THE STAR FORMATION HISTORIES OF LOCAL GROUP DWARF GALAXIES. II. SEARCHING FOR SIGNATURES OF REIONIZATION

Abstract

We search for signatures of reionization in the star formation histories\n(SFHs) of 38 Local Group dwarf galaxies (10$^4$ $<$ M$_{\\star}$ $<$ 10$^9$\nM$_{\\odot}$). The SFHs are derived from color-magnitude diagrams using archival\nHubble Space Telescope/Wide Field Planetary Camera 2 imaging. Only five\nquenched galaxies (And V, And VI, And XIII, Leo IV, Hercules) are consistent\nwith forming the bulk of their stars before reionization, when full\nuncertainties are considered. Observations of 13 of the predicted `true\nfossils' identified by Bovill & Ricotti show that only two (Hercules and Leo\nIV) indicate star formation quenched by reionization. However, both are within\nthe virial radius of the Milky Way and evidence of tidal disturbance\ncomplicates this interpretation. We argue that the late-time gas capture\nscenario posited by Ricotti for the low mass, gas-rich, and star-forming fossil\ncandidate Leo T is observationally indistinguishable from simple gas retention.\nGiven the ambiguity between environmental effects and reionization, the best\nreionization fossil candidates are quenched low mass field galaxies (e.g., KKR\n25).\n