Morphologies of high-redshift, dust-obscured galaxies from keck laser guide star adaptive optics

Spitzer MIPS images in the Boötes field of the NOAO Deep Wide-Field Survey have revealed a class of extremely dust-obscured galaxy (DOG) at z 2. The DOGs are defined by very red optical to mid-infrared (IR; observed-frame) colors, R - [24 μm]>14 mag, i.e. f _ν(24 μm)/f _ν(R)>1000. They are ultra-luminous infrared galaxies with L _{8-1000 μm} > 10¹²-10¹⁴ L, but typically have very faint optical (rest-frame UV) fluxes. We imaged three DOGs with the Keck laser guide star adaptive optics (LGSAO) system, obtaining 0.06″ resolution in the K′-band. One system was dominated by a point source, while the other two were clearly resolved. Of the resolved sources, one can be modeled as a exponential disk system. The other is consistent with a de Vaucouleurs profile typical of elliptical galaxies. The nonparametric measures of their concentration and asymmetry show the DOGs to be both compact and smooth. The AO images rule out double nuclei with separations of greater than 0.1″ (<1 kpc at z = 2), making it unlikely that ongoing major mergers (mass ratios of 1/3 and greater) are triggering the high-IR luminosities. By contrast, high-resolution images of z 2 SCUBA sources tend to show multiple components and a higher degree of asymmetry. We compare near-IR morphologies of the DOGs with a set of z = 1 luminous infrared galaxies (LIRGs; L _IR 10¹¹ L) imaged with Keck LGSAO by the Center for Adaptive Optics Treasury Survey. The DOGs in our sample have significantly smaller effective radii, 1/4 the size of the z = 1 LIRGs, and tend toward higher concentrations. The small sizes and high concentrations may help explain the globally obscured rest-frame blue-to-UV emission of the DOGs.