The morphologies and masses of extremely red galaxies in the Groth Strip

Abstract

We present a new catalogue of EROs from the Groth Strip and study the relation between their morphology and mass. With a selection criterion F814W−K_s≥ 4 and K_s≤ 21.0 we find 102 EROs, over a survey area of 155 arcmin^2, leading to a surface density of 0.66 arcmin^−2. The photometric data include U, B, F606W, F814W, J, Ks bands. Morphologies are based on a by eye classification and we distinguish between three basic classes: compact objects, targets with a disc and/or a bulge component and irregular or merger candidates. An additional group consists of the few objects which could not be classified. The majority of our targets has either a very compact morphology (34 ± 6 per cent), or show more or less distinct disc components (43 ± 6 per cent). 14 ± 4 per cent are merger or irregulars and seven objects (approximately 9 per cent) could not be classified. We also study the dependence of structural parameters (effective radius: r_eff, Sérsic index: n) on morphological appearance. As expected, EROs that are either compact or show a distinct bulge component have smaller median effective radii (1.22 ± 0.14 kpc and 3.31 ± 0.53 kpc) than disc dominated (5.50 ± 0.51 kpc) or possible irregular galaxies or merger candidates (4.92 ± 0.14 kpc). More importantly, the Sérsic index changes from 2.30 ± 0.34 and 3.24 ± 0.55, to 1.03 ± 0.24 and 1.54 ± 0.40, respectively. As found in previous studies, most the EROs in our sample have redshifts between z= 1 and 2; however, compact EROs in our sample are found at redshifts as low as z= 0.4 and as high as z= 2.8; the latter qualify as well as distant red galaxies (DRGs). Disc-like EROs are also found up to z= 2.8; however, those with a bulge-disc structure are only seen at z < 1.5. For each of these EROs we determined the stellar mass and mean population age by fitting synthetic Bruzual (2007) spectra to the photometric spectral energy distributions, via χ^2 minimization. Mass estimates were obtained by assuming an exponentially declining star formation rate with a wide set of parameters, e.g. decay time, redshift of last star formation, metallicity and optical depth. Total stellar masses for our sample are in the range 9.1 < log(M/M_⊙) < 11.6. We cannot detect significant differences between the stellar mass distribution of the morphological classes. EROs with masses of log(M/M_⊙) > 11.0 dominantly show compact morphologies, but also include a significant number of sources with a disc morphology.