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New Results from a Near-Infrared Search for Hidden Broad-Line Regions in Ultraluminous Infrared Galaxies

This paper reports the latest results from a near-infrared search for hidden broad-line regions (BLRs: ΔVFWHM ≳ 2000 km s-1) in ultraluminous infrared galaxies (ULIGs). The new sample contains 39 ULIGs from the 1 Jy sample selected for their lack of BLRs at optical wavelengths. Broad Paα emission is detected for the first time in two sources—F05189-2524 and F13305-1739. Broad Paα emission may also be present in three other sources—F13443+0802SW, F14394+5332, and F16156+0146—but new data are needed to make sure that H2 1.8665, 1.8721 μm are not contributing to this excess emission. The [Si VI] feature, a strong indicator of AGN activity, appears to be present in one object—F13305-1739—and perhaps also in Mrk 273 and F13454-2956. In addition, the presence of a hidden BLR is confirmed in the lower luminosity source F11058-1131. The results from this new study are combined with those from our previously published survey to produce a large database on 64 (non-Seyfert 1) ULIGs from the 1 Jy sample. All of the galaxies with strong evidence for a hidden BLR at near-infrared wavelengths present an optical Seyfert 2 spectrum. Overall, at least 50% (and perhaps up to 70%) of the optical Seyfert 2 galaxies in the combined sample present either a BLR or strong [Si VI] emission. In contrast, none of the 41 optically classified LINERs and H II galaxies in the sample shows any obvious signs of an energetically important AGN. Galaxies with "warm" IRAS colors (f25/f60 ≳ 0.2) show a tendency to harbor obscured BLRs in the near-infrared and to have large Paα-to-infrared luminosity ratios. These results support those of our earlier survey and suggest that the screen of dust in most warm Seyfert 2 galaxies is optically thin at 2 μm. 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 118 ULIGs, 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 at least 20%-25% but reaches 35%-50% for objects with LIR > 1012.3 L☉. Comparisons of the dereddened emission-line luminosities of the optical or obscured BLRs detected in the ULIGs of the 1 Jy sample with those of optical quasars indicate that the obscured AGNs/quasars in ULIGs is the main source of energy in at least 15%-25% of all ULIGs in the 1 Jy sample. This fraction is closer to 30%-50% among ULIGs with LIR > 1012.3 L☉. These results are compatible with those from recent mid-infrared spectroscopic surveys carried out with ISO.