abs/1707.03898

There... are... THREE... clouds !

Thanks everyone who voted on yesterday's poll (https://plus.google.com/u/0/+RhysTaylorRhysy/posts/h4ePJaDsHXf). The final score was :
1 group : 0%
2 groups : 7%
3 groups : 71 %
4 groups : 16%
More : 7%

... and the correct answer is.... there are FOUR LIGHTS three groups !

This diagram shows the location of galaxies in a particularly complicated part of the Virgo cluster, where, in addition to the main cluster, it's believed there are two other infalling sub-groups. Distance measurements to individual galaxies are hard but velocity (redshift) measurements are much easier. The distance measurements which we do have indicate that these three different groups are at different distances. It's also possible to see this using the velocity measurements, with each group centred on a different overall velocity (though with lots of scatter).

A colleague of mine disputed whether you'd really pick out three distinct groups without already knowing the distances. I thought it looked clear enough, but heck, this is the age of social media so why not test it ? The diagram I showed you yesterday was the same as the one below, just stripped of all distance information and rotated to a random angle. I deliberately gave you absolutely no information on the problem and didn't specify what size features count as significant - I wanted to see what you'd naturally guess without over-thinking what counts as a group.

Turns out the answer's three. Since I told you absolutely nothing, that makes the conclusion much stronger - not only can you pick out the three groups, but that's what most people naturally actually do. So you definitely don't need the distance measurements to realise there are three distinct groups here.

Clusters are messy places, so it's completely understandable that you might think there are more than three groups from this limited information. Even with the distance assignments, you can see that some objects at similar velocities apparently below to different groups. That's probably not actually the case - more likely, the distance assignments just didn't take this velocity information into account. There's clearly a lot of scatter in all cases - these aren't nice spheroidalish distributions - this is the real universe, messy and ugly and with galaxies lying in awkward locations. It's quite possible that a few of them do lie at quite different distances to the others; whether they would constitute separate groups or not is another matter (though not a terribly interesting one).

The busy life of an astronomer

EWASS is over and I'm still alive; the 1200-strong horde of barbarous astronomers has been sent back to the hellish netherworld from whence they came. I finally booked my summer holiday to see the total solar eclipse from Grand Teton (hello darkness, bye bye money - also, Expedia seems designed to make the whole process as nerve-inducing as watching a horror movie while receiving random electric shocks). My flat has been restored to something approaching normality. Today we have a scientific visitor hanging around after EWASS. Tonight/early tomorrow morning a friend arrives for a week of holiday and on Wednesday I'm giving a 90 minute public talk.

Wheeee.....