A big red dot: scattered light, host galaxy signatures, and multiphase gas flows in a luminous, heavily reddened quasar at cosmic noon

Journal article, 2024

We present a deep X-Shooter rest-frame ultraviolet (UV) to optical spectral analysis of the heavily reddened quasar, ULASJ2315 + 0143 at z = 2 . 566, known to reside in a major-merger host galaxy. The rest-frame optical is best fit by a dust- reddened quasar ( E ( B -V ) QSO = 1 . 55) with black-hole mass log 10 (H β, M BH [M ⊙]) = 10 . 26 ±0 . 05, bolometric luminosity L Bol = 10 48 . 16 erg s -1 and Eddington-scaled accretion rate log 10 ( λEdd ) = -0 . 19. We find remarkable similarities between ULASJ2315 + 0143 and the high-redshift little red dots (LRDs). The rest-frame UV cannot be explained by a dusty quasar component alone and requires an additional blue component consistent with either a star-forming host galaxy or scattered AGN light. We detect broad high-ionization emission lines in the rest-UV, supporting the scattered light interpretation for the UV excess. The scattering fraction represents just 0.05 per cent of the total luminosity of ULASJ2315 + 0143 . Analysis of the mid-infrared SED suggests an absence of hot dust on torus-scales similar to what is observed for LRDs. The obscuring medium is therefore likely on galaxy scales. We detect narrow, blueshifted associated absorption-line systems in C IV , N V , Si IV, and Si III . There is evidence for significant high-velocity ( > 1000 km s -1 ) outflows in both the broad- and narrow-line regions as traced by C IV and [O III ] emission. The kinetic power of the [O III ] wind is ϵion k = 10 44 . 61 erg s -1 ∼0 . 001 L Bol . ULASJ2315 + 0143 is likely in an important transition phase where star formation, black-hole accretion and multiphase gas flows are simultaneously occurring.