Seeking the Epoch of Maximum Luminosity for Dusty Quasars

TL;DR

This study investigates the maximum luminosity epoch of dusty quasars across redshifts up to 5, finding no clear maximum and suggesting most luminous quasars are less obscured, with implications for quasar evolution and obscuration.

Contribution

It provides new infrared luminosity measurements for SDSS quasars, identifies the lack of a maximum luminosity epoch below z=5, and compares obscured and unobscured quasar populations at z~2.1.

Findings

Infrared luminosity plateaus for z >~ 3 with L >~ 10^{47} erg/s.

No evidence of a maximum luminosity epoch below z=5.

Comparable numbers of obscured and unobscured quasars at z~2.1.

Abstract

Infrared luminosities vLv(7.8 um) arising from dust reradiation are determined for Sloan Digital Sky Survey (SDSS) quasars with 1.4 < z < 5 using detections at 22 um by the Wide-Field Infrared Survey Explorer. Infrared luminosity does not show a maximum at any redshift z < 5, reaching a plateau for z >~ 3 with maximum luminosity vLv(7.8 um) >~ 10^{47} erg per s; luminosity functions show one quasar per cubic Gpc having vLv(7.8 um) > 10^{46.6} erg per s for all 2 < z < 5. We conclude that the epoch when quasars first reached their maximum luminosity has not yet been identified at any redshift below 5. The most ultraviolet luminous quasars, defined by rest frame vLv(0.25 um), have the largest values of the ratio vLv(0.25 um)/vLv(7.8 um) with a maximum ratio at z = 2.9. From these results, we conclude that the quasars most luminous in the ultraviolet have the smallest dust content and appear luminous primarily because of lessened extinction. Observed ultraviolet/infrared luminosity ratios are used to define "obscured" quasars as those having > 5 magnitudes of ultraviolet extinction. We present a new summary of obscured quasars discovered with the Spitzer Infrared Spectrograph and determine the infrared luminosity function of these obscured quasars at z ~ 2.1. This is compared with infrared luminosity functions of optically discovered, unobscured quasars in the SDSS and in the AGN and Galaxy Evolution Survey. The comparison indicates comparable numbers of obscured and unobscured quasars at z ~ 2.1 with a possible excess of obscured quasars at fainter luminosities.