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

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 <SUB>AGN</SUB> and star formation rate (SFR) as a function of cosmic time, finding typical {L}<SUB>AGN</SUB>}∼ {10}<SUP>46</SUP>{--}{10}<SUP>47</SUP> erg s<SUP>-1</SUP> and SFR ∼50-1000 M <SUB>☉</SUB> yr<SUP>-1</SUP>. 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 <SUB>AGN</SUB> 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.