2021MNRAS.502.1527F

We present near-IR photometry and spectroscopy of 30 extremely luminous radio and mid-IR-selected galaxies. With bolometric luminosities exceeding ∼10¹³ L_☉ and redshifts ranging from z = 0.880 to 2.853, we use Very Large Telescope instruments X-shooter and Infrared Spectrometer and Array Camera to investigate this unique population of galaxies. Broad multicomponent emission lines are detected in 18 galaxies and we measure the near-IR lines H β, [O III] λλ4959,5007, and H αin 6, 15, and 13 galaxies, respectively, with 10 Lyαand 5 C IV lines additionally detected in the UVB arm. We use the broad [O III] λ5007 emission lines as a proxy for the bolometric active galactic nucleus luminosity, and derive lower limits to supermassive black hole masses of 10^7.9-10^9.4 M_☉ with expectations of corresponding host masses of 10^10.4-10^12.0 M_☉. We measure λ_Edd > 1 for eight of these sources at a 2σ significance. Near-IR photometry and SED fitting are used to compare stellar masses directly. We detect both Balmer lines in five galaxies and use these to infer a mean visual extinction of A_V = 2.68 mag. Due to non-detections and uncertainties in our H βemission line measurements, we simulate a broad H βline of FWHM = 1480 kms^–1 to estimate extinction for all sources with measured H αemission. We then use this to infer a mean A_V = 3.62 mag, demonstrating the highly obscured nature of these galaxies, with the consequence of increasing our estimates of black hole masses by a 0.5 orders of magnitude in the most extreme and obscured cases.