Optical Line Diagnostics ofz≈ 2 Optically Faint Ultraluminous Infrared Galaxies in theSpitzerBootes Survey

We present near-infrared spectroscopic observations for a sample of tenoptically faint luminous infrared galaxies (R-[24]> 14) using Keck NIRSPEC andGemini NIRI. The sample is selected from a 24 micron Spitzer MIPS imagingsurvey of the NDWFS Bootes field. We measure accurate redshifts in the range1.31900 km/s) Halpha or Hbeta emission lines; the remainingthree are type II AGN. Given their large mid-IR luminosities and faint opticalmagnitudes, we might expect these sources to be heavily extincted quasars, andtherefore only visible as type II AGN. The visibility of broad lines in 70% ofthe sources suggests that it is unlikely that these AGN are being viewedthrough the mid-plane of a dusty torus. For four of the sources we constrainthe Halpha/Hbeta Balmer decrement and estimate the extinction to the emissionline region to be large for both type I and type II AGN, with A_Halpha > 2.4-5mag. Since the narrow-line region is also extincted and the UV continuumemission from the host galaxies is extremely faint, this suggests that much ofthe obscuration is contributed by dust on large (~kpc) scales within the hostgalaxies. These sources may be examples of "host-obscured" AGN which could havespace densities comparable or greater to that of optically luminous type I AGNwith similar bolometric luminosities.Comment: 13 pages, ApJ accepte