An ALMA Band 7 survey of SDSS/Herschel quasars in Stripe 82: II. The nature of FIR-bright quasars

Abstract: We investigate the star formation properties and ultraviolet (UV)/optical spectral characteristics of 142 far-infrared (FIR)-bright quasars at redshifts 1 < z < 4 to probe supermassive black hole (SMBH) and host galaxy co-evolution. By combining multi-wavelength observations with high-resolution ALMA submillimeter (submm) data, we overcome source confusion and robustly constrain star formation rates (SFRs). The inclusion of ALMA data to the Spectral Energy Distribution fitting reduces SFR estimates by ~20% for quasars with single submm counterparts and by a factor >2 for FIR-blended multi-component systems, while reducing FIR luminosity uncertainties by ~18%. The revised host SFRs span 500 and 3000 solar masses per year (median 1080 solar masses per year), confirming these systems are extreme starbursts. The very shallow positive correlation between starburst and accretion luminosities suggests that SMBH accretion and star formation in the host are not coupled on the timescales probed by the observations. Compared to the general population of FIR-faint quasars, FIR-bright sources have a factor of three higher detection rates at 1.4 GHz, predominantly following the FIR-radio correlation, and reduced CIV and Lya equivalent widths with unaffected MgII, suggesting AGN-driven winds that leave MgII relatively unaffected. Together, these results place FIR-bright quasars in a brief transitional phase, bridging heavily obscured "blowout" galaxies and optically unobscured blue quasars, where bulge assembly and SMBH growth peak simultaneously during ongoing stellar assembly.