PANCHROMATIC SPECTRAL ENERGY DISTRIBUTIONS OF DUSTY GALAXIES WITH RADISHE. I. PREDICTIONS FOR HERSCHEL: CORRELATING COLORS WITH GALACTIC ENERGY SOURCES

We present three-dimensional, self-consistent radiative transfer solutions with a new Monte Carlo radiative equilibrium code. The code RADISHE can be applied to calculate the emergent spectral energy distributions (SEDs) and broadband images from optical to millimeter wavelengths of arbitrary density geometries with distributed sources of radiation. One of the primary uses of this code has been to interface with hydrodynamical codes to calculate emergent SEDs along a simulation time sequence. We focus the applications of this code in this paper on infrared bright dusty galaxies, but RADISHE is also ideal for calculating emission from star clusters or protostellar environments. The primary methodological focus of this paper is on the radiative equilibrium temperature calculation. We find that an iterative calculation of the temperature, which takes the sum of photon flight paths as the Monte Carlo estimator for the mean free intensity, is significantly faster than relaxation temperature calculation methods, particularly when large numbers of grid cells are required (i.e., in modeling three-dimensional geometries such as the dust envelopes of turbulent massive protostellar cores or infrared-bright galaxies). An accurate long-wavelength SED and corresponding temperature calculation will be essential for analyzing upcoming Herschel Space Observatory observations. We present simulated color-color plots for infrared-bright galaxies at a range of redshifts, and unfold these plots as color versus the fractional active galactic nucleus (AGN) luminosity, to demonstrate that Herschel will be able to effectively discriminate between submillimeter galaxies, where the energy source is dominated by AGN, and those where star formation dominates. We demarcate in particular the "Class II" or energetically active AGN evolutionary phase in Herschel color-color plots.