Spectral-based k-corrections and implications for the colour-magnitude relation of E/S0s and its evolution

TL;DR

This study uses spectral data from SDSS to accurately determine k-corrections for early-type galaxies, revealing how their colour-magnitude and colour-sigma relations evolve with redshift and are influenced by aperture definitions.

Contribution

It introduces a spectral-based method for precise k-corrections and analyzes their impact on galaxy relations and evolution, highlighting the importance of aperture effects.

Findings

Both CMR and CsigmaR evolve blueward with redshift.

The CMR slope depends on aperture type, but CsigmaR does not.

Colour gradients are linked to stellar population ages.

Abstract

We select a sample of 70378 E/S0 (early-type) galaxies at 0<z<0.36 from the Sloan Digital Sky Survey, excluding disk and star-forming galaxies. We estimate g and r magnitudes in the observer- and rest-frames directly from the SDSS DR6 spectra; this provides an object-by-object estimate of the k-correction. We use the k-corrections from the spectra to study the evolution of the rest-frame colour-magnitude relation (CMR) and colour-sigma (velocity dispersion) relation (CsigmaR). The evolution is very sensitive to the k-correction. Both the CMR and CsigmaR evolve blueward with increasing redshift, approximately in agreement with passive evolution models with age approx. 12 Gyr. The slope and zero-point of the CMR depend on whether colours were defined in fixed physical or angular apertures, a consequence of the fact that the centers of these objects tend to be redder: the relation is steeper for fixed angular apertures. One the other hand, the CsigmaR slope does not show this dependence on the aperture in which the colour was defined, suggesting that colour gradients are correlated with residuals from the sigma-M_r relation. As these residuals are age indicators, our findings suggest that colour gradients correlate with the age of the stellar population.