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

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.