Evolution of the Most Luminous Dusty Galaxies

TL;DR

This paper analyzes the evolution of luminous dusty galaxies with active galactic nuclei across a broad redshift range, revealing luminosity scaling laws and the detectability limits of AGN and starbursts with Spitzer.

Contribution

It provides new empirical scaling relations for infrared luminosities of obscured and unobscured AGN up to z=3.2, and discusses their evolution and detectability at higher redshifts.

Findings

Luminosity scales as (1+z)^2.6 up to z=2.8.

Obscured and unobscured AGN have similar total luminosities.

Luminous starbursts are undetectable beyond z=3 with current surveys.

Abstract

A summary of mid-infrared continuum luminosities arising from dust is given for very luminous galaxies, Lir > 10^12 solar luminosities, with 0.005 < z < 3.2 containing active galactic nuclei (AGN), including 115 obscured AGN and 60 unobscured (type 1) AGN. All sources have been observed with the Spitzer Infrared Spectrograph. Obscured AGN are defined as having optical depth > 0.7 in the 9.7 um silicate absorption feature and unobscured AGN show silicate in emission. Luminosity vLv(8 um) is found to scale as (1+z)^2.6 to z = 2.8, and luminosities vLv(8 um) are approximately 3 times greater for the most luminous unobscured AGN. Total infrared luminosities for the most luminous obscured AGN, Lir(AGN_obscured) in solar luminosities, scale as log Lir(AGN_obscured) = 12.3+-0.25 + 2.6(+-0.3)log(1+z), and for the most luminous unobscured AGN, scale as log Lir(AGN1) = 12.6+-0.15 + 2.6(+-0.3)log(1+z), indicating that the most luminous AGN are about 10 times more luminous than the most luminous starbursts. Results are consistent with obscured and unobscured AGN having the same total luminosities with differences arising only from orientation, such that the obscured AGN are observed through very dusty clouds which extinct about 50% of the intrinsic luminosity at 8 um. Both obscured and unobscured AGN should be detected to z ~ 6 by Spitzer surveys with fv(24 um) > 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.