Paper detail

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

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.