The Nature of Deeply Buried Ultraluminous Infrared Galaxies: A Unified Model for Highly Obscured Dusty Galaxy Emission

We present radiative transfer models of deeply buried ultraluminous infrared galaxy (ULIRG) spectral energy distributions and use them to construct a three-dimensional diagram for diagnosing the nature of ULIRG nuclei. Our diagnostic is based upon the properties dominating mid-IR continua of low-redshift ULIRGs: continuum slope, PAH equivalent width, and silicate feature strength. We use our diagnostic to analyze archival Spitzer Space Telescope IRS spectra of ULIRGs and find that: (1) >75% (in some cases 100%) of the bolometric luminosities of the most deeply buried ULIRGs must be powered by a hidden active galactic nucleus; (2) the observed absence of deeply buried ULIRGs with large PAH equivalent widths is naturally explained by our models showing that deep absorption features are quickly "filled-in" by small quantities of unobscured PAH emission at the level of ~1% the bolometric nuclear luminosity (e.g., as emitted by the host galaxy disk); and (3) an unobscured "keyhole" view through <~10% of the obscuring medium surrounding a deeply buried ULIRG is sufficient to make it appear nearly unobscured in the mid-IR. This modeling and analysis of deeply buried galaxy spectra also provides a powerful tool for interpreting the mid-IR spectra of high-redshift sources to be obtained with superb angular resolution using the James Webb Space Telescope.