ABSTRACT We estimate the star formation rates and the stellar masses of the extremely red objects (EROs) detected in a ≈180 arcmin 2Ks-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 < 19.5 mag EROs in this sample (I-K > 4 mag; 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 6.4 μJy (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 = 4.5 × 10 22 W Hz -1 and a star formation rate of SFR = 25 M ⊙ yr -1. Combining this result with photometric redshift estimates, we find a lower limit to the star formation rate density of 0.02 ± 0.01 M ⊙ yr -1 Mpc -3 for the K < 19.5 mag 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 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 = 19.5 mag.
- Galaxies: evolution
- Infrared: galaxies