The evolving disk galaxy population

In this contribution, I present a simplified overview of the evolution of the disk galaxy population since z=1, and a brief discussion of a few open questions. Galaxy evolution surveys have found that the disk galaxy population forms stars intensely at intermediate redshift. In particular, they dominate the cosmic star formation rate at z<1 -- the factor of ten drop in cosmic average comoving star formation rate in the last 8 Gyr is driven primarily by disk physics, not by a decreasing major merger rate. Despite this intense star formation, there has been little change in the stellar mass density in disk galaxies since z=1; large numbers of disk galaxies are being transformed into non-star-forming spheroid-dominated galaxies by galaxy interactions, AGN feedback, environmental effects, and other physical processes. Finally, despite this intense activity, the scaling relations of disk galaxies appear to evolve little. In particular, as individual galaxies grow in mass through the formation of stars, they appear to grow in radius (on average, the population grows inside-out), and they appear to evolve towards somewhat higher rotation velocity (i.e., mass is added at both small and large radii during this inside-out growth).Comment: Invited Review at the Vatican Symposium: Formation and Evolution of Galaxy Disks, October 2007, proceeding editors Jose G. Funes, SJ and Enrico M. Corsin

Stellar mass-to-light ratios from galaxy spectra: how accurate can they be?

Stellar masses play a crucial role in the exploration of galaxy properties and the evolution of the galaxy population. In this paper, we explore the minimum possible uncertainties in stellar mass-to-light (M/L) ratios from the assumed star formation history (SFH) and metallicity distribution, with the goals of providing a minimum set of requirements for observational studies. We use a large Monte Carlo library of SFHs to study as a function of galaxy spectral type and signal-to-noise ratio (S/N) the statistical uncertainties of M/L values using either absorption-line data or broad band colors. The accuracy of M/L estimates can be significantly improved by using metal-sensitive indices in combination with age-sensitive indices, in particular for galaxies with intermediate-age or young stellar populations. While M/L accuracy clearly depends on the spectral S/N ratio, there is no significant gain in improving the S/N much above 50/pix and limiting uncertainties of 0.03 dex are reached. Assuming that dust is accurately corrected or absent and that the redshift is known, color-based M/L estimates are only slightly more uncertain than spectroscopic estimates (at comparable spectroscopic and photometric quality), but are more easily affected by systematic biases. This is the case in particular for galaxies with bursty SFHs (high Hdelta at fixed D4000), the M/L of which cannot be constrained any better than 0.15 dex with any indicators explored here. Finally, we explore the effects of the assumed prior distribution in SFHs and metallicity, finding them to be higher for color-based estimates.Comment: accepted for publication on ApJ

The stellar populations of spiral galaxies

We have used a large sample of low-inclination spiral galaxies with radially-resolved optical and near-infrared photometry to investigate trends in star formation history with radius as a function of galaxy structural parameters. A maximum likelihood method was used to match all the available photometry of our sample to the colours predicted by stellar population synthesis models. The use of simplistic star formation histories, uncertainties in the stellar population models and regarding the importance of dust all compromise the absolute ages and metallicities derived in this work, however our conclusions are robust in a relative sense. We find that most spiral galaxies have stellar population gradients, in the sense that their inner regions are older and more metal rich than their outer regions. Our main conclusion is that the surface density of a galaxy drives its star formation history, perhaps through a local density dependence in the star formation law. The mass of a galaxy is a less important parameter; the age of a galaxy is relatively unaffected by its mass, however the metallicity of galaxies depends on both surface density and mass. This suggests that galaxy mass-dependent feedback is an important process in the chemical evolution of galaxies. In addition, there is significant cosmic scatter suggesting that mass and density may not be the only parameters affecting the star formation history of a galaxy.Comment: 25 pages; 17 figures; re-submitted to MNRAS after replying to referee's repor

A Bayesian Classifier for Photometric Redshifts: Identification of high redshift clusters

Photometric redshift classifiers provide a means of estimating galaxy redshifts from observations using a small number of broad-band filters. However, the accuracy with which redshifts can be determined is sensitive to the star formation history of the galaxy, for example the effects of age, metallicity and on-going star formation. We present a photometric classifier that explicitly takes into account the degeneracies implied by these variations, based on the flexible stellar population synthesis code of Kodama & Arimoto. The situation is encouraging since many of the variations in stellar populations introduce colour changes that are degenerate. We use a Bayesian inversion scheme to estimate the likely range of redshifts compatible with the observed colours. When applied to existing multi-band photometry for Abell 370, most of the cluster members are correctly recovered with little field contamination. The inverter is focussed on the recovery of a wide variety of galaxy populations in distant (z~1) clusters from broad band colours covering the 4000 angstrom break. It is found that this can be achieved with impressive accuracy ($|\Delta z| < 0.1$), allowing detailed investigation into the evolution of cluster galaxies with little selection bias.Comment: 18 pages, including 15 figures, Accepted for publication in MNRA