The Effects of Patchy Reionization on Satellite Galaxies of the Milky Way

TL;DR

This study uses high-resolution simulations to explore how patchy reionization influences the number and properties of faint satellite galaxies around the Milky Way, revealing significant model-dependent variations.

Contribution

It combines detailed reionization models with zoom-in simulations to quantify the impact of reionization patchiness on satellite galaxy populations.

Findings

Satellite counts vary by a factor of 3-4 across reionization models.

Faint satellite populations are most affected by reionization scenario.

Halo-to-halo scatter is comparable to reionization scenario differences.

Abstract

We combine the high-resolution Aquarius simulations with three-dimensional models of reionization based on the initial density field of the Aquarius parent simulation, Millennium-II, to study the impact of patchy reionization on the faint satellite population of Milky Way halos. Because the Aquarius suite consists of zoom-in simulations of halos in the Millennium-II volume, we follow the formation of substructure and the growth of reionization bubbles due to the larger environment simultaneously, and thereby determine the reionization redshifts of satellite candidates. We do this for four different reionization models, and also compare results to instantaneous reionization. Using a simple procedure for selecting satellites and assigning luminosities in the simulations, we compare the resulting satellite populations. We find that the overall number of satellites depends sensitively on the reionization model, with a factor of 3-4 variation between the four models for a given host halo, although the difference is entirely in the population of faint satellites (M_V > -10). In addition, we find that for a given reionization model the total number of satellites differs by 10%-20% between the patchy and homogeneous scenarios, provided that the redshift is chosen appropriately for the instantaneous case. However, the halo-halo scatter from the six Aquarius halos is large, up to a factor of 2-3, and so is comparable to the difference between reionization scenarios. In order to use the population of faint dwarf galaxies around the Milky Way as a probe of the local reionization history, then, it is necessary to first better understand the general distribution of substructure around Milky Way-mass halos.