The properties of field elliptical galaxies at intermediate redshift - I. Empirical scaling laws

We present measurements of the Fundamental Plane (FP) parameters (the effective radius, the mean effective surface brightness, and the central velocity dispersion) of six field elliptical galaxies at intermediate redshift. The imaging is taken from the Medium Deep Survey of the Hubble Space Telescope, while the kinematical data are obtained from long-slit spectroscopy using the 3.6-m ESO telescope. The Fundamental Plane appears well defined in the field even at redshift approximate to 0.3. The data show a shift in the FP zero-point with respect to the local relation, possibly indicating modest evolution, consistent with the result found for intermediate-redshift cluster samples. The FP slopes derived for pur field data, plus other cluster ellipticals at intermediate redshift taken from the literature, differ from the local ones, but are still consistent with the interpretation of the FP as a result of homology, of the virial theorem, and of the existence of a relation between luminosity and mass, L proportional to M-eta. We also derive the surface brightness versus effective radius relation for nine galaxies with redshifts up to z approximate to 0.6, and data from the literature; the evolution that can be inferred is consistent with what is found using the FP.