quantitative determination of the AGN content in local ULIRGs through L-band spectroscopy

TL;DR

This study quantitatively estimates the AGN and starburst contributions in local ULIRGs using L-band spectroscopy, revealing that most are starburst-powered with a minority hosting significant AGN activity.

Contribution

It introduces a diagnostic method combining L-band spectra and deconvolution to quantify AGN and starburst contributions in ULIRGs, providing new insights into their power sources.

Findings

At least 60% of ULIRGs host an active nucleus.

AGN contributes only 15-20% to the bolometric luminosity.

Most ULIRGs are primarily powered by starburst activity.

Abstract

We present a quantitative estimate of the relative AGN/starburst content in a sample of 59 nearby (z<0.15) infrared bright ULIRGs taken from the 1 Jy sample, based on infrared L-band (3-4 micron) spectra. By using diagnostic diagrams and a simple deconvolution model, we show that at least 60% of local ULIRGs contain an active nucleus, but the AGN contribution to the bolometric luminosity is relevant only in ~15-20% of the sources. Overall, ULIRGs appear to be powered by the starburst process, responsible for >85% of the observed infrared luminosity. The subsample of sources optically classified as LINERs (31 objects) shows a similar AGN/starburst distribution as the whole sample, indicating a composite nature for this class of objects. We also show that a few ULIRGs, optically classified as starbursts, have L-band spectral features suggesting the presence of a buried AGN.