2016MNRAS.463..811O

We present a study of nearby galaxies as a function of their [3.4]-[4.6] colour. Galaxies that are red in their [3.4]-[4.6] colour contain heated dust and the reddest systems ([3.4]-[4.6] > 0.5) are classified as active galactic nuclei (AGN) by some selection criteria. The sample discussed here includes nearby galaxies selected from the Sloan Digital Sky Survey (SDSS) that are also in the Wide-field Infrared Survey Explorer (WISE) catalogue. We calculate the number density of galaxies, in the form of the luminosity and mass functions, using the V/V_max method and a stepwise maximum likelihood method that has been modified to account for the additional colour selection. The reddest galaxies which have [3.4]-[4.6] > 0.8 and are sometimes classified as AGN by their colour make up 0.2 per cent of nearby galaxies. However, the reddest galaxies are a rising fraction of the low-mass galaxy population. Identifying the lowest mass (M < 10⁸ M_☉) red ([3.4]-[4.6] > 0.8) galaxies as AGN is surprising given that none are optical AGN or composites, in contrast with their more massive (M > 10¹⁰ M_☉) red galaxy counterparts that are dominated by optical AGN and composites (86.4 per cent). We also show that these low-mass red galaxies are associated with higher specific star formation rates than their bluer counterparts. While the properties of this relatively rare segment of nearby low-mass galaxies are intriguing, particularly if they are associated with AGN activity, there is not yet enough evidence to determine whether it is AGN or unusual star formation that is driving red colours in these systems.