The properties of field elliptical galaxies at intermediate redshift - II. Photometry and spectroscopy of an HST-selected sample

A sample of field early-type galaxies (E/S0) at intermediate redshift (z∼0.1–0.6) is selected, based on morphology and colours from HST-WFPC2 parallel images. Photometric structural parameters (effective radius Rₑ and effective surface brightness SBₑ) are derived through the F606W and F814W filters, using luminosity profile fitting and two-dimensional fitting techniques. The combined parameter that enters the Fundamental Plane (log Rₑ−βSBₑ, with β≈0.32) is shown to suffer from significantly smaller uncertainties (rms 0.03) than the individual structural parameters (e.g. ∼15 per cent rms on the effective radius). High signal-to-noise ratio, intermediate-resolution spectra, taken at the ESO 3.6-m telescope, yield redshifts for 35 galaxies and central velocity dispersions for 22 galaxies. Central velocity dispersions are derived using a library of stellar templates covering a wide range of spectral types, in order to study the effects of template mismatches. The average random error on the central velocity dispersion is found to be 8 per cent, and the average systematic error caused by template mismatch is found to be 5 per cent. The errors in the velocity dispersion measurement and the effects of template mismatches are studied by means of extensive Monte Carlo simulations. In addition, we investigate whether the determination of the velocity dispersion is sensitive to the spectral range used, finding that the value of velocity dispersion is unchanged when the spectral regions that include the absorption features Ca H and K and NaD are masked out during the fit