Broad-band host measurements of little red dots and similar compact nuclei require source-conditioned polychromatic PSFs

Sergio Bonaque-González

First listed 2026-09-01 | Last updated 2026-09-01

Abstract

Short digest

Bonaque-González shows that broad-band nucleus–host decompositions of little red dots can be systematically biased when empirical PSFs built from field stars are applied to LRDs with very different in-band spectral shapes. Reanalysing the Zhang et al. 217-object stacking framework with synthetic point-only LRDs, the paper finds that chromatic PSF mismatch alone can be absorbed by a compact Sérsic component, reaching 18.73% of the flux in F444W despite no input host. A source-conditioned polychromatic PSF, either computed from the nuclear spectrum or flexibly represented with LRD-derived diffraction modes, removes this false extended-light allocation while recovering injected hosts. The result puts rest-frame-optical host sizes, luminosities, and stellar masses inferred from stellar-PSF subtraction on a spectrum-dependent footing, with a provisional correction lowering the aggregate F444W host fraction from 11.8% to 4.6%.

Key figures to inspect

• Figure 1. This is the paper's central demonstration: a simulated LRD containing only an unresolved nucleus leaves wavelength-dependent residuals when fit with a stellar polychromatic PSF, and the decomposition assigns those residuals to a Sérsic host. The enclosed-energy comparison makes clear that small chromatic changes in the point-source light distribution grow toward F444W and can be amplified into an apparent rest-frame-optical host component.