How low does it go? Too few Galactic satellites with standard reionization quenching

TL;DR

This study uses cosmological simulations to show that the observed number of ultrafaint galaxies near the Milky Way implies galaxy formation in haloes with much lower velocities than standard reionization models predict, challenging existing theories.

Contribution

It demonstrates that ultrafaint galaxies likely form in haloes with peak velocities as low as 6 km/s, below the atomic cooling limit, contradicting standard reionization suppression models.

Findings

Fewer subhaloes with Vpeak > 20 km/s than observed ultrafaint galaxies.

Ultrafaint galaxies must inhabit haloes with Vpeak ~ 6 km/s.

Possible contribution of the Large Magellanic Cloud to satellite count.

Abstract

A standard prediction of galaxy formation theory is that the ionizing background suppresses galaxy formation in haloes with peak circular velocities smaller than Vpeak ~ 20 km/s, rendering the majority of haloes below this scale completely dark. We use a suite of cosmological zoom simulations of Milky Way-like haloes that include central Milky Way disk galaxy potentials to investigate the relationship between subhaloes and ultrafaint galaxies. We find that there are far too few subhaloes within 50 kpc of the Milky Way that had Vpeak > 20 km/s to account for the number of ultrafaint galaxies already known within that volume today. In order to match the observed count, we must populate subhaloes down to Vpeak ~ 6 km/s with ultrafaint dwarfs. The required haloes have peak virial temperatures as low as 1,500 K, well below the atomic hydrogen cooling limit of 10^4 K. Allowing for the possibility that the Large Magellanic Cloud contributes several of the satellites within 50 kpc could potentially raise this threshold to 10 km/s (4,000 K), still below the atomic cooling limit and far below the nominal reionization threshold.