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

Evanthia Hatziminaoglou, Hugo Messias, Duncan Farrah, Anna Feltre, Dennis Koopmans, Ismael Perez-Fournon, Rhimon Souza, Lingyu Wang

First listed 2026-09-16 | Last updated 2026-09-15

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.

Short digest

Using 0.8-arcsec ALMA Band 7 imaging to deconfuse 142 Herschel-selected SDSS quasars at 1 < z < 4 in Stripe 82, this paper revises the host-galaxy star-formation budget of FIR-bright quasars. ALMA lowers inferred SFRs by about 20% for single-counterpart systems and by more than a factor of two for SPIRE-blended systems, yet leaves a population of extreme starbursts with SFRs of 500-3000 solar masses per year and a median of 1080. Their weak starburst-accretion luminosity relation, enhanced 1.4-GHz detection rate consistent mainly with the FIR-radio correlation, and selectively weaker Lyα and C IV emission support a short transitional phase in which vigorous bulge growth and SMBH accretion coexist with AGN-driven winds. The result sharpens the case that these optically visible, FIR-bright quasars bridge obscured blowout systems and ordinary blue quasars rather than simply reflecting Herschel beam confusion.

Key figures to inspect

• Figure 3. This distribution directly quantifies why ALMA resolution changes the physical interpretation: single-counterpart quasars receive modest SFR revisions, whereas systems with multiple submillimeter counterparts can have strongly inflated Herschel-based SFRs. It is the clearest visual statement of the deblending correction underlying the paper's revised starburst census.
• Figure 4. The starburst-luminosity versus 1.4-GHz comparison tests whether the FIR emission is predominantly powered by star formation after ALMA deconfusion. Its placement relative to the Bell and redshift-dependent FIR-radio relations supports the conclusion that most radio-detected FIR-bright quasars remain consistent with star-forming hosts rather than being dominated by radio AGN.
• Figure 5. This is the key population-level diagnostic for the claimed weak coupling between host star formation and instantaneous SMBH accretion. The maximum-likelihood regression and intrinsic scatter show that even among exceptionally star-forming quasar hosts, starburst and accretion luminosities rise only very shallowly together.
• Figure 7. The equivalent-width distributions provide the concise spectral evidence for a distinct FIR-bright quasar phase: Lyα and C IV are reduced relative to the matched FIR-faint control, while Mg II is comparatively unchanged. That line-dependent difference is central to the proposed interpretation involving AGN-driven winds that affect high-ionization UV emission without comparably altering Mg II.