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 \la z \la 0.6 .
The observed b _ { J } ori 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 ( \la 1 M _ { \odot } 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 \simeq 0.4 , allowing only for passive luminosity evolution , is in excellent agreement with the local , ( \langle z \rangle = 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 .