GAMA/H-ATLAS : linking the properties of submm detected and undetected early-type galaxies - I. z <= 0.06 sample

Abstract

We present two large, nearby (0.013 <= z <= 0.06) samples of early-type galaxies (ETGs): a visually classified sample of 220 ETGs, created using source-matched data from the Galaxy and Mass Assembly (GAMA) data base with far-infrared/submm detections from Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS); and a visually classified sample of 551 ETGs which are undetected with H-ATLAS. Active galactic nuclei are removed from our samples using optical emission-line diagnostics. These samples are scrutinized to determine characteristics of submm detected versus undetected ETGs. We find similarities in the stellar mass distributions of the two ETG samples but testing other properties uncovers significant differences. The submm detected sample is shown to have lower concentration and Sersic indices than those in the undetected sample - a result which may be linked to the presence of dust in the former. Optical and ultraviolet-optical colours are also shown to be much bluer, indicating that the dust is linked with recent star formation. The intrinsic effective radii are on average 1.5 times larger for the submm detected ETGs. Surface densities and groups data from the GAMA data base are examined for the two samples, leading to the conclusion that dusty ETGs inhabit sparser environments than non-dusty ETGs in the nearby Universe, although environments of the brightest ETGs are shown to differ the least. Modified Planck functions are fit to the H-ATLAS detected Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) fluxes for ETGs with submm flux densities of at least 3 sigma in the 350 mu m SPIRE band, giving a resultant mean cold dust temperature of T-d = 22.1 K, with a range of 9-30 K. The corresponding mean dust mass is 1.8 x 10(7) M-circle dot, with a range of (0.08-35.0) x 10(7) M-circle dot. The dust masses calculated from these fits, normalized by stellar mass, are shown to increase with decreasing stellar mass and bluer colours. Based on visual classifications of elliptical and lenticular, we find similar dust properties for these two early-type morphologies. We conclude that there is a population of elliptical galaxies which exhibit larger dust masses, lower Sersic index and bluer colours than the more well-known, massive, red population of ellipticals.