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−Ks≥ 4 and Ks≤ 21.0 we find 102 EROs, over a survey area of 155 arcmin2, 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: reff, 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 χ2 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.