Dwarf Galaxies

Dwarf galaxies dominate the galaxy number density and are crucial for our understanding of galaxy evolution. They tend to have the least star formation per unit mass of any systems, making them interestingly pristine tests of small-scale cosmology. The surface-brightnesses of dwarfs (especially those at cosmological distances) makes them largely invisible in current large-scale surveys. The discovery of faint dwarfs has, therefore, been limited to the Local Group, where they can be detected in resolved stellar counts.

Given the dearth of dwarfs outside the local neighbourhood in past surveys, this regime remains largely unexplored, at least from a statistical point of view (and certainly from the point of view of probing the evolution of the dwarf population with redshift). Why is their star formation so inefficient? Does the molecular-cloud model of star formation break down in the dwarf regime? Do stars and AGN drive outflows in these systems? Does reionization photo-evaporate gas in the smallest dwarfs? Perhaps not unexpectedly, many of the tensions between our theoretical paradigm and observations lie in this regime, e.g. the overprediction of dwarfs in simulations (the 'substructure' problem) or the discrepancy between the luminosity of predicted and observed dwarfs (the 'too-big-to-fail' problem).

LSST will revolutionize the study of dwarfs by offering, for the first time, statistically significant samples of dwarfs at cosmological distances. While spectroscopic data may be scant in this regime (given the low surface brightnesses of these systems), photometric redshifts from deep data should have sufficient accuracy (fractional errors less than 10%) to enable statistical studies of galaxies (Fig 1). For very nearby dwarfs, LSST's excellent seeing and depth could also combine to yield distances through surface-brightness fluctuations (Fig 2).