A census of metals and baryons in stars in the local Universe

Abstract

We combine stellar metallicity and stellar mass estimates for a large sample of galaxies drawn from the SDSS DR2 spanning wide ranges in physical properties, in order to derive an inventory of the total mass of metals and baryons locked up in stars today. Physical parameter estimates are derived from galaxy spectra with high S/N (>20). Coadded spectra of galaxies with similar velocity dispersions, absolute r-band magnitudes and 4000Å-break values are used for those regions of parameter space where individual spectra have lower S/N. We estimate the total density of metals and of baryons in stars and, from these two quantities, we obtain a mass- and volume-averaged stellar metallicity of =1.04+-0.14 Z_sun, i.e. consistent with solar. We also study how metals are distributed in galaxies according to their mass, morphology and age, and we then compare these distributions with the corresponding distributions of stellar mass. We find that the bulk of metals locked up in stars in the local Universe reside in massive, bulge-dominated galaxies, with red colours and high 4000Å-break values corresponding to old stellar populations. Bulge-dominated and disc-dominated galaxies contribute similar amounts to the total stellar mass density, but have different fractional contributions to the mass density of metals in stars, in agreement with the mass-metallicity relation. Bulge-dominated galaxies contain roughly 40% of the total amount of metals in stars, while disc-dominated galaxies less than 25%. Finally, at a given galaxy stellar mass, we define two characteristic ages as the median of the distributions of mass and metals as a function of age. These characteristic ages decrease progressively from high-mass to low-mass galaxies, consistent with the high formation epochs of stars in massive galaxies.