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

Matthew Stepney, Manda Banerji, Shenli Tang, Matthew J. Temple, Paul C. Hewett

First listed 2026-01-24 | Last updated 2026-01-24

Abstract

We present a rest-ultraviolet to infrared spectral energy distribution (SED) analysis of 63 heavily reddened quasars (HRQs) at redshifts z=0.7-2.7 and with dust extinctions E(B-V)=0.4-1.8. Our analysis demonstrates that SEDs with red optical and blue UV continua are very common in HRQs, with more than 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 (0.3 per cent). Higher scattering fractions are ruled out by the (i-K)=2.5 colour-cut used to select HRQs which pre-dates the discovery of the JWST "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 AGN 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 anti-correlated 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.

Short digest

Analysis of rest-UV–IR SEDs for 63 heavily reddened quasars at z=0.7–2.7 (E(B−V)=0.4–1.8) shows that >82% display “V-shaped” SEDs with a UV excess over a reddened quasar continuum. Two-component fits favor a reddened quasar plus an unobscured scattered-light contribution with a small average scattering fraction (~0.3%), though UV light from a star-forming host cannot be excluded. Four HRQs meet LRD UV/optical slope criteria, and the sample exhibits a near-IR hot-dust deficit relative to blue quasars—consistent with a phase where AGN feedback has ejected inner-torus dust. The UV scattering fraction weakly tracks hot-dust strength and anti-correlates with E(B−V), supporting hot dust as the scatterer and ISM-scale dust along the line of sight.

Key figures to inspect

• Figure 1: Check where HRQs sit in the z–L3000 plane versus blue SDSS quasars and how E(B−V) varies across the locus; confirms the luminosity range of the V-shaped objects and selection space.
• Figure 2: Inspect the Eddington-ratio distributions to see HRQs skew toward (near-/super-)Eddington accretion compared to blue quasars, reinforcing a rapid-growth, feedback-prone phase.
• Figure 3: Compare the “confirmed/inconclusive/rejected” SED fits to visualize the UV upturn, the decomposition into reddened-quasar vs scattered/host components, and the photometric leverage needed blueward of the inflection.
• Figure 4: Examine the anti-correlation between scattering fraction and (i−K), showing how the legacy (i−K)=2.5 cut biases against high-scattering objects and why only a subset satisfy LRD-like slopes.