Deep ultraviolet luminosity functions at the infall region of the coma cluster

Abstract

We have used deep $\textit{GALEX}$ observations at the infall region of the Coma cluster to measure the faintest ultraviolet (UV) luminosity functions (LFs) presented for a rich galaxy cluster thus far. The Coma UV LFs are measured to $M_{UV} = –10.5$ in the $\textit{GALEX}$ FUV and NUV bands, or 3.5 mag fainter than previous studies, and reach the dwarf early-type galaxy population in Coma for the first time. The Schechter faint-end slopes ($\alpha \approx –1.39$ in both $\textit{GALEX}$ bands) are shallower than reported in previous Coma UV LF studies owing to a flatter LF at faint magnitudes. A Gaussian-plus-Schechter model provides a slightly better parameterization of the UV LFs resulting in a faint-end slope of $\alpha \approx –1.15$ in both $\textit{GALEX}$ bands. The two-component model gives faint-end slopes shallower than $\alpha = –1$ (a turnover) for the LFs constructed separately for passive and star-forming galaxies. The UV LFs for star-forming galaxies show a turnover at $M_{UV} \approx –14$ owing to a deficit of dwarf star-forming galaxies in Coma with stellar masses below $M_{*} = 108 M_{\odot}$. A similar turnover is identified in recent UV LFs measured for the Virgo cluster suggesting this may be a common feature of local galaxy clusters, whereas the field UV LFs continue to rise at faint magnitudes. We did not identify an excess of passive galaxies as would be expected if the missing dwarf star-forming galaxies were quenched inside the cluster. In fact, the LFs for both dwarf passive and star-forming galaxies show the same turnover at faint magnitudes. We discuss the possible origin of the missing dwarf star-forming galaxies in Coma and their expected properties based on comparisons to local field galaxies.