Exploring the active galactic nucleus and starburst content of local ultraluminous infrared galaxies through 5-8 micron spectroscopy

TL;DR

This study analyzes 5-8 micron spectra of 71 local ULIRGs to quantify the contributions of AGN and starbursts to their infrared emission, revealing AGN signatures in 70% of the sample and an increasing AGN role with luminosity.

Contribution

It provides a new quantitative method to estimate AGN and starburst contributions in ULIRGs using mid-infrared spectroscopy, highlighting the heterogeneity in dust structure.

Findings

70% of ULIRGs show AGN signatures

AGN contributes about 23% to total luminosity

AGN contribution increases with overall luminosity

Abstract

We present a 5-8 micron analysis of the Spitzer-IRS spectra of 71 ultraluminous infrared galaxies (ULIRGs) with redshift z < 0.15, devoted to the study of the role of active galactic nuclei (AGN) and starbursts (SB) as the power source of the extreme infrared emission. Around 5 micron an AGN is much brighter (by a factor 30) than a starburst of equal bolometric luminosity. This allows us to detect the presence of even faint accretion-driven cores inside ULIRGs: signatures of AGN activity are found in 70 per cent of our sample (50/71 sources). Through a simple analytical model we are also able to obtain a quantitative estimate of the AGN/SB contribution to the overall energy output of each source. Although the main fraction of ULIRG luminosity is confirmed to arise from star formation events, the AGN contribution is non-negligible (23 per cent) and is shown to increase with luminosity. The existence of a rather heterogeneous pattern in the composition and geometrical structure of the dust among ULIRGs is newly supported by the comparison between individual absorption features and continuum extinction.