A Big Red Dot: Scattered light, host galaxy signatures and multi-phase gas flows in a luminous, heavily reddened quasar at cosmic noon

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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 log₁₀(Hβ , M BH [ M]) = 10.26 ± 0.05, bolometric luminosity LBol = 1048.16\, erg\,\,s⁻¹ and Eddington-scaled accretion rate log₁₀( λ 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 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 ε ̇ₖⁱᵒⁿ = 1044.61 erg\, s⁻¹ ~ 0.001\, LBol. ULASJ2315+0143 is likely in an important transition phase where star formation, black-hole accretion and multiphase gas flows are simultaneously occurring.