VENUS: Two Faint Little Red Dots Separated by $\sim70\,\mathrm{pc}$ Hidden in a Single Lensed Galaxy at $z\sim7$

TL;DR

This study reports the discovery of two faint little red dots, Red Eyes, in a strongly lensed galaxy at $z\sim7$ identified by JWST/VENUS. Gravitational lens modeling reveals a magnification of $\mu\sim20$ for the SW image, and source-plane reconstruction shows two distinct LRDs separated by $\sim70$ pc, offset by $\sim460$ pc from the host star-forming galaxy center. SED fitting with BAGPIPES, including a blackbody component with $T_{\rm BB}\sim4000$--$4500$ K, indicates intrinsically faint LRDs with $M_{\rm UV,int}\gtrsim -16$ coexisting with a brighter $M_{\rm UV,int}\sim -19$ SFG; the LRDs could be powered by accreting intermediate-mass black holes (IMBHs). The inferred $L_{\rm bol}$ and $M_{BH}$ values, together with a dynamical-friction timescale of $t_{\rm df}\sim0.15$ Gyr, imply that such off-nuclear IMBHs could migrate toward the center and contribute to SMBH growth, suggesting that hidden LRDs in typical high-$z$ galaxies may play a role in early black hole assembly.

Abstract

We report the identification of a pair of faint little red dots (LRDs), dubbed Red Eyes, in a strongly-lensed galaxy at $z\sim7$ behind the PLCKG004.5-10.5 cluster, identified from the JWST Treasury program VENUS. Red Eyes are spatially resolved on the image plane with distinct colors, while the critical curve lies far north of Red Eyes, clearly requiring two different LRDs rather than a single LRD. Red Eyes is an extremely close pair of LRDs separated by $\sim70\,\mathrm{pc}$ in the source plane with a magnification of $μ\sim20$, which consistently explains another counter-image detected to the north-west. Red Eyes is hosted in a typical star-forming galaxy with $M_{\mathrm{UV,int}}\sim -19$, but its own UV emission is very faint ($M_{\mathrm{UV,int}} \gtrsim -16$). Moreover, Red Eyes does not reside at the galaxy center but lies at an offset position of approximately one effective radius $R_{\mathrm{e}}$ away from the galaxy center. If observed without lensing, Red Eyes would appear as a typical star-forming galaxy at $z\sim 7$ with $M_{\mathrm{UV}}\sim -19$, showing no apparent LRD signatures in either morphology or SED. These results suggest that multiple off-center LRDs, similar to Red Eyes, may be commonly hidden in a typical high-$z$ star-forming galaxy. In this case, various plausible scenarios may emerge, one of which is that intermediate-mass black holes (IMBHs) with $M_\mathrm{BH}\sim10^{4\text{--}6}\,M_\odot$ may form in star clusters on a stellar disk and contribute to the growth of the central supermassive black hole via mergers, with some IMBHs detectable as luminous LRDs in a sufficiently active and massive phase.

Figures (11)

• Figure 1: RGB composite image (R: F356W+F444W, G: F200W+F277W, B: F115W+F150W) of the PLCK G004-10.5 cluster. The red contours indicate the critical curves at $z=7.4$. LRD and SFG images are highlighted by annotated inset figures.
• Figure 2: LRD selection using colors (the top panels) and slope fitting (the bottom panels). The black lines show the LRD selection threshold. The red and blue symbols indicate the LRD and SFG images, respectively. The circles, diamonds, and squares represent the SW, NW, and NE images, respectively. The LRD-SW1, SW2, and NW satisfy all of the LRD selection criteria, while the SFG components do not. The cyan stars denote the intrinsic colors and slopes of the whole system (i.e., corrected for the gravitational lensing; Section \ref{['sec:sourceplane']}). Without gravitational lensing, Red Eyes would not be identified as an LRD.
• Figure 3: (Top) HST and NIRCam cutouts of the SW image. The rightmost panel represents the segmentation image. (Bottom) Photometry and best-fit SEDs for the SW images. The blue points indicate the observed photometry, with upper limits shown at the $3\sigma$ level. The photometry and upper limit values are not corrected for the magnification. The red curves show the best-fit SEDs consisting of galaxy and blackbody components, while the red circles indicate the best-fit photometry. The gray curves in the bottom two panels represent the best-fit SEDs without the blackbody component. The models without the blackbody component favor the solutions invoking dusty SFGs, which significantly underestimate the UV continuum due to the strong dust attenuation. The posterior redshift distribution is shown in the top-left corner of each panel.
• Figure 4: Same as Figure \ref{['fig:sed-sw']}, but for the NW image.
• Figure 5: Same as Figure \ref{['fig:sed-sw']}, but for the NE image.