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

Hiroto Yanagisawa, Masami Ouchi+48 more|arXiv (Cornell University)|Jan 9, 2026

Galaxies: Formation, Evolution, Phenomena被引用 1

一句话总结

本论文报告在一个强透镜放大的 z~7 星系中发现了两个极其微弱的“小红点”，讨论它们作为潜在的非核 IMBH 的性质，并探讨对 SMBH 增长的影响。

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 $M_\mathrm{BH}\sim10^{4 ext{--}6}\,M_\odot$ 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.

研究动机与目标

• Identify faint off-nuclear red sources (LRDs) in a z~7 galaxy behind a lensing cluster using JWST/NIRCam data.
• Characterize the SEDs and colors to distinguish LRDs from star-forming components and assess red optical/blue UV nature.
• Reconstruct the source-plane configuration with lens modeling to determine physical separation and association with the host galaxy.
• Estimate black hole masses for the LRDs under plausible Eddington ratios and discuss implications for IMBH formation and SMBH growth.

提出的方法

• Detect objects with LRD color criteria using F150W-F200W and F277W-F356W/F277W-F444W color cuts.
• Perform SED fitting with BAGPIPES using BPASS stellar population models plus a blackbody component to account for red optical continuum.
• Model nebular emission with Cloudy grids and include Balmer break features; test fits with and without blackbody to compare scenarios.
• Construct a gravitational lens model of PLCK G004.5-10.5 to obtain magnifications and reconstruct the source-plane brightness distribution (Lenstronomy-based).
• Describe intrinsic SEDs and morphologies by multi-band fitting, fixing structural parameters to F444W-based best-fit values across bands.

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.

实验结果

研究问题

• RQ1 Can extremely faint, off-nuclear red sources (LRDs) exist within a typical high-z star-forming galaxy when magnified by strong lensing?
• RQ2 What are the physical properties (SEDs, colors, and Balmer-break features) of these LRDs and their host SFG in the source plane?
• RQ3 Do the LRDs represent two distinct objects or multiple images of a single source under lensing?
• RQ4 What are the inferred black hole masses and Eddington ratios for the LRDs, and what do they imply about IMBH formation and SMBH growth at z~7?

主要发现

• Identification of Red Eyes: two faint LRDs separated by ~70 pc in the source plane within a lensed z~7 galaxy.
• Intrinsic UV magnitudes of the LRDs are M_UV,int ≳ -16, about 20 times fainter than the surrounding SFG (M_UV,int ≈ -19).
• Lens modeling yields magnifications μ ≈ 20 (SW), 8 (NW), and 3 (NE) for the images at z~7.4.
• Best-fit SEDs require a blue UV continuum plus a red optical continuum modeled with a blackbody of T_BB ≈ 4000–4500 K; without blackbody, dusty SFGs are needed but underpredict UV.
• In the source plane, the LRDs are off-center (~460 pc) from the SFG center within a ~410 pc half-light radius host; LRD1 and LRD2 have inferred black hole masses log M_BH/M_sun ≈ 4.4^{+0.7}_{-0.7} and 4.1^{+0.8}_{-0.4} (assuming Eddington ratios 0.1–1).
• Without lensing, such LRDs would be undetectable in a typical z~7 SFG, suggesting many similar off-nuclear IMBHs could be hidden in ordinary high-z galaxies.

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.