A long time to say "I told you so"

Back in 1997, Raul Jimenez predicted that some galaxies could be completely optically dark, or nearly so. Such a "dark galaxy" would be a big disc of hydrogen spinning serenely inside a dark matter halo, but with such a low density that star formation was at worst an occasional irritation, and at best averted completely, rather than the more typical state of galaxies as star formation factories that are good for only one thing. Here, with the assistance of some delightful nominative determinism of Alan Heavens, he revisits his predictions.

I can't claim to fully understand the details of modelling, but the gist of it is simple enough. Atomic hydrogen needs to reach a certain density before its self-gravity reaches the point where it overwhelms any other counteracting forces and collapses into stars. One such counteracting force is the thermal temperature of the gas, generally around 10,000 K, equivalent to random motions of 10 km/s. Another is the spin, which keeps the gas on stable circular orbits, and this can be much larger - up to ~500 km/s in extreme cases. The "spin parameter" is a measure of how much of a role the rotation plays in preventing collapse. It's a bit more complicated than ordinary rotation, but the two are roughly equivalent : the faster the rotation, the harder it will be for gas to collapse and form stars.

Twenty-three years ago, Jimenez predicted the value of the spin parameter above which galaxies ought to remain entirely dark. Back then detecting the gas of such galaxies was a formidable challenge indeed, but two decades of improvements have allowed his predictions to be tested. In particular, the gas-rich "Ultra Diffuse Galaxies" from the ALFALFA survey are natural targets for comparison : they have a few stars, but far less than typical galaxies of comparable size.

Jimenez finds that these UDGs are indeed well-described by the high-spin tail model, where the most extreme spin-dominated galaxies remain optically dark in the conventional cold dark matter scenario. The number of discoveries (22) is in excellent agreement with the prediction (24). They're also in good quantitative agreement for his prediction of just how bright - allowing "dark" to be a synonym for "very dim" - they should be and what colours they should have. He says that ALFALFA has probably detected nearly all the largest dark halos and that only a survey of an even larger volume would detect any more. In contrast, detecting less massive galaxies requires a more sensitive survey.

What does this mean for my beloved clouds in the much deeper AGES survey ? I'm not sure yet, but the spin parameter would be interesting to compare. Those clouds are totally optically dark, but one of the main doubts about their galaxian nature is that we only found them in the Virgo cluster... on the other hand, Virgo is uniquely dense so this would be the best place to detect rare objects.

Even more interesting would be what this means for the very smallest objects, i.e. the missing satellite problem that prompted the whole "dark galaxy" thing in the first place. Jimenez doesn't describe this, or the other predictions of dark galaxies that were in vogue at the time. Still, this is potentially a super-interesting result. My main question is : what about all those other UDGs that don't have gas ? How do they fit into the model, and why don't more of them have gas ? But after twenty years, this is probably interesting enough already.

The distribution of dark galaxies and spin bias

In the light of the discovery of numerous (almost) dark galaxies from the ALFALAFA and LITTLE THINGS surveys, we revisit the predictions of Jimenez et al. 1997, based on the Toomre stability of rapidly-spinning gas disks. We have updated the predictions for $Λ$CDM with parameters given by Planck18, computing the expected number densities of dark objects, and their spin parameter and mass distributions.