ApJ, in press, astro-ph/0509641 Bruzual
- M. Franx
We introduce Spoc, a new code for constraining the physical properties of observed galaxies through a Bayesian likelihood comparison with galaxies drawn from simulations. Spoc inputs an object’s photometry and outputs probability distributions of stellar mass, star formation rate (SFR), age, metallicity, dust extinction, and redshift (if none is given) for that galaxy. We apply Spoc, employing model galaxies drawn from ΛCDM cosmological hydrodynamic simulations, to Abell 2218 KESR (z ≈ 6.7) and five other z > 5.5 galaxies for which published rest-frame ultraviolet and optical measurements are available. We compare the outcome of using our simulated galaxies’ star formation histories (SFHs) versus using simple one-parameter SFHs such as constant, exponentially-decaying, and rising (a new form we introduce motivated by typical SFHs seen in our simulated galaxies). We show that simulated galaxies match these observations at least as well as simple SFHs, with similar favored values obtained for the intrinsic physical parameters such as stellar mass and SFR, but with substantially smaller uncertainties. This shows that the existence of galaxies at z > 5.5 with properties as observed is straightforwardly accomodated within current hierarchical structure formation scenarios. Our Spoc-derived photometric redshifts agree well with the available spectroscopic redshifts. We examine several models for galactic outflows and reddening, and show that most inferred physical properties are insensitive to these choices. Hence Spoc provides a robust tool for optimally utilizing hydrodynamic simulations (or any model that predicts galaxy SFHs) to constrain the physical properties of individual galaxies having only photometric data.