A lack of midgets

Ah, another paper on everyone's favourite problem with local cosmology : the chronic lack of dwarf galaxies. By rights, the Local Group should be more crowded with dwarfs than a Smurf convention or the mines of Moria in their heyday. There ought, say simulations, to be hundreds and hundreds of the little hairy sparkly buggers running around orbiting the Milky Way, but there aren't.

An ever-present question as to the significance of this is whether the Milky Way has been visited by the galactic equivalent of Gargamel or if all galaxies have this same problem. Are they all missing their expected dwarfs, or did something peculiar happen to the Milky Way ? Without knowing what things are like for galaxies in general, any purported explanation is largely speculative.

This paper extends this to the nearby spiral M101. At 6 Mpc, it's well within the Local Volume but well outside the Local Group. So if the dwarf-killer is peculiar to our most local environment, M101 shouldn't be affected. From a previous survey they identified potential candidate satellite galaxies of M101, and here they try and establish the distance to the four faintest objects using Hubble.

All four objects appear to be distant background galaxies. Known satellite galaxies at this distance are resolved into individual stars by Hubble observations, but these ones remain stubbornly diffuse. They even simulate what the colour-magnitude diagram for these objects should look like if they were as close as M101, and it's clearly different.

While they don't rule out that future surveys might change their results, since completeness at these very faint magnitudes is always a problem, for now it looks like M101 has an even worse problem than the Milky Way. If the Milky Way doesn't have enough dwarfs, then M101 is positively racist. They also show a comparison with other nearby galaxies where it's possible to measure dwarf abundances with some accuracy (the Milky Way, Andromeda, M94, M81, and Centaurus A). All paint much the same picture.

Wisely, they don't speculate or comment on the astrophysical significance of this. So I'll do their dirty work and point out that this does not constitute any kind of crisis or catastrophe for the standard model. First, we knew this was a problem for the Milky Way anyway, and second, virtually all so-called "problems" for the dark matter paradigm are nothing of the sort : they're problems for the baryonic physics. If Gargamel can't be everywhere at once, it's perfectly plausible that every galaxy has its own Gargamel - a universal property of galaxy formation that prevents star formation in the smallest dark matter halos. Yes, it could also be that the whole dang model is wrong, but much more likely we just don't understand the complicated lives of those pesky little dwarfs. We'll just have to wait and see.

The Satellite Luminosity Function of M101 into the Ultra-Faint Dwarf Galaxy Regime

We have obtained deep Hubble Space Telescope (HST) imaging of four faint and ultra-faint dwarf galaxy candidates in the vicinity of M101 - Dw21, Dw22, Dw23 and Dw35, originally discovered by Bennet et al. (2017).