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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 367; no. 1; pp. 331 - 338
• Main Authors: Georgakakis, A., Hopkins, A. M., Afonso, J., Sullivan, M., Mobasher, B., Cram, L. E.
• Format: Journal Article
• Language: English
• Published: 23 Ainslie Place, Edinburgh EH3 6AJ, UK. Telephone 226 7232 Fax 226 3803 Blackwell Science Ltd 01.03.2006; Blackwell Science; Oxford University Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; galaxies: evolution; infrared: galaxies; Radio astronomy; Star & galaxy formation; surveys; Galaxy evolution; Lower bound; Red shift; infrared: galaxies; galaxies: evolution; Luminosity; surveys; Formation rate; Radio observation; Light emission; Flux density; Star formation; Stellar mass; Attenuation; Selection criterion; Radio flux; Infrared galaxies; Emission line; Early type galaxies
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2005.09954.x

We estimate the star formation rates and the stellar masses of the extremely red objects (EROs) detected in a ≈180 arcmin2Ks-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 L1.4 = 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.