The Dependence of the Mean Spectral Energy Distributions on the Accretion Rate for Quasars with $z < 0.75$ from the Sloan Digital Sky Survey

TL;DR

This study constructs mean spectral energy distributions for a large quasar sample, revealing how SED shapes vary with accretion rate, spectral features, and viewing angle, highlighting the complexity of quasar emission mechanisms.

Contribution

It provides the first comprehensive analysis of how quasar SEDs depend on accretion rate and spectral properties using a large SDSS sample.

Findings

Quasars with higher $R_{Fe II}$ have redder UV, optical, MIR, and NIR continua.

Increasing Eddington ratio or accretion rate makes MIR, NIR, and UV continua redder.

Optical continuum becomes bluer with higher accretion rates, indicating complex emission regions.

Abstract

We construct mean spectral energy distributions (SEDs) for a substantial sample of 56,969 Sloan Digital Sky Survey DR16 quasars with $z < 0.75$, utilizing multiwavelength data from the mid-infrared (MIR) to ultraviolet (UV). These SEDs are built on eigenvector 1 parameters -- the relative optical $\rm Fe~ II$ strength ($R_{\rm Fe~II}$) and the H$\beta$ line width ($\rm H\beta$) -- that capture the principal spectral variance of quasar spectra. From three $R_{\rm Fe~II}$-dependent mean SEDs we find that quasars with a larger $R_{\rm Fe~II}$ exhibit redder UV and optical and redder MIR and near-infrared (NIR) continua, indicating more dust emission. We also split our sample directly into Eddington ratio $L_{\rm Bol} /L_{\rm Edd}$ (or dimensionless accretion rate $\dot{\mathscr{M}}$) bins to construct different mean SEDs and find that the continua become increasingly red with increasing $L_{\rm Bol} /L_{\rm Edd}$ (or $\dot{\mathscr{M}}$) in the MIR, NIR, and UV bands. This demonstrates that the shapes of Type 1 AGN SEDs depend on the accretion rate. However, the optical continuum shows the opposite trend (becoming harder and bluer), indicating the complexity of the optical emission region. From $\rm FWHM_{H\beta}$-dependent mean SEDs we find that quasars with a larger $\rm FWHM_{H\beta}$ show redder optical and NIR continua and bluer UV and MIR continua. The bluer MIR continuum suggests that a larger angle between of the line of sight and the torus plane results in weaker torus emission in the MIR.