Limitations on Morphological Fitting for JWST “Little Red Dots”

Abstract Early results from JWST have uncovered a peculiar class of objects referred to as “little red dots” (LRDs). The extremely compact morphologies of these LRDs are often invoked to point towards an active galactic nucleus (AGN)-dominated picture in the context of their conflicting multiwavelength properties. In this work, we assess the capability of pysersic and GALFIT —commonly used tools in LRD morphological studies—to recover input parameters for a simulated suite of LRD-like objects in the F444W band. We find that (1) these tools have difficulty recovering input parameters for simulated images with signal-to-noise ratio (SNR) ≲ 25; (2) estimated point-spread function (PSF) fraction could be a more robust physically motivated description of LRD compactness; and (3) almost all permutations of modeled LRDs with SNR ≲ 50 cannot be differentiated from a point source, regardless of intrinsic extent. This has serious implications on how we interpret morphological results for increasingly large photometric samples of LRDs, especially at extremely high z or in relatively shallow fields. We present results of Sérsic and two-component fitting to a sample of observed LRDs to compare with our mock sample fitting. We find that ∼85% of observed LRDs are PSF dominated, consistent with the AGN-dominated interpretation. The remaining ∼15% have low estimated PSF fractions (two-component fit) and sizes ≳150 pc (Sérsic fit). This morphological diversity of LRDs suggests that that the population likely is not homogeneous. It possibly has a primary subset of LRDs consistent with the AGN-dominated hypothesis, and a secondary population more consistent with arising perhaps from extremely compact starbursts.