The CALIFA survey across the Hubble sequence: Spatially resolved stellar population properties in galaxies

Abstract

© ESO, 2015. Various different physical processes contribute to the star formation and stellar mass assembly histories of galaxies. One important approach to understanding the significance of these different processes on galaxy evolution is the study of the stellar population content of today's galaxies in a spatially resolved manner. The aim of this paper is to characterize in detail the radial structure of stellar population properties of galaxies in the nearby universe, based on a uniquely large galaxy sample, considering the quality and coverage of the data. The sample under study was drawn from the CALIFA survey and contains 300 galaxies observed with integral field spectroscopy. These cover a wide range of Hubble types, from spheroids to spiral galaxies, while stellar masses range from M<inf>Black star</inf> ∼ 10⁹ to 7 × 10¹¹ M<inf>⊙</inf>. We apply the fossil record method based on spectral synthesis techniques to recover the following physical properties for each spatial resolution element in our target galaxies: the stellar mass surface density (μ<inf>Black star</inf>), stellar extinction (A<inf>V</inf>), light-weighted and mass-weighted ages ('log age'<inf>L</inf>, 'log age'<inf>M</inf>), and mass-weighted metallicity ('log Z<inf>Black star</inf>'<inf>M</inf>). To study mean trends with overall galaxy properties, the individual radial profiles are stacked in seven bins of galaxy morphology (E, S0, Sa, Sb, Sbc, Sc, and Sd). We confirm that more massive galaxies are more compact, older, more metal rich, and less reddened by dust. Additionally, we find that these trends are preserved spatially with the radial distance to the nucleus. Deviations from these relations appear correlated with Hubble type: earlier types are more compact, older, and more metal rich for a given M<inf>Black star</inf>, which is evidence that quenching is related to morphology, but not driven by mass. Negative gradients of 'log age'<inf>L</inf> are consistent with an inside-out growth of galaxies, with the largest 'log age'<inf>L</inf> gradients in Sb-Sbc galaxies. Further, the mean stellar ages of disks and bulges are correlated and with disks covering a wider range of ages, and late-type spirals hosting younger disks. However, age gradients are only mildly negative or flat beyond R ∼ 2 HLR (half light radius), indicating that star formation is more uniformly distributed or that stellar migration is important at these distances. The gradients in stellar mass surface density depend mostly on stellar mass, in the sense that more massive galaxies are more centrally concentrated. Whatever sets the concentration indices of galaxies obviously depends less on quenching/morphology than on the depth of the potential well. There is a secondary correlation in the sense that at the same M<inf>Black star</inf> early-type galaxies have steeper gradients. The μ<inf>Black star</inf> gradients outside 1 HLR show no dependence on Hubble type. We find mildly negative 'log Z<inf>Black star</inf>'<inf>M</inf> gradients, which are shallower than predicted from models of galaxy evolution in isolation. In general, metallicity gradients depend on stellar mass, and less on morphology, hinting that metallicity is affected by both - the depth of the potential well and morphology/quenching.