The Phoenix Deep Survey: the star formation rates and the stellar masses of extremely red objects

ABSTRACT

We estimate the star formation rates and the stellar masses of the extremely red objects (EROs) detected in a  ≈180 arcmin² Ks-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 × 10²² 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 × 10¹⁰ 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 × 10¹⁰ 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 .