Evolution of the most massive galaxies to z= 0.6 - I. A new method for physical parameter estimation

Abstract

We use principal component analysis (PCA) to estimate stellar masses, mean stellar ages, star formation histories (SFHs), dust extinctions and stellar velocity dispersions for ~290,000 galaxies with stellar masses greater than $10^{11}Msun and redshifts in the range 0.4<z<0.7 from the Baryon Oscillation Spectroscopic Survey (BOSS). We find the fraction of galaxies with active star formation first declines with increasing stellar mass, but then flattens above a stellar mass of 10^{11.5}Msun at z~0.6. This is in striking contrast to z~0.1, where the fraction of galaxies with active star formation declines monotonically with stellar mass. At stellar masses of 10^{12}Msun, therefore, the evolution in the fraction of star-forming galaxies from z~0.6 to the present-day reaches a factor of ~10. When we stack the spectra of the most massive, star-forming galaxies at z~0.6, we find that half of their [OIII] emission is produced by AGNs. The black holes in these galaxies are accreting on average at ~0.01 the Eddington rate. To obtain these results, we use the stellar population synthesis models of Bruzual & Charlot (2003) to generate a library of model spectra with a broad range of SFHs, metallicities, dust extinctions and stellar velocity dispersions. The PCA is run on this library to identify its principal components over the rest-frame wavelength range 3700-5500A. We demonstrate that linear combinations of these components can recover information equivalent to traditional spectral indices such as the 4000A break strength and HdA, with greatly improved S/N. This method is able to recover physical parameters such as stellar mass-to-light ratio, mean stellar age, velocity dispersion and dust extinction from the relatively low S/N BOSS spectra. We examine the sensitivity of our stellar mass estimates to the input parameters in our model library and the different stellar population synthesis models.