A Near‐Infrared Search for Hidden Broad‐Line Regions in Ultraluminous Infrared Galaxies

Abstract

We report the results of a sensitive new near-infrared search for hidden broad-line regions (BLRs: ΔVFWHM ≳ 2000 km s-1) in a sample of 25 ultraluminous infrared galaxies (ULIGs) selected for their lack of BLRs at optical wavelengths. These objects constitute a representative subset of the (non-Seyfert 1) 1 Jy sample of 111 ULIGs (Kim & Sanders), both in terms of their redshift and infrared luminosity distributions. In contrast to previous studies of ULIGs at lower redshift, the redshifts for our current subsample (z ~ 0.1-0.2) allow us to search for broad-line emission from the strong Paα λ1.8751 μm and [Si VI] λ1.962 μm emission lines, two powerful AGN diagnostic lines which are generally inaccessible in lower redshift objects. Broad Paα emission is detected for the first time in two sources—PKS 1345+12, F23499+2423 (object names that begin with "F" are sources identified in the IRAS Faint Source Catalog, Version 2), and the presence of a hidden BLR is confirmed in two additional sources—F20460+1925, F23060+0505. Broad Paα emission may also be present in three other sources—F08559+1053, F17179+5444, F23233+2817—but new data are needed to make sure that H2 λλ1.8665, 1.8721 is not contributing to this excess emission. In addition, the [Si VI] feature appears to be present in three objects—F12072-0444, PKS 1345+12, F23233+2817—and perhaps also in F17179+5444.