Elliptical galaxies : structure, stellar content, and evolution

A near-infrared imaging survey of 341 nearby early-type galaxies, combined with optical imaging and spectroscopic data from the literature, are used to construct the global scaling relations for this population of galaxies. These data demonstrate a number of important features of the early-type galaxy sequence: (1) the slope of the fundamental plane (FP) correlations systematically increases with wavelength; (2) the slope of these FP correlations deviates from the virial expectation at all wavelengths, implying a breakdown of either or both of the assumptions of constant mass-to-light ratio and homology; (3) the intrinsic scatter of the FP correlations is small but resolved at all wavelengths, implying a small cosmic scatter of earlytype galaxy properties at any position in the galaxy sequence and contradicting any model in which various stellar populations parameters “conspire” with each other to keep the correlations thin at optical wavelengths; (4) there is no correlation among residuals of the metallicity-independent near-infrared FP and the metallicity-sensitive Mg2-j0 relation, implying that both age and metallicity variations contribute to the cosmic scatter of both correlations; and (5) the effective radii systematically decrease with increasing wavelength, fully consistent with the general presence of stellar population gradients in early-type galaxies. A comprehensive and self-consistent model is described that simultaneously explains these and other global properties of the early-type galaxy sequence. This model demonstrates that age, metallicity, deviations from a dynamically homologous family, and population gradients are all contributing to the form of the global scaling relations; the latter two effects are the least constrained by the observations and model, respectively. Additional constraints on the role of age in the global scaling relations are obtained by observing early-type galaxies at higher redshifts. This has been studied using three approaches: color evolution, evolution of the slope and intercept of the K-band FP correlations, and evolution of the absorption-line strengths. The galaxies in each cluster are identified in a systematic way using two-color (three bandpass) imaging—sampling approximately the U, V, and I bandpasses in the rest frame—to eliminate late-type cluster member and general field interloper galaxies. This method is more than 90% effective in identifying early-type galaxies at the target cluster redshift. Weak color evolution in rest-frame has been de(U V )0 tected in 26 rich clusters of galaxies. The bluing trend in color is mag at , mag at , 0.05 0.03 z 0.2 0.12 0.04 z 0.4 and ∼ mag at . Using stellar population 0.23 0.05 z 0.54 synthesis models from the literature, this color evolution is fully consistent with the galaxy population as a whole having formed at . 1 ! z ! 5 f The FP correlations are studied for 128 galaxies in eight rich clusters at using moderate dispersion spectros0.1 ! z ! 0.6 copy (110 of the measurements are new) and imaging in the near-infrared K band. These data more than quadruple the data in the literature that can be used to study the FP at high redshift. The near-infrared FP is constructed at high redshifts for the first time. The intercept of the FP on the surface brightness axis is observed to dim with redshift, as expected for the Tolman signal in an expanding world model. A small amount of luminosity evolution mag is detected DK ≈ 2.5 log (1 z) on top of the Tolman signal, which is consistent with the passive evolution of a stellar population that formed at high redshift. The slope of the near-infrared FP is observed to flatten with redshift, implying that the least luminous galaxies are evolving faster than the luminous galaxies. This is strong evidence that low-luminosity elliptical galaxies have a stellar content that is up to a factor of 2 times younger than high-luminosity elliptical galaxies, but age spreads much larger than this appear to be excluded, and age spreads somewhat smaller than this are still allowed depending on the choice of nearby galaxy sample used in the comparison. The absorption-line strengths of the galaxies are observed to evolve slowly with redshift: the Mg2 and AFeS indices weaken while the HbG index strengthens. The latter effect in particular is another strong indication that the mean age of the stellar populations in early-type galaxies formed at redshifts 3 ! . All of these properties are moderately consistent with z ! 5 f the models derived above based on the samples of nearby galaxies, implying that age is an important physical parameter underlying the global scaling relations for early-type galaxies.