2009ApJ...698.1682W

A summary of mid-infrared continuum luminosities arising from dust is given for very luminous galaxies, L_IR> 10¹² L_☉, with 0.005 <z < 3.2 containing active galactic nuclei (AGNs), including 115 obscured AGNs and 60 unobscured (type 1) AGNs. All sources have been observed with the Spitzer Infrared Spectrograph. Obscured AGNs are defined as having optical depth τ > 0.7 in the 9.7 µm silicate absorption feature (i.e., half of the continuum is absorbed) and having equivalent width of the 6.2 µm polycyclic aromatic hydrocarbon feature < 0.1 µm (to avoid sources with a significant starburst component). Unobscured AGNs are defined as those that show silicate in emission. Luminosity νL_ν(8 µm) for the most luminous obscured AGNs is found to scale as (1+z)^2.6 to z = 2.8. For unobscured AGNs, the scaling with redshift is similar, but luminosities νL _ν(8 µm) are approximately three times greater for the most luminous sources. Using both obscured and unobscured AGNs having total infrared fluxes from the Infrared Astronomical Satellite, empirical relations are found between νL_ν(8 µm) and L_IR. Combining these relations with the redshift scaling of luminosity, we conclude that the total infrared luminosities for the most luminous obscured AGNs, L_IR(AGN_obscured) in L_☉, scale as log L_IR(AGN_obscured) = 12.3±0.25 + 2.6(±0.3)log(1+z), and for the most luminous unobscured AGNs, scale as log L_IR(AGN1) = 12.6(±0.15) + 2.6(±0.3)log(1+z). We previously determined that the most luminous starbursts scale as log L_IR(SB) = 11.8±0.3 + 2.5(±0.3)log(1+z), indicating that the most luminous AGNs are about 10 times more luminous than the most luminous starbursts.
Result. are consistent with obscured and unobscured AGNs having the same total luminosities with differences arising only from orientation, such that the obscured AGNs are observed through very dusty clouds which extinct about 50% of the intrinsic luminosity at 8 µm. Extrapolations of observable f_ν(24 µm) to z = 6 are made using evolution results for these luminous sources. Both obscured and unobscured AGNs should be detected to z ∼ 6 by Spitzer surveys with f_ν(24 µm) > 0.3 mJy, even without luminosity evolution for z > 2.5. By contrast, the most luminous starbursts cannot be detected for z > 3, even if luminosity evolution continues beyond z = 2.5.