Coeval star formation in ultra diffuse galaxies

Abstract

Ultra diffuse galaxies (UDGs) have enjoyed a resurgence in the discovery space over the last decade thanks to improved telescope sensitivity, but their formation pathways remain unclear. The spatially resolved spectroscopy that is needed to map their stellar population properties is expensive due to their inherent faintness, with the result that only 2 UDGs have previously been observed in this way. Anna Ferré-Mateu and colleagues add four cluster UDGs and ‘nearly-UDGs’ to this sample in order to investigate whether these systems evolve in an entirely different way from classical dwarfs or instead form the faint tail-end of that population.

The authors analyse Keck Cosmic Web Imager (KCWI) spectra, newly obtained as part of the Analysis of Galaxies At The Extremes (AGATE) collaboration, to show that all four galaxies exhibit flat age profiles with old stellar populations that formed and quenched rapidly, and either flat or mildly rising metallicity profiles. This places them at the extreme end of the dwarf galaxy population, which the authors construct using classical dwarfs from the literature and simulated dwarfs from FIRE-2, and implies that UDGs may form ‘everything everywhere all at once’ in contrast to the outside–in star formation that is understood to take place in classical dwarfs.