2021ApJ...906...12S

Ultraluminous quasars (M₁₄₅₀ <= -29) provide us with a rare view into the nature of the most massive and most rapidly accreting supermassive black holes (SMBHs). Following the discovery of two of these extreme sources, J0341+1720 (M₁₄₅₀ = -29.56, z = 3.71) and J2125-1719 (M₁₄₅₀ = -29.39, z = 3.90), in the Extremely Luminous Quasar Survey (ELQS) and its extension to the Pan-STARRS 1 footprint (PS-ELQS), we herein present an analysis of their rest-frame UV to optical spectroscopy. Both quasars harbor very massive SMBHs with M_BH=6.73_–0.83^+0.75×10⁹M_☉ and M_BH=5.45_–0.55^+0.60×10⁹M_☉, respectively, showing evidence of accretion above the Eddington limit (L_bol/L_Edd=2.74_–0.27^+0.39 and L_bol/L_Edd=3.01_–0.30^+0.34). NOEMA 3 millimeter observations of J0341+1720 reveal a highly star-forming (SFR ≃ 1500 M_☉ yr^–1), ultraluminous infrared galaxy (L_IR ≃ 1.0 x 10¹³ L_☉) host, which, based on an estimate of its dynamical mass, is only ∼30 times more massive than the SMBH it harbors at its center. As examples of luminous super-Eddington accretion, these two quasars provide support for theories that explain the existence of billion solar mass SMBHs ∼700 million years after the Big Bang by moderate super-Eddington growth from standard SMBH seeds.