EROs found behind lensing clusters: II. Empirical properties, classification, and SED modelling based on multi-wavelength observations

Aims. We study the properties and nature of extremely red galaxies (FRO, R-K >= 5.6) found behind two lensing clusters and compare them with other known galaxy populations.
Methods. New HST/ACS, Spitzer (TRAC and MIPS), and Chandra/ACIS observations of the two lensing clusters Abell 1835 and AC 114 have been obtained, combined with our earlier optical and near-IR observations and used to study EROs in these deep fields.
Results. We have found 6 and 9 EROs in Abell 1835 and AC 114. Several (7) of these objects are undetected up to the I and/or Z(850) band, and are hence "optical" drop-out sources (at a 3(T limit). The photometric redshifts of most of our sources (80%) are z similar to 0.7-1.5. According to simple colour-colour diagrams, the majority of our objects would be classified as hosting old stellar populations ("ellipticals"). However, there are clear signs of dusty starbursts for several among them. These objects correspond to the most extreme ones in R - K colour. After correcting for lens magnification, we estimate a surface density of (0.97 +/- 0.31) arcmin(-2) for EROs with (R - K >= 5.6) at K < 20.5. As in earlier Studies, an overlap of different populations is found. Among our 15 EROs 6 (40%) also classify as distant red galaxies. Eleven of 13 EROs (85%) with available IRAC photometry also fulfil the selection criteria for TRAC selected EROs (IEROs) of Yan et al. (2004, ApJ, 616, 63). SED modelling shows that similar to 36% of the IEROs in our sample are luminous or Ultra-luminous infrared galaxies ((U)LIRG). Some very red DRGs are found to be very dusty starbursts, even (U)LIRGs. No AGNs is found, although faint activity cannot be excluded for all objects. From mid-IR and X-ray data, 5 objects are clearly classified as starbursts. The derived properties are quite similar to those of DRGs and IEROs, except for 5 extreme objects in terms of colours, for which a very high extinction (Av greater than or similar to 3) is found.