Unveiling Extended Components of 'Little Red Dots' in Rest-Frame Optical

Yiyang Zhang, Xuheng Ding, Lilan Yang, Erini Lambrides, Hollis Akins, Andrew J. Battisti, Caitlin M. Casey, Chang-hao Chen, Isa Cox, Andreas Faisst, Maximilien Franco, Aryana Haghjoo, Luis C. Ho, Kohei Inayoshi, Shuowen Jin, Mitchell Karmen, Anton M. Koekemoer, Jeyhan S. Kartaltepe, Kai Liao, Ghassem Gozaliasl, Masafusa Onoue, Vasily Kokorev, Namrata Roy, R. Michael Rich, John D. Silverman, Takumi S. Tanaka, Bei You, Hassen M. Yesuf, Jorge A. Zavala

First listed 2025-10-29 | Last updated 2025-10-29

Abstract

Recent JWST observations have revealed a population of red, compact, high-redshift objects called 'Little Red Dots'(LRD), whose host components have remained largely unconstrained, possibly due to their extreme compactness. Current morphological studies have been limited by small samples, as well as by insufficient imaging depth, which may not allow reliable separation between point-like and extended components, leaving the existence and properties of extended components in LRD largely unconstrained. Here, we perform the image stacking analysis of 217 LRDs in four NIRCam bands, representing the largest and homogeneous sample observed from COSMOS-Web survey to date. Our results reveal an unambiguous detection of faint extended emission in the F444W band, with a typical size of ~200 parsecs and magnitude of ~27.7 AB at z~6.5. We perform four-band photometric SED fitting based on galaxy templates and derive a stellar mass of 8.91+-~0.1 logM_sun. Given this stellar mass, the host galaxy is compact, i.e., ~2.5 times smaller than star-forming populations at similar mass, and the typical black hole mass of LRDs is elevated by ~1.5 dex above the local MBH-M* relation. This work provides direct observational evidence for the existence of LRD host galaxies and offers crucial insights into the growth of the host galaxy and the co-evolution of galaxies and their black holes within the first billion years after the Big Bang.

Short digest

Stacking 217 Little Red Dots from COSMOS-Web across F115W–F444W and decomposing with PS+Sérsic, the authors unambiguously recover faint rest‑optical host emission, including a ring-like residual in F444W with total SNR ~72. The mean extended component has Re ≈ 200 pc and m_F444W ≈ 27.7 AB at z ≈ 6.5; four-band host photometry fit with Bagpipes yields log10(M*) = 8.91 ± 0.1. The extended-to-total flux fraction declines from ~60% (F115W) to ~12% (F444W), underscoring the dominance of the compact nucleus. Hosts are ≈2.5× smaller than SFGs at similar mass, and implied MBH are ~1.5 dex above the local MBH–M* relation, pointing to overmassive early black holes and compact stellar hosts.

Key figures to inspect

• Figure 1: Inspect the stacked residual “ring” after PS-only subtraction and its SNR map (~72), and compare with the star-control stack to verify the extended component is not a PSF artifact.
• Figure 2: Check the PS+Sérsic fits per NIRCam band and the azimuthal profiles versus the PSF to read off the mean effective radius (~200 pc) and any wavelength-dependent morphology of the host light.
• Figure 3: From the Bagpipes SED fit to the isolated host fluxes, verify the inferred m_F444W ≈ 27.7 AB and log10(M*) ≈ 8.91, and gauge the uncertainty envelope on the stellar component.
• Figure 4: Use the size–mass panel to see that LRD hosts are ~2.5× smaller than SFGs at the same M*, and the MBH–M* panel to appreciate the ~1.5 dex MBH excess relative to local relations and to quasars plotted for context.