H-ATLAS/GAMA : The nature and characteristics of optically red galaxies detected at submillimetre wavelengths

Abstract

We combine Herschel/SPIRE sub-millimeter (submm) observations with existing multi-wavelength data to investigate the characteristics of low redshift, optically red galaxies detected in submm bands. We select a sample of galaxies in the redshift range 0.01$\leq$z$\leq$0.2, having >5$\sigma$ detections in the SPIRE 250 micron submm waveband. Sources are then divided into two sub-samples of $red$ and $blue$ galaxies, based on their UV-optical colours. Galaxies in the $red$ sample account for $\approx$4.2 per cent of the total number of sources with stellar masses M$_{*}\gtrsim$10$^{10}$ Solar-mass. Following visual classification of the $red$ galaxies, we find that $\gtrsim$30 per cent of them are early-type galaxies and $\gtrsim$40 per cent are spirals. The colour of the $red$-spiral galaxies could be the result of their highly inclined orientation and/or a strong contribution of the old stellar population. It is found that irrespective of their morphological types, $red$ and $blue$ sources occupy environments with more or less similar densities (i.e., the $\Sigma_5$ parameter). From the analysis of the spectral energy distributions (SEDs) of galaxies in our samples based on MAGPHYS, we find that galaxies in the $red$ sample (of any morphological type) have dust masses similar to those in the $blue$ sample (i.e. normal spiral/star-forming systems). However, in comparison to the $red$-spirals and in particular $blue$ systems, $red$-ellipticals have lower mean dust-to-stellar mass ratios. Besides galaxies in the $red$-elliptical sample have much lower mean star-formation/specific-star-formation rates in contrast to their counterparts in the $blue$ sample. Our results support a scenario where dust in early-type systems is likely to be of an external origin.