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

We present a deep X-Shooter rest-frame UV to optical spectral analysis of the heavily reddened quasar, ULASJ2315+043 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(Hbeta, MBH [M_sol]) = 10.26 +\- 0.05, bolometric luminosity L_Bol = 10^48.16 erg s^-1 and Eddington-scaled accretion rate log10(\lambda_Edd) = -0.19. We find remarkable similarities between ULASJ2315+043 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-ionisation emission lines in the rest-UV, supporting the scattered light interpretation for the UV excess. The scattering fraction represents just 0.05% of the total luminosity of ULASJ2315+043. Analysis of the mid infra-red 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 CIV, NV, SiIV and SiIII. There is evidence for significant high-velocity (>1000 km s^-1) outflows in both the broad and narrow line regions as traced by CIV and [OIII] emission. The kinetic power of the [OIII] wind is e_ion = 10^44.61 erg s^-1 ~ 0.001 L_Bol. ULASJ2315+043 is likely in an important transition phase where star formation, black-hole accretion and multi-phase gas flows are simultaneously occurring.