The Primeval Populations of the Ultra-Faint Dwarf Galaxies

TL;DR

This study uses deep Hubble Space Telescope photometry to investigate the star formation histories of ultra-faint dwarf galaxies, finding they are ancient, coeval, and likely affected by early universe events like reionization.

Contribution

It provides new constraints on the ages and star formation histories of ultra-faint dwarf galaxies, suggesting they are true fossils from the early universe.

Findings

UFDs are as old as globular cluster M92.

No evidence of intermediate-age populations in studied UFDs.

UFDs' star formation was likely truncated by reionization.

Abstract

We present new constraints on the star formation histories of the ultra-faint dwarf (UFD) galaxies, using deep photometry obtained with the Hubble Space Telescope (HST). A galaxy class recently discovered in the Sloan Digital Sky Survey, the UFDs appear to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in efforts to understand the missing satellite problem. They are the least luminous, most dark-matter dominated, and least chemically-evolved galaxies known. Our HST survey of six UFDs seeks to determine if these galaxies are true fossils from the early universe. We present here the preliminary analysis of three UFD galaxies: Hercules, Leo IV, and Ursa Major I. Classical dwarf spheroidals of the Local Group exhibit extended star formation histories, but these three Milky Way satellites are at least as old as the ancient globular cluster M92, with no evidence for intermediate-age populations. Their ages also appear to be synchronized to within ~1 Gyr of each other, as might be expected if their star formation was truncated by a global event, such as reionization.