VENUS: When Red meets Blue -- A multiply imaged Little Red Dot with an apparent blue companion behind the galaxy cluster Abell 383

TL;DR

We report the discovery of a multiply-imaged Little Red Dot behind the cluster Abell 383, designated A383-LRD1. JWST/NIRCam imaging reveals two components—a red LRD and a nearby blue companion—both at $z=6.027$, lensed with $μ_A=16.2±1.2$ and $μ_B=9.0±0.6$, and a predicted gravitation time delay of $Δt_{ ext{grav}}=5.20±0.14$ yr. SED modeling favors a BH* AGN component plus a substantial host galaxy for the LRD, while the blue companion appears as an extremely young, emission-line dominated nebula; ALMA detects [C II] 158 μm emission, constraining the system further. The high magnification enables tight intrinsic size limits ($r_{ ext{intr}} \\lesssim 40$ pc for the LRD) and positions A383-LRD1 as a prime target for deep JWST spectroscopy and high-resolution ALMA follow-up to study black hole growth and host-galaxy evolution at cosmic dawn.

Abstract

We report the discovery of a doubly-imaged Little Red Dot (LRD) candidate behind the galaxy cluster Abell 383, which we dub A383-LRD1. Initially classified as a dropout galaxy in HST imaging with several ground-based emission line detections placing it at $z_{\mathrm{spec}}=6.027$, new JWST/NIRCam observations taken as part of the cycle 4 VENUS survey now reveal that the source consists of two underlying components: A red point-source with a V-shaped SED consistent with LRD selection criteria, and a nearby ($\sim 380$ pc) compact blue companion which was the main contributor to the previous rest-frame UV detections. Based on lensing symmetry and its SED, the LRD appears to lie at a similar redshift as well. The magnification of the two images of A383-LRD1 is $μ_{\mathrm{A}}=16.2\pm1.2$ and $μ_\mathrm{B}=9.0\pm0.6$, respectively, and the predicted time delay between them is $Δt_{\mathrm{grav}}=5.20\pm0.14$ yr ($\sim0.7$ yr in the rest-frame). After correcting for the lensing magnification, we derive an absolute magnitude of $M_{\mathrm{UV,LRD}}=-16.8\pm 0.3$ for the LRD, and $M_{\mathrm{UV,BC}}=-18.2\pm 0.2$ for the blue companion. We perform SED fits to both components, revealing the LRD to be best fitted with a black hole star (BH*) model and a substantial host galaxy, and the blue companion with an extremely young, emission-line dominated star-forming nebula. A383-LRD1 represents the second known multiply-imaged LRD detected to date, following A2744-QSO1, and to our knowledge, the first LRD system with a confirmed detection of [C $_{II}$]$\lambda158 \ μ$m emission from ALMA observations. Thanks to lensing magnification, this system opens a unique door to study the relation between a LRD, its host galaxy, and its environment, and represents a prime candidate for deep JWST spectroscopy and high-resolution ALMA follow-up observations.

Figures (5)

• Figure 1: Abell383-LRD1 -- a new multiply-imaged LRD candidate. The critical curves for $z_{\mathrm{s}}=6.027$ from our new SL model (see section \ref{['lensing']}) are shown in orange, over a color-composite image of A383 (${z_{\mathrm{d}}=0.187}$), constructed using the JWST/NIRCam imaging from the VENUS survey (Blue: F090W, Green: F277W, Red: F444W). The predicted position of the third de-magnified image of the system is marked with a black box. The third image is obscured by the BCG and predicted to be too demagnified to be observed. The inlets correspond to $3\arcsec\times3\arcsec$ cutouts and show the two multiple images of A383-LRD1, with the blue companion clearly seen in both images.
• Figure 2: Cutouts of image A of A383–LRD1 in the JWST/NIRCam bands from the VENUS program and in the available HST bands. The upper–left panel shows a stacked image from the ACS bands-F435W+F475W+F606W+F625W+F775W, where the source remains undetected. Each cutout covers a $3 \arcsec \times 3\arcsec$ field of view. Only the blue companion is visible in the F814W image, while the LRD becomes more prominently detectable towards redder wavelengths. The blue companion has a noticeable flux excesses in F356W and F444W, likely due to strong H$\beta$+[O iii] and H$\alpha$ emission lines, respectively, at $z=6.027$.
• Figure 3: pysersic fits to the LRD (in the F410M band) and the blue companion (in the F200W band). We show a point-mass model fit for image A, and a Sersic profile fit for both images A and B. The small white region near the center indicates the masked component in each fit. For both the LRD and the blue companion, the two model types yield an excellent reconstruction, and the size (e.g., Sersic effective radius) remains the same between the two multiple images despite a factor of $\sim1.7$ in magnification ratio. This is also seen by simply comparing their model-independent light-profiles to each other and to the PSF profile, as seen in the bottom subfigures. See section \ref{['size']} for more details.
• Figure 4: Photometry and SED-modeling of A383-LRD1 image A, with composite-color image cutouts on the right. The de-magnified observed fluxes are shown in blue, with $2\sigma$ upper limits in the case of a non-detection. The best-fit (maximum-a-posteriori) fluxes are shown in red, and the best-fit SED in black. Top: Bagpipes LRD fit to the LRD component. The LRD SED model comprises a BH*-AGN component shown in red, and a host galaxy shown in purple. We also show the BEAGLE DSFG fit as the orange dotted line. Bottom:BEAGLE star-forming galaxy fit to the blue companion.
• Figure 5: Contours of the ALMA [C ii]$\lambda158 \ \mu$m detection from Knudsen2016, overlaid over the JWST color image of A383-LRD1. Contours show 2, 3, 4, 5$\sigma$ of the integrated spectral line, and -2$\sigma$ are shown as a dashed line. The obtained resolution is presented as a red ellipse on the bottom left. The contours are centered slightly off the blue companion, with the LRD within the 4$\sigma$ contour.