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I continue archiving my G+ posts in the (hopefully !) more permanent blogger format. I've added a bunch of posts here from the Slightly Social Scientists collection, limited to those which are somewhat science-based. Most of the additions are under the deliberately generic "Status updates" label :
https://llittlephysicists.blogspot.com/search/label/Status%20updates

I also collated my IRAM observing trip posts into a single thread :
https://llittlephysicists.blogspot.com/2017/12/at-iram-30-m-telescope.html

I believe that should cover the majority of posts which included my own images. So the remaining updates (for this one that will be Philosophy of Science posts) will mainly consist of selecting which ones are relevant and replacing any links to G+ posts. It's a lot of work, but not too awful so far...

A reminder of where you can find me in a post-Google+ world :
https://wheretofind.me/@Rhysy
Little Physicists
Little Physicists
llittlephysicists.blogspot.com
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First new post on Little Physicists : over 13 years of observations now complete.
AGES : The End Of The Beginning
AGES : The End Of The Beginning
llittlephysicists.blogspot.com
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NAZI FARMER MILFS IN SPAAAAACE !

Because sometimes you have to grab Godwin's Law and kick it inna ribs. This is my take on a series of papers (in which I was involved, and third author on the latest) on breeding and farming aboard a multi-generational spaceship. Also features space cows.
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How to kill a galaxy in your web browser

Ram pressure stripping explained with torture and bees ! And I managed to keep the length down to something short enough to actually READ !

... but seriously, why am I still gaining followers ? Look, I know this place is lovely and has the best interface ever, and you don't have to go home, but you can't stay here.
Ram Pressure Stripping Made Easy
Ram Pressure Stripping Made Easy
astrorhysy.blogspot.com
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Ask An Astronomer Anything At All About Astronomy, Part 48

The last hurAAAAAAAA, I guess... in all likelihood, this will be my last proper post on Google Plus. With notifications well and truly broken, there's just no point in starting anything that deserves a detailed discussion. So here's the latest batch of collected astronomy Q&A's from social media. This week featuring aliens, more aliens, the Chinese and a journey into the deepest void.

From here on in, all subsequent posts are probably going to be about migration-only, except possibly for the occasional (arguably) hilarious meme. I'll still be here - this account will be open until the bitter end, and I'll still be checking it regularly. But I won't be starting much in the way of new posts myself. Maybe something when the very final end is near.

I'm going to start signing up to alternative social media sources shortly. You can already find me answering astronomy questions on Quora : https://www.quora.com/profile/Rhys-Taylor-20
And, as you may already know, there's a pinned post in my profile about where else to find me. I'll be posting updates every week or so with reminders, since I know not everyone is very active.

This definitely isn't the end of the AAAAAAAAs, but it is the final one for Google Plus. So please enjoy this final bunch of questions and sarcastic answers !
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If anyone's got a subscription to "Room", you can read about multi-generational spaceships here.
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Extreme chemistry in an extreme galaxy

Plus seems to be approaching its hitherto bizarre description of a ghost town ever more rapidly... I'm still here for now. Sometime in the next few weeks I'll migrate to Pluspora. I don't really want to go through a double-posting phase. I'd rather export all my G+ posts and be done with it, rather than having to continually make backups. There's quite a bit of content that currently exists only here that I'd rather not lose. I did a backup some months ago and I can't be bothered doing this continuously. Still haven't decided if I'll move to other social networks as well yet. We'll see.

Anyway, I normally steer clear of papers on astrochemistry because even plain atomic hydrogen is complicated enough. I made an exception for this one, though, because a) it's short and b) it's about an ultra diffuse galaxy.

UDGs are hard to measure because they're big and faint. Even getting their total mass is still extremely difficult, so measuring their chemical composition isn't much fun. But these guys seem to have managed it, using the giant Keck telescope and a fancy Integrated Field Unit. Traditional spectroscopy - which gives you the chemical composition and velocity information - would give you measurements only at a single point or along a slit, but IFUs give you spectral information at every pixel. 3D optical data cubes, because science.

This also lets them measure the kinematics. This particular UDG has a velocity dispersion of 56 km/s. For comparison the Milky Way has a rotation speed of ~220 km/s. Speed is a good proxy for total mass (i.e. how much dark matter is present) but it also depends on where you make that measurement. For disc galaxies, we can use the gas to probe regions far outside the stellar disc. We can't do that for UDGs - at least not this one - so we can't really get a good estimate of its total mass (there are methods of extrapolating, but the authors didn't try any). What they do show, however, is that the mass of dark matter within the measurable region is much, much higher than for more typical galaxies. It's way off the usual relation. If I were less cautious, I'd say that indicates it might be a very massive object indeed, but the authors (probably wisely) don't comment.

