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

TL;DR

This paper develops a simple yet comprehensive model for deeply buried ultraluminous infrared galaxies (ULIRGs), explaining their observed properties and the dominance of obscured active galactic nuclei, with implications for high-redshift observations.

Contribution

The paper introduces a unified spectral energy distribution model for ULIRGs that explains their diverse mid-IR features and the dominance of obscured AGNs, aiding interpretation of high-redshift data.

Findings

Most deeply buried ULIRGs are powered by obscured AGNs.

A small unobscured line of sight can make ULIRGs appear less obscured.

Deep absorption features are filled-in by foreground PAH emission.

Abstract

We present models of deeply buried ultraluminous infrared galaxy (ULIRG) spectral energy distributions (SEDs) and use them to construct a three-dimensional diagram for diagnosing the nature of observed ULIRGs. Our goal is to construct a suite of SEDs for a very simple model ULIRG structure, and to explore how well this simple model can (by itself) explain the full range of observed ULIRG properties. We use our diagnostic to analyze archival Spitzer Space Telescope IRS spectra of ULIRGs and find that: (1) In general, our model does provide a comprehensive explanation of the distribution of mid-IR ULIRG properties; (2) >75% (in some cases 100%) of the bolometric luminosities of the most deeply buried ULIRGs must be powered by a dust-enshrouded active galactic nucleus; (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; and (4) the observed absence of deeply buried ULIRGs with large PAH equivalent widths is naturally explained by our models showing that deep absorption features are "filled-in" by small quantities of foreground unobscured PAH emission (e.g., from the host galaxy disk) at the level of ~1% the bolometric nuclear luminosity. The modeling and analysis we present will also serve as a powerful tool for interpreting the high angular resolution spectra of high-redshift sources to be obtained with the James Webb Space Telescope.