The Physical Nature of the Off-centered Extended Emission Associated with the Little Red Dots

Chang-Hao Chen, Luis C. Ho, Ruancun Li, Kohei Inayoshi

First listed 2025-05-06 | Last updated 2025-05-06

Abstract

A significant fraction of little red dots (LRDs) exhibit nearby extended emission of unknown origin. If physically associated with the LRD, this component may trace stellar emission from an off-centered host galaxy, neighboring companions, or nebular gas illuminated by the active nucleus. We investigate the detailed spectral energy distribution of the extended emission near four LRDs in the JWST UNCOVER and MegaScience surveys. We accurately decompose the extended emission from the dominant point source by simultaneously fitting the images in eight broad-band and nine medium-band filters. After considering both the results from photometric redshift fitting and the probability of galaxies at different redshift overlapping, we confirm that the off-centered blobs in three sources are physically associated with the LRDs, with two of them showing strong [\ion{O}{3}] $λλ4959,\,5007$ emission captured by the medium-band filters. While the spectral energy distributions of all three blobs can be modeled assuming star-forming galaxies with stellar mass $\sim 10^8\,M_{\odot}$, the exceptionally strong [\ion{O}{3}] emission of two sources is best interpreted as pure nebular emission from low-density ($n<10\, {\rm cm}^{-3}$), low-metallicity ($Z\approx 0.05\,Z_{\odot}$) gas photoionized by the ultraviolet radiation from the nearby LRD. Adopting LRD halo masses constrained by clustering measurements and theoretical considerations, we estimate a typical baryonic halo mass accretion rate of $\sim 2-9\, M_{\odot}\,{\rm yr}^{-1}$. If the halo accretion rate is sustained to $z=4$ and stars form with an efficiency of 10\%, the accreted gas would form a galaxy with stellar mass $\sim 10^9\,M_{\odot}$, potentially rendering them spatially resolved at lower redshift.

Short digest

This Letter decomposes NIRCam imaging in 8 broad and 9 medium bands to isolate off-centered emission around four LRDs in Abell 2744 (UNCOVER + MegaScience). Three blobs are confirmed to be physically associated with their LRDs, with two showing medium-band excess consistent with very strong [O III] λλ4959,5007. While all three SEDs can be fit as ~10^8 Msun star-forming systems, the two [O III]-extreme cases are best explained as pure nebular emission from low-density, low-metallicity gas photoionized by the nearby LRD. Adopting LRD halo masses, the implied baryonic halo accretion rates are ~2–9 Msun/yr, suggesting growth to ~10^9 Msun by z≈4 if sustained at 10% star-formation efficiency.

Key figures to inspect

• Figure 1 (multi-band decomposition for MSA10686): Inspect the data–model–residual panels across filters to verify that the PSF+Sérsic fit robustly separates the off-centered component, and note in which bands the blob emerges most clearly after nucleus subtraction.
• Figure 2 (overlap probability vs redshift): Use this to gauge how likely a chance superposition is across z; the low overlap probability at the LRD redshifts supports physical association for three blobs.
• Figure 3 (SED fits from GalfitS): Compare the EAzY photo-z model to the forced LRD-redshift model; identify the two blobs with medium-band spikes pinpointing [O III] λλ4959,5007, read off best-fit M*, and use the inset SFH to judge whether a stellar continuum is required versus nebular-dominated flux.
• Figure 4 (scaling relations): See where the blobs fall relative to the z≳6 main sequence and mass–size relation compared to EELGs; note if the [O III]-extreme blobs deviate toward nebular-dominated loci while any stellar-continuum cases align more closely with EELGs.