Chemically the galaxy is odd too. It seems to have had a prolonged duration of star formation lasting about 10 Gyr (don't ask me how they measure this). Given all the expected supernovae, that should make it enriched in iron - but it isn't. In fact it's magnesium-iron ratio is way, way off, even compared to other UDGs.

How could this be ? It may depends on the formation of the galaxy and when the different types of supernovae exploded. Early supernovae (from short-lived massive stars) may have blasted most of the galaxy's gas out into intergalactic space. This could also remove part of the dark matter through a "gravity tractor" since the mass of the gas could be initially very high. Whatever gas was left would have been rich in magnesium. Then later on, supernovae from accreting material in binary star systems (which take much longer) would have exploded, but since the mass of the galaxy was now much less, most of their iron-rich ejecta would have escaped.

That's their best guess for now, at any rate. But we're still very ignorant of even the basic properties of UDGs. Other exotic possibilities like continuous accretion of gas might be possible; it's also hard to see how the supposed loss of dark matter can be reconciled with its apparently heavily dark matter-dominated nature. Further research is very definitely needed.
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SPACE NAZIS BECAUSE SPACE NAZIS

I dunno why it took 7 weeks to appear on arXiv after acceptance, but it did. Will blog this WITH OH GOD SO MANY NAZI REFERENCES once I finish blogging the last paper, but it's a pretty accessible read.

In the first papers of our series on interstellar generation ships we have demonstrated that the numerical code HERITAGE is able to calculate the success rate of multi-generational space missions. Thanks to the social and breeding constraints we examined, a multi-generational crew can safely reach an exoplanet after centuries of deep space travel without risks of consanguinity or genetic disorders. We now turn to addressing an equally important question : how to feed the crew? Dried food stocks are not a viable option due to the deterioration of vitamins with time and the tremendous quantities that would be required for long-term storage. The best option relies on farming aboard the spaceship. Using an updated version of HERITAGE that now accounts for age-dependent biological characteristics such as height and weight, and features related to the varying number of colonists, such as infertility, pregnancy and miscarriage rates, we can estimate the annual caloric requirements aboard using the Harris-Benedict principle. By comparing those numbers with conventional and modern farming techniques we are able to predict the size of artificial land to be allocated in the vessel for agricultural purposes. We find that, for an heterogeneous crew of 500 people living on an omnivorous, balanced diet, 0.45 km2 of artificial land would suffice in order to grow all the necessary food using a combination of aeroponics (for fruits, vegetables, starch, sugar, and oil) and conventional farming (for meat, fish, dairy, and honey).
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Two's Company : A Second Galaxy Without Dark Matter

A short letter submitted to ApJ by the same team who brought you the first galaxy without dark matter. This second discovery is very similar to the first : it's in the same group of galaxies, at the same distance, has a low surface brightness, is very extended, and has a similar smooth and boring-looking morphology. Its all-important velocity dispersion is even lower than the first, at a mere 6 km/s compared to 8-10 km/s. Which is again consistent with its dynamics being completely dominated by its stellar mass, with little or no additional dark matter needed.

This is very strange indeed. During tidal encounters between galaxies, it's possible for the gravity to tear off enough material to form a brand new (low mass) galaxy without its own dark matter component - that's been known for ages. But such a process ought to be messy. It shouldn't be able to form big, smooth objects that move very slowly. There ought to be debris all over the place : stellar streams and other weird-looking structures. It just shouldn't be able to make anything that looks this damn boring. Well, not quite true : after a good long while things should settle down and most of the crazier stuff ought to disperse, but to form very smooth things like these galaxies should take a very, very long time indeed because their motions are so low. And galaxies produced by this mechanism are chemically different to other galaxies, whereas this one isn't.

Could this just be a normal (though faint) galaxy observed close to face-on where we wouldn't be able to detect any rotation ? It was possible with one object, but that becomes highly unlikely with two - where are all the faint edge-on galaxies, eh ? Similarly, while tidal encounters can act to strip away large amounts of dark matter from ordinary galaxies, it seems incredibly unlikely that we'd find two such objects in a group without the expected tidal debris.

