Red Type-1 Quasars after Cosmic Noon and Impact on $L_{\rm UV}$-related Quasar Statistics

TL;DR

This study investigates the prevalence of red, dust-obscured type-1 quasars using spectral energy distribution fitting, revealing their impact on quasar luminosity functions and suggesting future surveys could uncover more obscured quasars.

Contribution

It provides the first comprehensive analysis of red type-1 quasars' fraction and their effect on quasar statistics using optical-to-MIR data and SED fitting.

Findings

Approximately 10% of type-1 quasars are red due to dust obscuration.

Red quasars are more common among brighter MIR luminosities.

Reddening correction slightly increases the bright-end slope of the quasar luminosity function.

Abstract

Over the past decades, nearly a million quasars have been explored to shed light on the evolution of supermassive black holes and galaxies. The ultraviolet-to-optical spectra of type-1 quasars particularly offer insights into their black hole activities. Recent findings, however, raise questions about the prevalence of red type-1 quasars of which colors might be due to dust-obscuration and their potential influence on luminosity-related properties of quasars. We examine the fraction of red type-1 quasars within the redshift range of $0.68\leq z < 2.20$, applying spectral energy distribution (SED) fitting using optical-to-MIR photometric data of Sloan Digital Sky Survey Data Release 14 quasars. Approximately 10\,\% of the type-1 quasars exhibit red colors suggestive of dust obscuration. There is an association between the brightness of the MIR luminosity and a higher fraction of red type-1 quasars, albeit with negligible redshift evolution. By employing $E(B-V)$ values from the SED fitting, we obtained dereddened luminosity of the red type-1 quasars and reassess the quasar luminosity function (QLF) and black hole mass ($M_{\rm BH}$) estimates. Result shows a modest increase in the number density of bright quasars, linking to more flatten bright-end slope of QLFs, while $M_{\rm BH}$ adjustments are minimal. Current SDSS selections in optical could miss a significant population of heavily dust-obscured quasars. As future MIR surveys like SPHEREx expand, they may reveal enough obscured quasars to prompt a more profound revision of fundamental quasar properties.