The rest-ultraviolet to infrared spectral energy distributions of heavily reddened quasars are ‘V-shaped’ and hot-dust poor

Abstract

We present a rest-ultraviolet to infrared spectral energy distribution (SED) analysis of 63 heavily reddened quasars (HRQs) at redshifts $0.7<\rm z_{sys}< 2.7$ and with dust extinctions $0.4< E(B-V)< 1.8$. Our analysis demonstrates that SEDs with red optical and blue ultraviolet (UV) continua are very common in HRQs, with $>$82 per cent of the sample showing a UV-excess relative to the reddened quasar continuum. We model the SEDs by combining a reddened quasar and an unobscured scattered light component, though contributions from a star-forming host galaxy cannot be ruled out. The average scattering fraction is small ($\sim$0.3 per cent). Higher scattering fractions are ruled out by the $(i-K) \gt 2.5$ colour-cut used to select HRQs, which pre-dates the discovery of the James Webb Space Telescope ‘Little Red Dot’ (LRD) population. Hence, LRDs generally have bluer UV continua. Nevertheless, four HRQs satisfy the LRD UV/optical continuum slope selections and are therefore massive cosmic noon analogues of LRDs. Analysis of the near-infrared SEDs of HRQs reveals a deficit of hot dust relative to blue quasars, similar to what is observed in LRDs. This suggests HRQs trace a phase where strong active galactic nucleus feedback processes eject dust from the inner torus. The UV scattering fraction of HRQs is weakly correlated with the amount of hot dust emission and anticorrelated with the line-of-sight extinction, $E(B-V)$. This is consistent with the hot dust acting as the scattering medium, and the line-of-sight extinction being dominated by dust on interstellar medium scales in the host galaxy.

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