Obscuring active galactic nuclei with nuclear starburst disks

We assess the potential of nuclear starburst disks to obscure the Seyfert-like AGNs that dominate the hard X-ray background at z similar to 1. Over 1200 starburst disk models, based on the theory developed by Thompson et al., are calculated for five input parameters: the black hole mass, the radial size of the starburst disk, the dust-to-gas ratio, the efficiency of angular momentum transport in the disk, and the gas fraction at the outer disk radius. We find that a large dust-to-gas ratio, a relatively small starburst disk, a significant gas mass fraction, and efficient angular momentum transport are all important to produce a starburst disk that can potentially obscure an AGN. The typical maximum star formation rate in the disks is similar to 10 M-circle dot yr(-1). Assuming no mass loss due to outflows, the starburst disks feed gas onto the black hole at rates sufficient to produce hard X-ray luminosities of 10(43)-10(44) erg s(-1). The starburst disks themselves should be detectable at mid-infrared and radio wavelengths; at z - 0.8, the predicted fluxes are similar to 1 mJy at 24 mu m and similar to 10-30 mu Jy at 1.4 GHz. Thus, we predict a large fraction of radio/X-ray matches in future deep radio surveys. The starburst disks should be easily distinguished from AGNs in future 100 mu m surveys by the Herschel Space Observatory with expected fluxes of similar to 5 mJy. Any AGN-obscuring starbursts will be associated with hot dust, independent of AGN heating, resulting in observable signatures for separating galactic and nuclear star formation. Finally, because of the competition between gas and star formation, nuclear starbursts will be associated with lower luminosity AGNs. Thus, this phenomenon is a natural explanation for the observed decrease in the fraction of obscured AGNs with luminosity.