Red Misfits in the Sloan Digital Sky Survey: Properties of Star-Forming Red Galaxies

Abstract

Galaxies in the Universe are primarily blue and star-forming or red and passively evolving. Here we study an outlier population of red, star-forming galaxies in the local Universe which we call Red Misfits. These galaxies are classified based on inclination-corrected optical colours and specific star formation rates derived from the Sloan Digital Sky Survey Data Release 7. We find that $\sim$11 per cent of galaxies at all stellar masses are classified as red in colour yet actively star-forming. Using the wealth of information provided by the SDSS and related products we explore a number of properties of these galaxies and demonstrate that Red Misfits are a distinct population of galaxies in the Universe and not simply blue star-forming galaxies whose colours are reddened by intrinsic dust extinction. Red Misfit galaxies exhibit intermediate, bulge-dominated disk morphologies, intermediate stellar ages, slightly enhanced dust extinction and gas-phase metallicities, and an enhanced likelihood of hosting an active galactic nucleus. The proportion of Red Misfits in galaxy groups remains constant irrespective of group halo mass or projected distance to the group centre. We conclude that Red Misfits are a transition population being gradually quenched on their way to the red sequence and that this quenching is dominated by internal mechanisms rather than environmentally-driven processes.