Constraining local UV field geometry at reionization using Milky Way satellites

TL;DR

This paper introduces a semi-analytical model to study how the geometry of reionization affects the distribution and properties of Milky Way satellite galaxies, providing insights into the epoch of reionization.

Contribution

It compares external and internal reionization scenarios, showing the internal scenario better matches observed satellite distributions and offers a new method to probe reionization geometry.

Findings

Internal reionization scenario produces a more extended satellite distribution.

Both scenarios match luminosity functions and mass-to-light ratios.

The satellite radial distribution can reveal reionization history.

Abstract

We present a new semi-analytical model of the population of satellite galaxies of the Milky Way, aimed at estimating the effect of the geometry of reionization at galaxy scale on the properties of the satellites. In this model reionization can be either: (A) externally-driven and uniform, or (B) internally-driven, by the most massive progenitor of the Milky Way. In the latter scenario the propagation of the ionisation front and photon dilution introduce a delay in the photo-evaporation of the outer satellites' gas with respect to the inner satellites. As a consequence, outer satellites experience a longer period of star formation than those in the inner halo. We use simple models to account for star formation, the propagation of the ionisation front, photo-evaporation and observational biases. Both scenarios yield a model satellite population that matches the observed luminosity function and mass-to-light ratios. However, the predicted population for scenario (B) is significantly more extended spatially than for scenario (A), by about 0.3 dex in distance, resulting in a much better match to the observations. The survival of the signature left by the local UV field during reionization on the radial distribution of satellites makes it a promising tool for studying the reionization epoch at galaxy scale in the Milky Way and nearby galaxies resolved in stars with forthcoming large surveys.