Luminous early-type field galaxies at z 0.4 -- II. Star formation history and space density

We present a combined photometric and spectroscopic analysis of the star formation history and space density of a sample of 485 luminous, MV− 5 log h < −20.5, field early-type galaxies at redshifts 0.3 ≲z≲ 0.6. The observed bJori colours as a function of redshift, mean absorption-line strengths and [O ii]λ3727 emission properties are used to constrain the star formation history of the galaxies. The mean star formation history of the early-type galaxy sample is consistent with an old (zf > 1), passively evolving luminosity-weighted stellar population. 21 per cent of the sample possess detectable [O ii]λ3727 emission consistent with a low level (≲1 M⊙ yr−1) of on-going star formation. Parametric and non-parametric estimates of the space density of the sample are derived. The integrated luminosity density at z≃ 0.4, allowing only for passive luminosity evolution, is in excellent agreement with the local, (〈z〉= 0.1), luminosity density of early-type galaxies. Overall, the sample properties are consistent with a galaxy formation scenario in which the majority of luminous field early-type galaxies formed at redshifts z > 1 and have largely evolved passively since the formation epoch.