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

Abstract

We present near-infrared spectroscopic observations for a sample of ten optically faint luminous infrared galaxies (R-[24]> 14) using Keck NIRSPEC and Gemini NIRI. The sample is selected from a 24 micron Spitzer MIPS imaging survey of the NDWFS Bootes 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 ten galaxies harbor luminous AGN. Seven sources are type I AGN, exhibiting broad (>1900 km/s) Halpha or Hbeta emission lines; the remaining three are type II AGN. 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 AGN. The visibility of broad lines in 70% of the sources suggests that it is unlikely that these AGN are being viewed through the mid-plane of a dusty torus. For four of the sources we constrain the Halpha/Hbeta Balmer decrement and estimate the extinction to the emission line region to be large for both type I and type II AGN, with A_Halpha > 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 (~kpc) scales within the host galaxies. These sources may be examples of "host-obscured" AGN which could have space densities comparable or greater to that of optically luminous type I AGN with similar bolometric luminosities.