EVOLUTION OF THE MOST LUMINOUS DUSTY GALAXIES

A summary of mid-infrared continuum luminosities arising from dust is given for very luminous galaxies, L(IR) > 10(12)L(circle dot), 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 tau > 0.7 in the 9.7 mu m silicate absorption feature (i.e., half of the continuum is absorbed) and having equivalent width of the 6.2 mu m polycyclic aromatic hydrocarbon feature < 0.1 mu m (to avoid sources with a significant starburst component). Unobscured AGNs are defined as those that show silicate in emission. Luminosity nu L(nu) (8 mu m) 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 nu L(nu) (8 mu m) 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 nu L(nu) (8 mu m) 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(circle dot), 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. Results 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 mu m. Extrapolations of observable f(nu)(24 mu m) to z = 6 are made using evolution results for these luminous sources. Both obscured and unobscured AGNs should be detected to z similar to 6 by Spitzer surveys with f(nu)(24 mu m) > 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.