(Re)solving the complex multi-scale morphology and V-shaped SED of a newly discovered strongly-lensed Little Red Dot in Abell 383

TL;DR

This paper reports the discovery and detailed characterization of a strongly lensed Little Red Dot at $z=6.027$ behind Abell 383, resolved into two ultra-compact components (blue and red) connected by a bridge within a diffuse, line-emitting cloud. Using JWST/NIRCam imaging and lens-model reconstructions, the authors decompose the system into spatially distinct stellar populations, deriving intrinsic sizes of order tens of parsecs and a total stellar mass around $M_\approx 6\times10^9\,M_$, with a blue component of ~$M_\approx 3\times10^8\,M_$ and a red component of ~$M_\approx 6\times10^{10}\,M_$. The V-shaped LRD SED arises from the superposition of the two components, not from a single compact region, and the red component remains a key candidate for either a reddened AGN or an old stellar population. The findings demonstrate the power of strong lensing to resolve sub-kpc structure in the early universe and provide constraints on the physical nature of LRDs, with implications for their formation and for interpreting unlensed LRDs. Future high-resolution spectroscopy will be essential to disentangle stellar and non-stellar emission and to map the kinematics of these extreme systems.

Abstract

We present a luminous Little Red Dot (LRD) at $z=6.027$, doubly imaged by the galaxy cluster Abell 383 and observed with JWST/NIRCam. The source shows the characteristic "V-shaped" SED and pronounced Balmer break that define the LRD population. Owing to its large magnifications, $μ\sim11$ for image S1 and $μ\sim7$ for S2, the system is exceptionally bright and highly stretched, providing a rare, spatially resolved view of an LRD. The images reveal a complex morphology with a compact red dot, a spatially offset blue dot, and faint emission bridging and surrounding the two. After correcting for lensing, we find that both dots are extremely small but resolved, with rest-frame UV sizes of $\sim 20$ pc (red) and $\sim60$ pc (blue). These compact dots are embedded in a more extended, line-dominated cloud traced most clearly in F356W ([OIII]+H$β$), which reaches scales of order $\sim$1 kpc. SED decomposition shows that the blue component has a flat UV continuum consistent with a young stellar population, whereas the red component has a steep red SED that can be interpreted as either an evolved stellar population with high stellar mass ($\log M_\star/M_\odot>10$) or a reddened AGN. If this object is representative of the LRD population, our results imply that the V-shaped SEDs of LRDs do not arise from individual compact sources but instead from the superposition of two physically distinct components. Separated by only $\sim300$ pc in the source plane, these components would blend into a single compact source in unlensed observations with the canonical LRD colors. This system therefore provides a rare opportunity to resolve the internal structure of an LRD and to gain direct insight into the physical nature of this population.

Figures (8)

• Figure 1: Top: RGB composite image of Abell 383, constructed using F090W + F115W + F150W for blue, F200W + F210M for green, and F277W + F356W + F444W for red. Inset cutouts show the doubly-lensed system (S1 and S2) and are $1.6\arcsec \times 1.6\arcsec$. Bottom: Multi-band cutouts ($1.0\arcsec \times 1.0\arcsec$) for images S1 and S2 across HST and JWST filters.
• Figure 1: Aperture fluxes (in nJy) for S1 and S2 across all JWST filters. Coordinates correspond to the F444W centroid positions of each image.
• Figure 3: RGB image stamps constructed using filters F200W, F150W, and F115W for the two lensed images: S1 (left) and S2 (right). Both images are $1 \arcsec\times 1\arcsec$. In this work, we focus on image S2, as it lies in a relatively dark region of the sky with minimal contamination from intracluster light (ICL), whereas S1 is embedded in a brighter ICL background. We identify a prominent blue component, red component, and a connecting bridge-like structure, and model their surface brightness profiles as described in the text.
• Figure 4: GALFIT best-fit models for S2. For each filter, we show (from left to right): the original image, fitting mask, best-fit model, and residual. The overall fits are good, with minimal residuals in most bands.
• Figure 5: SED decomposition of the spatially resolved components derived from GALFIT modeling (red, blue), shown as colored points. Solid lines show best-fit EAZY fits, which assume simple stellar population templates. The red component has a steeply rising SED, while the blue component shows a relatively flat rest-frame UV continuum characteristic of young, unobscured star formation.