Physically Modeling High-Redshift Ultraluminous Infrared Galaxies

Abstract

We have used a combination of hydrodynamical simulations, dust radiative transfer, and an empirically based analytical model for galaxy number densities and merger rates in order to physically model the bright high-redshift submillimeter-selected galaxy (SMG) population. We report the results of three projects: In the first we study the dependence of a galaxy’s observed-frame submillimeter (submm) flux on its physical properties. One of our principal conclusions is that the submm flux scales significantly more weakly with star formation rate for starbursts than for quiescently star-forming galaxies. Consequently, we argue that the SMG population is not exclusively merger-induced starbursts but rather a mix of merger-induced starbursts, early-stage mergers where two quiescently star-forming disk galaxies are blended into one submm source ("galaxy-pair SMGs"), and isolated disk galaxies. In the second work we present testable predictions of this model by demonstrating how quiescently star-forming and starburst SMGs can be distinguished from integrated data alone. Starbursts tend to have higher luminosity, effective dust temperature, global star formation efficiency \((L_{IR}/M_{gas})\), and infrared excess \((L_{IR}/L_{FUV})\) and tend to lie significantly above the star formation rate-stellar mass relation defined by quiescently star-forming galaxies. These diagnostics can be used to observationally determine the relative contribution of quiescently star-forming and starburst galaxies to the SMG population. In the final work we present the SMG number density, cumulative number counts, and redshift distribution predicted by our model. We show that, contrary to previous claims, the observed SMG number counts do not provide evidence for a top-heavy initial mass function. We also show that starbursts and galaxy-pair SMGs both contribute significantly to the bright SMG counts, whereas isolated disks contribute significantly only at the faint end.