The Phoenix Deep Survey: The star-formation rates and the stellar masses of EROs

We estimate the star-formation rates and the stellar masses of the Extremely Red Objects (EROs) detected in a ≈ 180 arcmin Ks-band survey (Ks ≈ 20mag). This sample is complemented by sensitive 1.4GHz radio observations (12μJy 1σ rms) and multiwaveband photometric data (UBV RIJ) 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 earlytype 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 subsamples 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 L1.4 = 4.5 × 10 W/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 −1 Mpc 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× 10 10 M⊙. We also find that EROs represent a sizable fraction (about 50 per cent) of the number density of galaxies more massive than M = 5 × 10 M⊙ 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.