A near-infrared search for hidden broad-line regions in ultraluminous infrared galaxies

We report the results of a sensitive new near-infrared search for hidden bread-line regions (BLRs: Delta V-FWHM greater than or similar to 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 similar to 0.1-0.2) allow us to search for broad-line emission from the strong Pa alpha lambda 1.8751 mu m and [Si VI] lambda 1.962 mu m emission lines, two powerful AGN diagnostic lines which are generally inaccessible in lower redshift objects. Broad Pa alpha 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 cofirmed in two additional sources-F20460 + 1925, F23060 + 0505. Broad Pa alpha emission may also be present in three other sources-F08559 + 1053, F17179 + 5444, F23233 + 2817-but new data are needed to make sure that H-2 lambda lambda 1.8665, 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. Combining our new data with previously published spectra for Mrk 463E, we find that all of the galaxies with evidence for a hidden BLR at near-infrared wavelengths present an optical Seyfert 2 spectrum. Overall, seven (and perhaps nine) of the 10 optical Seyfert 2 galaxies in our sample present either a BLR or strong [Si VI] emission. Also, galaxies with ''warm'' IRAS colors (f(25)/f(60) > 0.2) (the quantities f(25), f(60) are the IRAS flux densities in Jy at 25 and 60 mu m, respectively) show a tendency to harbor obscured BLRs in the near-infrared and to have large Pa alpha-to-infrared luminosity ratios. These results suggest that the screen of dust in most ''warm'' Seyfert 2 galaxies is optically thin at 2 mu m. However, no correlation is found between narrow-line extinctions and the presence of a hidden BLR. The narrow emission lines in our spectra mainly reflect the properties of the circumnuclear gas rather than the dust content of the material along the line of sight to the nucleus. The dereddened emission-line luminosities of the obscured BLRs detected in the ''warm'' Seyfert 2 ULIGs are similar to those of optically selected quasars at similar bolometric luminosities, which suggests that an important fraction of the bolometric luminosity in these ULIGs is powered by the same mechanism as that in optical quasars. These objects therefore truly appear to be ''buried quasars,'' spanning the gap between ''cool'' ULIGs and optical quasars. None of the 15 optically classified LINERs and H II galaxies in our sample shows any obvious signs of an obscured BLR or strong [Si VI] emission. The LINERs typically have ''cool'' IRAS colors (f(25)/f(60) < 0.2) and a deficit of Pa alpha emission, which suggest that the optical thickness due to dust is still significant at 2 mu m and may be sufficient to hide an AGN in the cores of these ULIGs. The H II galaxies span a wide range of f(25)/f(60) ratios and are strong emitters of narrow Pa alpha. Dust obscuration therefore appears to be relatively unimportant in many of these objects. When the results from this near-infrared survey are combined with those from a recent optical spectroscopic study of the entire 1 Jy sample of 111 ULIGs (Veilleux, Kim, & Sanders), we find that the fraction of all ULIGs with optical or near-infrared signs of genuine AGN activity (either a BLR or [Si VI] emission) is 25%-30%. This fraction increases with increasing infrared luminosity.