3-5 micron spectroscopy of obscured AGNs in ULIRGs

TL;DR

This study uses infrared spectroscopy to analyze the obscuration and characteristics of AGNs in ULIRGs, revealing correlations between continuum slope and CO absorption, and showing AGN dominance in near-infrared emission.

Contribution

First detailed infrared spectroscopic analysis distinguishing obscured and unobscured AGNs in ULIRGs with new insights into their spectral features.

Findings

Obscured AGNs show steep, reddened continua with absorption features.

Unobscured AGNs have flat continua without absorption features.

Deep CO absorption correlates with highly obscured AGNs.

Abstract

We present the results of infrared L-band (3-4 micron) and M-band (4-5 micron) VLT-ISAAC spectroscopy of five bright Ultraluminous InfraRed Galaxies (ULIRGs) hosting an AGN. From our analysis we distinguish two types of sources: ULIRGs where the AGN is unobscured (with a flat continuum and no absorption features at 3.4 micron and 4.6 micron), and those with highly obscured AGNs (with a steep, reddened continuum and absorption features due to hydrocarbons and CO). Starburst activity is also present in all of the sources as inferred from the 3.3 micron PAH emission line. A strong correlation is found between continuum slope and CO optical depth, which suggests that deep carbon monoxide absorption is a common feature of highly obscured ULIRG AGN. Finally we show that the AGN dominates the 3-4 micron emission, even if its contribution to the bolometric luminosity is small.