Spectro-Polarimetric search for hidden AGNs in four southern Ultraluminous Infrared Galaxies

Abstract

We report on a spectro-polarimetric analysis of four southern Ultra-Luminous Infrared Galaxies (ULIRGs), aimed at constraining the presence of hidden broad AGN lines. For IRAS 19254--7245 ({\sl The Superantennae}) we find evidence for a significant level of polarized light in the H$α$ line with FWHM$>$2300 km/s. Some degree of polarization is also detected in IRAS 20551-4250, though with lower significance. In the two other sources (IRAS 20100--4156 and IRAS 22491--1808) no polarized signals are detected. Although it is unclear from the present data if the origin of polarization is due to reflected light from an AGN or more simply to dichroic transmission by a dust slab, we find interesting correlation between the presence of polarized components in the optical spectra and independent evidence for AGN emissions in hard X-rays and the far-IR.