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 } \geq 4 and K _ { s } \leq 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,F 606 W,F 814 W,J,K _ { s } bands .
Morphologies are based on a by eye classification and we distinguish between 3 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 ( 33 \pm 6 \% ) , or show more or less distinct disc components ( 41 \pm 6 \% ) .
14 \pm 4 \% are merger or irregulars and 7 objects ( approximately 10 % ) could not be classified . We also study the dependence of structural parameters ( effective radius : r _ { \mathrm { 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 \pm 0.14 kpc and 3.31 \pm 0.53 kpc ) than disc dominated ( 5.50 \pm 0.51 kpc ) or possible irregular galaxies or merger candidates ( 4.92 \pm 0.14 kpc ) .
More importantly , the Sérsic index changes from 2.30 \pm 0.34 and 3.24 \pm 0.55 , to 1.03 \pm 0.24 and 1.54 \pm 0.40 respectively . As found in previous studies , most the EROs in our sample have redshifts between z = 1 and z = 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 objects ( 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 spectra to the photometric spectral energy distributions , via \chi ^ { 2 } minimisation .
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 _ { \odot } ) < 11.6 .
We can not detect significant differences between the stellar mass distribution of the morphological classes .
EROs with masses of \log ( M / M _ { \odot } ) > 11.0 dominantly show compact morphologies , but also include a significant number of sources with a disc morphology .