Redshift Evolution of the Stellar Populations in Elliptical Galaxies

Abstract

Velocity dispersions σ and Mg absorption line strengths Mg_b have been measured for a sample of 16 ellipticals in three clusters at a redshift of 0.37. Like local cluster ellipticals, these objects show a correlation between Mg_b and σ. However, at any given σ, the mean Mg_b of the ellipticals at z = 0.37 is weaker than the mean Mg_b of their local relatives in the Coma and Virgo clusters. The Mg_b weakening is smallest for the most luminous ellipticals and larger for the fainter objects. This is unambiguous evidence for small but significant passive evolution of the stellar populations of elliptical galaxies with redshift. It requires that the bulk of the stars in cluster ellipticals has formed at z > 2. The most luminous objects may even have formed at z > 4. The Mg_b-σ test is a very reliable estimator for the evolution of old stellar populations because it is virtually independent from the stellar initial mass function (IMF) and from the metallicities of the galaxies. Furthermore, the influence of selection effects is minimal. Consistent with the weakening of Mg_b we find evidence for a B-band luminosity evolution of about 0.5 ± 0.1 mag at z = 0.37 from the Faber-Jackson relation. The combined information about the evolution of Mg_b and luminosity allows us to constrain both the slope of the IMF in ellipticals and the cosmological deceleration parameter q₀. Our present measurements are compatible with a standard Salpeter IMF and a q₀ of 0.5 ± 0.5.