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

Abstract

ABSTRACT 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)$^{\rm QSO}= 1.55$) with black-hole mass $\rm log_{10}(H\beta , M{\small BH} [{\rm M}_{\odot }]) = 10.26 \pm 0.05$, bolometric luminosity $\rm L_{Bol}$  = $\rm 10^{48.16}\, erg\,\,s^{-1}$ and Eddington-scaled accretion rate log$_{10}(\rm \lambda _{Edd}) = -0.19$. 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 $\rm km\, s^{-1}$) 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 $\dot{\epsilon }_{k}^{ion} = 10^{44.61} \rm erg\, s^{-1} \sim 0.001\, L_{Bol}$. ULASJ2315+0143 is likely in an important transition phase where star formation, black-hole accretion and multiphase gas flows are simultaneously occurring.