The Composite Spectral Energy Distribution of Quasars is Surprisingly Universal Since Cosmic Noon

TL;DR

This study constructs composite spectral energy distributions of quasars across a wide redshift range, revealing a surprising universality and redshift independence in their properties from optical to ultraviolet wavelengths.

Contribution

It demonstrates that quasar SEDs are remarkably consistent across redshift, black hole mass, and Eddington ratio, and supports a truncated disk model over the standard thin disk for the quasar central engine.

Findings

Quasar SEDs are redder at shorter wavelengths than previously thought.

The composite SEDs are redshift-independent from optical to UV.

A truncated disk model is favored for the quasar central engine.

Abstract

Leveraging the photometric data of the Sloan Digital Sky Survey and the Galaxy Evolution Explorer (GALEX), we construct mean/median spectral energy distributions (SEDs) for unique bright quasars in redshift bins of 0.2 and up to $z \simeq 3$, after taking the GALEX non-detection into account. Further correcting for the absorption of the intergalactic medium, these mean/median quasar SEDs constitute a surprisingly redshift-independent mean/median composite SED from the rest-frame optical down to $\simeq 500~{\rm \mathring A}$ for quasars with bolometric luminosity brighter than $10^{45.5}~{\rm erg s^{-1}}$. Moreover, the mean/median composite quasar SED is plausibly also independent of black hole mass and Eddington ratio, and suggests similar properties of dust and gas in the quasar host galaxies since cosmic noon. Both the mean and median composite SEDs are nicely consistent with previous mean composite quasar spectra at wavelengths beyond $\simeq 1000~{\rm \mathring A}$, but at shorter wavelengths, are redder, indicating, on average, less ionizing radiation than previously expected. Through comparing the model-predicted to the observed composite quasar SEDs, we favor a simply truncated disk model, rather than a standard thin disk model, for the quasar central engine, though we request more sophisticated disk models. Future deep ultraviolet facilities, such as the China Space Station Telescope and the Ultraviolet Explorer, would prompt revolutions in many aspects, including the quasar central engine, production of the broad emission lines in quasars, and cosmic reionization.