What about all those recent ultra diffuse galaxies ? If we want to know if they're dwarfs or giants, which will affect theories of galaxy formation, we'll need to know if they're spinning or supported by dispersion. This paper looks at a sample from simulations.
The paper gives a very good overview of the current state of play for UDGs, noting the interesting shenanigans going on with deviations from the Tully Fisher relation, the diversity of the sample, the different ways to measure their dimensions, and the different formation mechanisms proposed. Essentially there are two main ideas : they could form due to internal processes, or they could form in clusters (where most UDGs have been found) due to unique environmental processes. The later doesn't seem likely, or at least it certainly isn't sufficient, since we now know of isolated UDGs that have never even see a cluster.
The one major thing I don't like about this cluster is that they're unclear about their simulations and sample selection. I don't even like reading these bits of simulation papers and I normally just skim them, but even I found this too brief. It's important to know what the mass resolution is and what they mean when they say they selected an "isolated" sample - how you define isolation is basically arbitrary. They don't even say how many galaxies were in their simulation in total or how large a cosmological volume they simulated. Sure, I could consult the original simulation papers, but I shouldn't have to : these parameters are fundamental to this paper.
That aside, they find that almost exactly half of their sample of 38 isolated UDGs are dispersion dominated and the other half rotation supported. They tend to be gas rich (though "rich-most" is an ugly phrase indeed), though dispersion dominated galaxies are more gas poor. This is partly a selection effect, in that rotation is better at supporting more extended systems at any given mass. Also, their objects are not part of the high-spin tail of normal galaxies, as one of the most popular early papers claimed.
Interestingly, at these masses, supernovae feedback is expected to be highly efficient, but they find that this plays only a secondary role in determining whether a UDG is rotation or dispersion supported. The major driver appears to be how the gas accretes. If it happens to align with the existing rotation of a proto-galaxy, the accreting gas helps increase the rotation. This lets it maintain a steady accretion of gas without any destructive starbursts. If, however, the accreting gas is misaligned, then chaotic accretion leads to bursts of star formation and supernovae feedback that quickly reduces any further gas inflow. So rotationally supported systems tend to have later gas accumulation.
Prior to the lockdown I was writing a paper on gas observations of UDGs. Although the deviation from the Tully-Fisher of some objects is weird and interesting, quite a few don't show this : they look like normal rotating discs. So this paper makes a lot of sense to me, but I would have liked more information. How extended are these galaxies using different radius measurements ? What's their typical star formation rate - do they have much molecular gas ? How are they still accumulating gas when they're in isolation ? On the other hand, if they'd given all that I might have dismissed the paper for being too long and not read it at all...
NIHAO XXIV: Rotation or pressure supported systems? Simulated Ultra Diffuse Galaxies show a broad distribution in their stellar kinematics
In recent years a new window on galaxy evolution opened, thanks to the increasing discovery of galaxies with a low surface brightness, such as Ultra Diffuse Galaxies (UDGs). The formation mechanism of these systems is still a much debated question, and so are their kinematical properties.
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