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.
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