Spectral decomposition of starbursts and AGNs in 5-8 micron Spitzer IRS spectra of local ULIRGs

TL;DR

This study analyzes 5-8 micron spectra of 68 local ULIRGs to distinguish and quantify the contributions of AGNs and starbursts, revealing that star formation dominates but AGNs are prevalent.

Contribution

The paper introduces a spectral decomposition method that effectively separates AGN and starburst components in mid-IR spectra of ULIRGs, providing quantitative luminosity estimates.

Findings

Star formation accounts for ~85% of ULIRG luminosity.

AGNs are ~30 times brighter at 6 microns than starbursts with the same bolometric luminosity.

Majority of ULIRGs host an AGN despite starburst dominance.

Abstract

We present an analysis of the 5-8 micron Spitzer-IRS spectra of a sample of 68 local Ultraluminous Infrared Galaxies (ULIRGs). Our diagnostic technique allows a clear separation of the active galactic nucleus (AGN) and starburst (SB) components in the observed mid-IR emission, and a simple analytic model provides a quantitative estimate of the AGN/starburst contribution to the bolometric luminosity. We show that AGNs are ~30 times brighter at 6 micron than starbursts with the same bolometric luminosity, so that even faint AGNs can be detected. Star formation events are confirmed as the dominant power source for extreme infrared activity, since ~85% of ULIRG luminosity arises from the SB component. Nonetheless an AGN is present in the majority (46/68) of our sources.