Composite Spectral Energy Distributions and Infrared-Optical Colors of Type 1 and Type 2 Quasars

TL;DR

This study compares the spectral energy distributions and colors of Type 1 and Type 2 quasars, demonstrating that mid-infrared colors can effectively identify luminous quasars, including obscured ones, through large statistical analysis.

Contribution

It provides composite SEDs for Type 1 and 2 quasars using multi-wavelength data and shows how MIR colors can be used to select luminous and obscured quasars effectively.

Findings

Type 1 and 2 quasar SEDs are remarkably similar, with differences mainly due to dust extinction.

Luminous quasars can be selected using MIR color criteria like W1-W2 > 0.7.

Obscured quasars can be identified using optical-IR color thresholds such as (u-W3) > 1.4(W1-W2)+3.2.

Abstract

We present observed mid-infrared and optical colors and composite spectral energy distributions (SEDs) of type 1 (broad-line) and 2 (narrow-line) quasars selected from Sloan Digital Sky Survey (SDSS) spectroscopy. A significant fraction of powerful quasars are obscured by dust, and are difficult to detect in optical photometric or spectroscopic surveys. However these may be more easily identified on the basis of mid-infrared (MIR) colors and SEDs. Using samples of SDSS type 1 type 2 matched in redshift and [OIII] luminosity, we produce composite rest-frame 0.2-15 micron SEDs based on SDSS, UKIDSS, and Wide-Field Infrared Survey Explorer (WISE) photometry and perform model fits using simple galaxy and quasar SED templates. The SEDs of type 1 and 2 quasars are remarkably similar, with the differences explained primarily by the extinction of the quasar component in the type 2 systems. For both types of quasar, the flux of the AGN relative to the host galaxy increases with AGN luminosity (L_[OIII]) and redder observed MIR color, but we find only weak dependencies of the composite SEDs on mechanical jet power as determined through radio luminosity. We conclude that luminous quasars can be effectively selected using simple MIR color criteria similar to those identified previously (W1-W2 > 0.7 [Vega]), although these criteria miss many heavily obscured objects. Obscured quasars can be further identified based on optical-IR colors (for example, (u-W3 [AB]) > 1.4(W1-W2 [Vega])+3.2). These results illustrate the power of large statistical studies of obscured quasars selected on the basis of mid-IR and optical photometry.