Satellite mass functions and the faint end of the galaxy mass-halo mass relation in LCDM

TL;DR

This paper compares two models of galaxy formation at the faint end within LCDM cosmology, showing how satellite counts can distinguish between a sharp cutoff and a steep power-law relation.

Contribution

It introduces and contrasts two models for the stellar mass-halo mass relation at low masses, highlighting their different predictions for ultra-faint dwarf populations.

Findings

Both models match current satellite observations around Milky Way-like galaxies.

They predict different numbers of ultra-faint dwarfs and satellites around dwarf galaxies.

Satellite mass functions can help determine the galaxy formation threshold in low-mass halos.

Abstract

The abundance of the faintest galaxies provides insight into the nature of dark matter and the process of dwarf galaxy formation. In the LCDM scenario, low mass halos are so numerous that the efficiency of dwarf formation must decline sharply with decreasing halo mass in order to accommodate the relative scarcity of observed dwarfs and satellites in the Local Group. The nature of this decline contains important clues to the mechanisms regulating the onset of galaxy formation in the faintest systems. We explore here two possible models for the stellar mass ($M_*$)-halo mass ($M_{200}$) relation at the faint end, motivated by some of the latest LCDM cosmological hydrodynamical simulations. One model includes a sharp mass threshold below which no luminous galaxies form, as expected if galaxy formation proceeds only in systems above the Hydrogen-cooling limit. In the second model, $M_*$ scales as a steep power-law of $M_{200}$ with no explicit cutoff, as suggested by recent semianalytic work. Although both models predict satellite numbers around Milky Way-like galaxies consistent with current observations, they predict vastly different numbers of ultra-faint dwarfs and of satellites around isolated dwarf galaxies. Our results illustrate how the satellite mass function around dwarfs may be used to probe the $M_*$-$M_{200}$ relation at the faint end and to elucidate the mechanisms that determine which low-mass halos "light up" or remain dark in the LCDM scenario.