2006MNRAS.367..331G

We estimate the star formation rates and the stellar masses of the extremely red objects (EROs) detected in a ~180arcmin²Ks-band survey (Ks~20 mag). This sample is complemented by sensitive 1.4-GHz radio observations (12 µJy 1σ rms) and multiwaveband photometric data (UBVRIJ) as part of the Phoenix Deep Survey. For bright K < 19.5mag EROs in this sample (I-K > 4mag; total of 177), we use photometric methods to discriminate dust-enshrouded active systems from early-type galaxies and to constrain their redshifts. Radio stacking is then employed to estimate mean radio flux densities of ~8.6 (3σ) and 6.4µJy (2.4σ) for the dusty and early-type sub-samples, respectively. Assuming that dust-enshrouded active EROs are powered by star formation, the above radio flux density at the median redshift of z=1 translates to a radio luminosity of L_1.4=4.5x10²²W/Hz and a star formation rate of SFR=25M_☉/yr. Combining this result with photometric redshift estimates, we find a lower limit to the star formation rate density of 0.02±0.01M_☉/yr/Mpc3 for the K<19.5mag dusty EROs in the range z=0.85-1.35. Comparison with the star formation rate density estimated for previous ERO samples (with similar selection criteria) using optical emission lines, suffering dust attenuation, suggests a mean dust reddening of at least E(B-V)~0.5 for this population. We further use the Ks-band luminosity as proxy to stellar mass and argue that the dust-enshrouded starburst EROs in our sample are massive systems, M≳5 x10¹⁰M_☉. We also find that EROs represent a sizable fraction (about 50 per cent) of the number density of galaxies more massive than M=5x10¹⁰M_☉at z~1, with almost equal contributions from dusty and early-type systems. Similarly, we find that EROs contribute about half of the mass density of the Universe at z~1 (with almost equal contributions from dusty and early types), after taking into account incompleteness because of the magnitude limit K=19.5mag.