Optical line diagnostics of z ≈ 2 optically faint ultraluminous infrared galaxies in the Spitzer Bootes 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 mu m Spitzer MIPS imaging survey of the NDWFS Bootes field. We measure accurate redshifts in the range 1: 3 less than or similar to z less than or similar to 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)) H alpha or H beta 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 H alpha/H beta Balmer decrement and estimate the extinction to the emission-line region to be large for both type I and type II AGNs, with A(H alpha) greater than or similar to 2.4-5mag. 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 ( similar to kiloparsec) scales within the host galaxies. These sources may be examples of "hostobscured'' AGNs, which could have space densities comparable to or greater than that of optically luminous type I AGNs with similar bolometric luminosities.