Thread by @MBKplus

OK, my people, time for the tweet storm you neither need nor deserve (nor want?), but that you’ll get anyway. It’s about the paper by T. Kelley (@UCIastro) out today on the arXiv; arxiv.org/abs/1811.12413

Once upon a time (~1999), the dark matter satellites expected to orbit around the Milky Way (MW) were missing.

Only 11 satellite galaxies were known, whereas simulations (Klypin ++, Moore ++) predicted 10-1000x more dark matter subhalos, depending on how one compared masses

Already by 2000, a likely candidate for their absence was proposed by @jbprime ++: cosmic reionization would impose a floor on galaxy formation and leave most of the "missing (dark matter) satellites" completely star-free.

The SDSS & successor surveys found many more satellite galaxies (the total count is closer to 50 now), almost all of which are fainter than any of the previously known satellite galaxies.

The detection limits of modern surveys still aren't sensitive enough to see the faintest, lowest surface brightness systems throughout the MW, however.

So, while we can make approximations and educated guesses about how many satellite galaxies exist around the MW, the total number is still highly uncertain (at the factor of ~5-10 level).

This is important for a number of reasons. One is that different dark matter models end up predicting different numbers of subhalos around the MW, so counting satellites can constrain dark matter models ....

... if we understand where the "floor" of galaxy formation lies.

That is, many models of reionization-based solutions to the missing satellites problem predict that most galaxies form in halos where metal-poor gas can cool via atomic processes (V_halo > 20 km/s, T_halo > 10,000 K).

If galaxies can form with relative ease in lower-mass systems (where cooling would proceed via molecular processes), then there should be many more faint galaxies.

We re-examine this issue by taking dark-matter-only simulations & modifying them to include the gravitational potential of the MW disk + bulge. This accounts for the destruction of subhalos that have the misfortune of coming close enough to the Galaxy to be shredded by tides.

Tyler's results: after including the Galaxy's disruptive effects -- which are very large in the inner region of the halo -- the simulations do excellent job of reproducing the distribution of pericenters of known MW satellites. This isn't true for dark-matter-only simulations.

He also matches the radial distribution of satellites in the region where observations are complete. Nice! And again, this is not the case with dark-matter-only simulations.

An interesting requirement for these matches, however, is that satellite galaxies from in very low-mass halos -- those with V_halo of 7 km/s (T_halo ~ 2,000 K). This is far below the atomic cooling limit.

This low threshold for galaxy formation would also imply a large population of undiscovered galaxies at larger radii: we expect almost 1000 satellites within 300 kpc of the MW in this scenario, which is 5-10x more than many recent predictions!

So, while there is a natural understanding of the missing satellites problem in LCDM, our understanding of the details of this solution are surprisingly poor (which limits our ability to constrain dark matter models with satellite counts).

A major take-away of Tyler's work is that the Galaxy affects the number, spatial distribution, and orbits of satellites in the inner halo at a very significant level. This also has implications for, e.g., using stellar streams to constrain the existence of dark matter subhalos.

Tyler's paper has many more interesting results, so please go ahead and read it for yourself! arxiv.org/abs/1811.12413