Investigating the central engine and excitation mechanisms of ultraluminous infrared galaxies: near infrared spectroscopy

Abstract

We present NIR observations of a sample of mainly interacting ULIRGs, comprising H and K band spectra. Our main aims are to investigate the power source of these extremely luminous objects and the various excitation mechanisms of the strong molecular hydrogen emission often seen in such objects. Broadened emission lines were only detected in one object, IRAS 23498, consistent with previous results for this galaxy. The [SiVI] emission line was detected in IRAS17179 and IRAS20210, both classified as Sy2s. Two of the sample were unclassified, IRAS00150 and IRAS23420, which exhibit neither [SiVI] emission or broadened HI emission. However this does not rule out the presence of an obscured AGN. Analysis of the molecular hydrogen emission showed that the major excitation mechanism for most objects was thermal. Modelling of the more luminous objects indicate for IRAS20210 10 per cent, and for both IRAS23365 and IRAS23420, 30 per cent of their 1-0S(1) line emission has a non-thermal origin.