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, M_V - 5 log h < -20.5, field early-type galaxies at redshifts 0.3 < z < 0.6. The observed b_Jori colours as a function of redshift, mean absorption line strengths and [OII] 3727 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 (z_f>1), passively evolving luminosity-weighted stellar population. Twenty-one percent of the sample possess detectable [OII] 3727 emission consistent with a low level (<1 M_solar/yr) 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. 
J.P. Willis (1 and 2), P.C. Hewett (3), S.J. Warren (4), G.F. Lewis   (5) ((1) PUC, Chile, (2) ESO, Chile, (3) IoA, Cambridg...