The evolution of field early-type galaxies in the FDF and WHDF

We explore the properties of 24 field early-type galaxies in the redshift range 0.20 < z < 0.75 down to M(B) <= -19.30 in a sample extracted from the FORS Deep Field and the William Herschel Deep Field. Target galaxies were selected on the basis of a combination of luminosity, spectrophotometric type, morphology and photometric redshift or broad-band colours. High signal-to-noise ratio intermediate-resolution spectroscopy has been acquired at the Very Large Telescope, complemented by deep high-resolution imaging with the Advanced Camera for Surveys onboard the Hubble Space Telescope (HST) and additional ground-based multiband photometry. All galaxy spectra were observed under subarcsecond conditions and allow us to derive accurate kinematics and stellar population properties of the galaxies. To clarify the low level of star formation detected in some galaxies, we identify the amount of active galactic nuclei (AGN) activity in our sample using archive data of Chandra and XMM- Newton X-ray surveys. None of the galaxies in our sample was identified as secure AGN source based on their X-ray emission. The rest-frame B- and K-band scaling relations of the Faber-Jackson relation and the Fundamental Plane display a moderate evolution for the field early-type galaxies. Lenticular (S0) galaxies feature on average a stronger luminosity evolution and bluer rest-frame colours which can be explained that they comprise more diverse stellar populations compared to elliptical galaxies. The evolution of the FP can be interpreted as an average change in the dynamical ( effective) mass-to-light ratio of our galaxies as (triangle log(M/L(B))/z ) = -0.74 +/- 0.08. The M/L evolution of these field galaxies suggests a continuous mass assembly of field early-type galaxies during the last 5 Gyr, which gets supported by recent studies of field galaxies up to z similar to 1. Independent evidence for recent star formation activity is provided by spectroscopic ([O II] emission, H delta) and photometric (rest-frame broad-band colours) diagnostics. Based on the Hd absorption feature we detect a weak residual star formation for galaxies that accounts for 5-10 per cent in the total stellar mass of these galaxies. The co-evolution in the luminosity and mass of our galaxies favours a downsizing formation process. We find some evidence that our galaxies experienced a period of star formation quenching, possible triggered by AGN activity that is in good agreement with recent results on both observational and theoretical side.