Evolution and Environment of Early-Type Galaxies

We study the photometric and spectral properties of 39320 early-type galaxieswithin the SDSS, as a function of both local environment and redshift. Thedistance to the nearest cluster of galaxies and the distance to the 10thnearest luminous neighbor were used to define two extremes in environment. Theproperties of early-type galaxies are weakly but significantly different inthese two extremes: objects in dense environments are less than ~1 Gyr olderand alpha-enhanced by ~0.02 relative to their counterparts of the same velocitydispersion in less dense regions, but the metallicities show no dependence onenvironment. This suggests that, in dense regions, the stars in early-typegalaxies formed at slightly earlier times, and on a slightly shorter timescale,than in less dense regions. In addition, objects at lower redshifts are older,but have the same metallicities and alpha-enhancements as their counterparts ofthe same sigma at higher redshifts, as expected if the low redshift sample is apassively-aged version of the sample at higher redshifts. These significantconclusions are derived from several pieces of evidence discussed in thispaper, including the FP of early-types, the observed evolution of absorptionlinestrengths (measured from a large ensemble of composite spectra which weprovide in tabular form) over small lookback times, and comparison with thepredictions of single burst (non-solar abundance) stellar population synthesismodels (which also indicate that age, metallicity and alpha-enhancement allincrease with velocity dispersion).