WHAT DOES A SUBMILLIMETER GALAXY SELECTION ACTUALLY SELECT? THE DEPENDENCE OF SUBMILLIMETER FLUX DENSITY ON STAR FORMATION RATE AND DUST MASS

Abstract

We perform 3-D dust radiative transfer (RT) calculations on hydrodynamic\nsimulations of isolated and merging disk galaxies in order to quantitatively\nstudy the dependence of observed-frame submillimeter (submm) flux density on\ngalaxy properties. We find that submm flux density and star formation rate\n(SFR) are related in dramatically different ways for quiescently star-forming\ngalaxies and starbursts. Because the stars formed in the merger-induced\nstarburst do not dominate the bolometric luminosity and the rapid drop in dust\nmass and more compact geometry cause a sharp increase in dust temperature\nduring the burst, starbursts are very inefficient at boosting submm flux\ndensity (e.g., a $\\ga16$x boost in SFR yields a $\\la 2$x boost in submm flux\ndensity). Moreover, the ratio of submm flux density to SFR differs\nsignificantly between the two modes; thus one cannot assume that the galaxies\nwith highest submm flux density are necessarily those with the highest\nbolometric luminosity or SFR. These results have important consequences for the\nbright submillimeter-selected galaxy (SMG) population. Among them are: 1. The\nSMG population is heterogeneous. In addition to merger-driven starbursts, there\nis a subpopulation of galaxy pairs, where two disks undergoing a major merger\nbut not yet strongly interacting are blended into one submm source because of\nthe large ($\\ga 15$", or $\\sim 130$ kpc at $z = 2$) beam of single-dish submm\ntelescopes. 2. SMGs must be very massive ($M_{\\star} \\ga 6 \\times 10^{10}\n\\msun$). 3. The infall phase makes the SMG duty cycle a factor of a few greater\nthan what is expected for a merger-driven starburst. (Abridged.)\n