Ultra-diffuse galaxies: the high-spin tail of the abundant dwarf galaxy population

Recent observations have revealed the existence of an abundant population of faint, low surface brightness (SB) galaxies, which appear to be numerous and ubiquitous in nearby galaxy clusters, including the Virgo, Coma and Fornax clusters. With median stellar masses of dwarf galaxies, these ultra-diffuse galaxies (UDGs) have unexpectedly large sizes, corresponding to a mean SB of $24\lesssim\langle\mu_e\rangle_r\ {\rm mag}^{-1} {\rm arcsec}^2\lesssim27$ within the effective radius. We show that the UDG population represents the tail of galaxies formed in dwarf-sized haloes with higher-than-average angular momentum. By adopting the standard model of disk formation -- in which the size of galaxies is set by the spin of the halo -- we recover both the abundance of UDGs as a function of the host cluster mass and the distribution of sizes within the UDG population. According to this model, UDGs are not failed $L_*$ galaxies, but genuine dwarfs, and their low SB is not uniquely connected to the harsh cluster environment. We therefore expect a correspondingly abundant population of UDGs in the field, with likely different morphologies and colours.