JWST has revealed an abundance of compact, red objects at z≈5−8 dubbed "little red dots" (LRDs), whose SEDs display a faint blue UV continuum followed by a steep rise in the optical. Despite extensive study of their characteristic V-shaped SEDs, the nature of LRDs remains unknown. We present a new analysis of the NIRSpec/PRISM spectrum of A2744-QSO1, a triply imaged LRD at z=7.04 from the UNCOVER survey. The spectrum shows a strong Balmer break and broad Balmer emission lines, both of which are difficult to explain with models invoking exclusively AGN or stellar contributions. Our fiducial model decomposes the spectrum into a post-starburst galaxy dominating the UV-optical continuum and a reddened AGN being sub-dominant at all wavelength and contributing at ∼20% level. However, our most credible model infers a stellar mass of M⋆≈4×109M⊙ within a radius of re<30pc, driving its central density to the highest among observations to date. This high central density could be explained if A2744-QSO-1 is the early-forming core of a modern-day massive elliptical galaxy that later puffed up via the inside-out growth channel. The models also necessitate an unusually steep dust law to preserve the strong break strength, though this steepness may be explained by a deficit of large dust grains. It is also probable that these challenges reflect our ignorance of A2744-QSO1's true nature. Future variability and reverberation mapping studies could help disentangle the galaxy and AGN contribution to the continuum, and deeper redder observations could also unveil the dust properties in LRDs.
@article{arxiv.2410.06257,
title = {UNCOVER: 404 Error -- Models Not Found for the Triply Imaged Little Red Dot A2744-QSO1},
author = {Yilun Ma and Jenny E. Greene and David J. Setton and Marta Volonteri and Joel Leja and Bingjie Wang and Rachel Bezanson and Gabriel Brammer and Sam E. Cutler and Pratika Dayal and Pieter van Dokkum and Lukas J. Furtak and Karl Glazebrook and Andy D. Goulding and Anna de Graaff and Vasily Kokorev and Ivo Labbe and Richard Pan and Sedona H. Price and John R. Weaver and Christina C. Williams and Katherine E. Whitaker and Adi Zitrin},
journal= {arXiv preprint arXiv:2410.06257},
year = {2025}
}