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

Abstract

Characterizing compact sources in the early Universe is difficult because light from physically distinct components can overlap within the same diffraction-limited image. A common method uses field stars to model how a single point of light appears and from this estimates the intrinsic structure and contributions of the physical components of the distant object under study. Many important properties inferred for high-redshift objects depend on the accuracy of this method. Using the little red dots (LRDs) analysed by Zhang et al. as a test case, this work shows that applying the method through broad filters to objects whose spectra differ from those of the field stars can systematically misassign light, overestimating an existing extended component or creating an apparent one where none exists. At the measured level, this error is large enough to alter the inferred sizes, luminosities and masses, and therefore the physical interpretation of these systems. The physical and mathematical origin of the error is established and a correction applicable to other compact distant sources is demonstrated.