Probing the Epoch of Reionization with Milky Way Satellites

TL;DR

This paper models Milky Way satellites to understand the epoch of reionization, revealing how low-mass galaxy formation and feedback processes inform the observed satellite population and their star formation histories.

Contribution

It introduces a detailed model linking MW satellite properties to early universe processes, including H_2 cooling and inhomogeneous reionization feedback, providing new insights into galaxy formation history.

Findings

Existence of faint satellites supports H_2 cooling in low-mass halos.

Most stars in bright satellites likely formed after reionization.

Model predicts larger M_300 dispersion and low-mass luminous satellites.

Abstract

While the connection between high-redshift star formation and the local universe has recently been used to understand the observed population of faint dwarf galaxies in the Milky Way (MW) halo, we explore how well these nearby objects can probe the epoch of first light. We construct a detailed, physically motivated model for the MW satellites based on the state-of-the-art Via Lactea II dark-matter simulations. Our model incorporates molecular hydrogen (H_2) cooling in low-mass systems and inhomogeneous photo-heating feedback during the internal reionization of our own galaxy. We find that the existence of MW satellites fainter than M_V ~ -5 is strong evidence for H_2 cooling in low-mass halos, while satellites with -5 > M_V > -9 were affected by hydrogen cooling and photoheating feedback. The age of stars in very low-luminosity systems and the minimum luminosity of these satellites are key predictions of our model. Most of the stars populating the brightest MW satellites could have formed after the epoch of reionization. Our models also predict a significantly larger dispersion in M_300 values than observed and a number of luminous satellites with M_300 as low as 10^6 solar masses.