Broadband Spectral Energy Distributions of SDSS-selected Quasars and of Their Host Galaxies: Intense Activity at the Onset of AGN Feedback

Abstract

Abstract We present the mean spectral energy distribution (SED) of a sample of optically selected quasars (QSOs) at redshifts of 1 ≤ z ≤ 5. To derive it, we exploit photometric information from the Sloan Digital Sky Survey, UKIRT Infrared Deep Sky Survey, and Wide-field Infrared Survey Explorer surveys in combination with a stacking analysis of Herschel, AKARI, and Planck maps at the location of the QSOs. The near-UV and optical parts of the reconstructed mean rest-frame SED are similar to those found in other studies. However, the SED shows an excess at 1–2 μm (when compared to the aforementioned SEDs normalized in the near UV) and a prominent bump around 4–6 μm, followed by a decrease out to ∼20 μm and a subsequent far-IR bump. From the fitted SEDs, we estimate the average active galactic nucleus (AGN) luminosity L AGN and star formation rate (SFR) as a function of cosmic time, finding typical erg s−1 and SFR ∼50–1000 M ⊙ yr−1. We develop mid-IR-based criteria to split the QSO sample, finding that these allow us to move along the average relationship in the SFR versus L AGN diagram toward increasing AGN luminosities. When interpreted in the context of the in situ coevolution scenario presented by Lapi et al., our results suggest that the detection in the far-IR band is an effective criterion to select objects where the star formation is on the verge of being affected by energy/momentum feedback from the central AGN.