The other weird feature of these objects is that they have a large number of globular clusters given their low stellar mass. What the connection might be with the lack of dark matter is anyone's guess, but it does make it even more likely that they're part of a distinct population rather than being weird but rare exotica. Having a second object changes the picture considerably, but we need many more objects to have any kind of statistical view.

What I'm a bit surprised at is that no-one is talking much about the other (rather large !) population of objects which are known to show unusually low velocity dispersion from their gas measurements : ultra-diffuse galaxies (https://plus.google.com/u/0/+RhysTaylorRhysy/posts/V6mMZo57hj3). I'm going to e-mail van Dokkum about that because it seems like something that should be mentioned more in the literature somewhere.
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Missing Matter Still Even More Missing, Study Finds

Last year a galaxy that seemed to have no dark matter was doing the rounds because that's freakin' weird. Virtually all galaxies appear to be heavily mass dominated by dark matter : it's arguably the best way to define a galaxy as opposed to a giant star cluster. The whole mainstream basis of galaxy formation and evolutionary theory depends on dark matter as an integral feature. While there are some cases of dwarf galaxies formed by tidal encounters that don't have much (or any) dark matter, these tend to be still embedded in the debris associated with their formation. As far as I know there are no good cases of a dark matter free galaxy just sitting there minding its own business.

If such an object were to be found, it would raise awkward questions for the standard theories of galaxy evolution but make life even more difficult for the main alternative : modified gravity. Tidal encounters between galaxies can strip away dark matter, so it's at least possible to reduce the dark matter content in standard theories (but remove it completely ? I doubt it). For modified gravity, on the other hand, any two star systems of the same size and shape ought to have the same dynamics : gravity should work the same everywhere, more or less. A nice control test where one can compare similar objects is not so easy to find as you might think, but such systems have been found - and the results don't look good for modified gravity :
https://plus.google.com/u/0/+RhysTaylorRhysy/posts/DCV3csgBMAQ

Then there is this galaxy, NGC1052-DF2. This is an ultra-diffuse galaxy, meaning it's very large but with few stars per unit area. That makes it difficult to measure how fast its stars are moving, which is what you need to work out its total mass. So previously astronomers used its globular clusters, which are much brighter and easier to measure (though there are only a few of them). They found a velocity dispersion of 8-10 km/s - so low that it's consistent with the galaxy having no dark matter at all.

This made a lot of people very upset. Claims were made that the distance measurements were wrong and that would mean the galaxy was perfectly normal, but then an independent team came along and said nope, the distance measurements are correct, this galaxy really is weird.

Still, having only 10 globular clusters has always raised concerns that the estimate of the velocity resolution is reliant on small number statistics. Other teams have questioned the rigour of the claim for such a low dispersion, though in my opinion the original van Dokkum claim always looked stronger. Now, two teams have used extremely powerful instruments to measure the velocity dispersion of the stars directly. Note that both papers are still under review.

The first paper was by an independent group and came out just before Christmas :
http://adsabs.harvard.edu/abs/2018arXiv181207345E

I read this, but didn't bother writing about it because I found it rather badly-worded : I could not easily extract the main point about just how massive the galaxy is supposed to be. Fortunately the new paper (by the original team) is much more clearly written and comments on the Emsellem work.

The bottom line is that this galaxy does indeed seem to have very little or possibly no dark matter whatsoever. This is in conflict with the Emsellem claim in two ways : first, Emsellem claimed that the velocity dispersion could be much higher (13-27 km/s), whereas this paper says it's 8 km/s (just as the original globular cluster measurements indicated); second, Emsellem claimed the galaxy is rotating (albeit slowly) whereas this paper finds no evidence of that. The authors comment directly on the disagreement , noting that they aren't able to explain it. The only hint is that this latest study has a much greater velocity resolution than the Emesllem paper so it should be more accurate. And their fitted velocity dispersion profiles do seem to match the data extremely well.

As for how well this galaxy does or doesn't fit with modified gravity, as usual there's the complication of the external field effect. In modified dynamics, the presence of nearby galaxy can change the velocity dispersion in a very different way to conventional theories. Based on this, the earlier prediction was that the dispersion should be 13 km/s. This is not consistent with the new results, and only just about consistent with the Emsellem range which is actually more favourable to the galaxy having some dark matter than the modified gravity prediction.

I would expect a great deal of back-and-forth on this issue. My money's on the original van Dokkum team. Though a very strange result, it does seem to stand up to scrutiny so far. Watch this space.
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