The Star Formation Histories of Local Group Dwarf Galaxies II. Searching For Signatures of Reionization

TL;DR

This study investigates whether signatures of cosmic reionization are observable in the star formation histories of 38 Local Group dwarf galaxies, finding limited evidence for reionization-driven quenching and highlighting environmental effects as confounding factors.

Contribution

It provides the first systematic search for reionization signatures in Local Group dwarf galaxies using Hubble data, and discusses the challenges in distinguishing reionization effects from environmental influences.

Findings

Only five galaxies are consistent with early star formation before reionization.

Two out of 13 predicted reionization fossils show quenched star formation, but environmental effects complicate interpretation.

Quenched low-mass field galaxies are the best candidates for reionization fossils.

Abstract

We search for signatures of reionization in the star formation histories (SFHs) of 38 Local Group dwarf galaxies (10$^4$ $<$ M$_{\star}$ $<$ 10$^9$ M$_{\odot}$). The SFHs are derived from color-magnitude diagrams using archival Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. Only five quenched galaxies (And V, And VI, And XIII, Leo IV, Hercules) are consistent with forming the bulk of their stars before reionization, when full uncertainties are considered. Observations of 13 of the predicted `true fossils' identified by Bovill & Ricotti show that only two (Hercules and Leo IV) indicate star formation quenched by reionization. However, both are within the virial radius of the Milky Way and evidence of tidal disturbance complicates this interpretation. We argue that the late-time gas capture scenario posited by Ricotti for the low mass, gas-rich, and star-forming fossil candidate Leo T is observationally indistinguishable from simple gas retention. Given the ambiguity between environmental effects and reionization, the best reionization fossil candidates are quenched low mass field galaxies (e.g., KKR 25).