Optical Line Diagnostics of z ≈ 2 Optically Faint Ultraluminous Infrared Galaxies in the Spitzer Boötes Survey

We present near-infrared spectroscopic observations for a sample of 10 optically faint luminous infrared galaxies (R - [24] ≥ 14) using Keck NIRSPEC and Gemini NIRI. The sample is selected from a 24 μm Spitzer MIPS imaging survey of the NDWFS Boötes field. We measure accurate redshifts in the range 1.3 ≲ z ≲ 3.4. Based on either emission-line widths or line diagnostics, we find that all 10 galaxies harbor luminous active galactic nuclei (AGNs). Seven sources are type I AGNs, exhibiting broad (>1900 km s-1) Hα or Hβ emission lines; the remaining three are type II AGNs. Given their large mid-IR luminosities and faint optical magnitudes, we might expect these sources to be heavily extincted quasars, and therefore only visible as type II AGNs. The visibility of broad lines in 70% of the sources suggests that it is unlikely that these AGNs are being viewed through the midplane of a dusty torus. For four of the sources we constrain the Hα/Hβ Balmer decrement and estimate the extinction to the emission-line region to be large for both type I and type II AGNs, with AHα ≳ 2.4-5 mag. Since the narrow-line region is also extincted and the UV continuum emission from the host galaxies is extremely faint, this suggests that much of the obscuration is contributed by dust on large (~kiloparsec) scales within the host galaxies. These sources may be examples of "host-obscured" AGNs, which could have space densities comparable to or greater than that of optically luminous type I AGNs with similar bolometric luminosities.