A PANORAMIC of UV-optical morphologies of "Little Red Dots": Two groups of LRDs distinguished by UV half-light radius

Aidan P. Cloonan, Katherine E. Whitaker, Sinclaire M. Manning, Christina C. Williams, Jenny E. Greene, Pascal A. Oesch, Andrea Weibel, Gabriel Brammer, Anna de Graaff, Raphael E. Hviding, Pratika Dayal, Christian Kragh Jespersen, Zhiyuan Ji, Ivo Labbe, Mengyuan Xiao, Yunchong Zhang

First listed 2026-03-25 | Last updated 2026-03-25

Abstract

Among the most remarkable results from JWST is the discovery of abundant, compact, and very red sources in the early Universe known as "Little Red Dots" (LRDs). The relative degree to which starlight and active galactic nuclei (AGN) drive the rest-frame UV and optical emission from LRDs remains unclear. With a large sample of LRDs selected photometrically from the pure-parallel PANORAMIC survey, we study their morphology as a function of rest-wavelength and find that the rest-UV light is typically more extended than the rest-optical. This result holds both when measuring LRD sizes with a single Sérsic profile and when comparing the fraction of light from a point source via joint PSF+Sérsic modeling. A shift occurs at the Balmer break, with LRDs becoming highly compact and unresolved ($R_{50,\rm{opt}}\lesssim100\;\rm{pc}$) in the rest-optical relative to the rest-UV. When splitting the sample at the Balmer break into those that are resolved and unresolved, a stacking analysis demonstrates that the latter are compact ($R_{50}\lesssim100\;\rm{pc}$) on average across the full rest-UV-optical spectrum. Conversely, those LRDs resolved at the break show extended UV emission ($R_{50,\rm{UV}}>200\;\rm{pc}$) on average. We find a similar dichotomy when repeating with a spectroscopic sample. Altogether, these results are consistent with the rest-UV emission driven by a combination of emission from starlight and a dense, dust-poor cloud of hydrogen gas enveloping an AGN. Differences between LRDs in the relative contribution from the AGN and starlight could reflect an ensemble of black hole seed masses, where a heavier seed produces an LRD of smaller $R_{50,\rm{UV}}$.

Short digest

Using PANORAMIC pure‑parallel NIRCam imaging, the authors assemble 181 photometric LRDs and map morphology from rest‑UV to optical with single‑Sérsic and joint PSF+Sérsic fits. They find a sharp transition at the Balmer break: rest‑optical light is highly compact/unresolved (R50,opt ≲ 100 pc) while rest‑UV is typically more extended; stacking confirms red‑band profiles track the PSF whereas blue‑band profiles are diffuse. Splitting at the break reveals two groups—objects unresolved there remain compact across the spectrum, while those resolved show extended UV emission (R50,UV > 200 pc)—a dichotomy also seen in a spectroscopic subsample. The patterns favor UV emission from starlight plus a dense, dust‑poor H envelope around an AGN and point to a spread in seed black‑hole masses, with heavier seeds yielding smaller UV half‑light radii.

Key figures to inspect

• Figure 1: Inspect the color–magnitude and color–compactness cuts (plus brown‑dwarf pruning) to see how the 181 LRDs were isolated and how the ‘red1/red2’ color tracks bias the sample toward Balmer‑break systems at higher redshift.
• Figure 2: Check the median SED’s ‘v‑shape’ and the location of the Balmer break across F115W–F444W; the small bumps attributed to H lines illustrate how line contamination ties into photometric‑z and the UV/optical split used for morphology.
• Figure 3: Size vs. rest‑wavelength shows the collapse of R50 at the Balmer break—UV sizes scatter to extended values while optical sizes hug the resolution limit; the side histograms visualize the two groups separated by UV half‑light radius.
• Figure 4: Stacked surface‑brightness profiles contrast blue (diffuse, shallower than PSF) and red (compact, PSF‑like) bands; use this to validate that the compactness in the optical is population‑wide, not fit‑by‑fit noise.