SIMBAD references

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 arcmin², leading to a surface density of 0.66/arcmin2. The photometric data include U, B, F606W, F814W, J, K_sbands. 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 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.