A Supermagnified Little Red Dot with a Resolved Host

Abstract

Little red dots (LRDs) are an intriguing new class of sources in the early universe discovered by the James Webb Space Telescope (JWST) characterized by V-shaped spectra and extremely compact sizes. Their underlying mechanisms still remain the subject of ongoing debate. We present JWST observations of a gravitationally lensed LRD nicknamed "Best Friend" at $z_{\rm spec} = 8.297\pm0.002$. Its high magnification factor of $μ= 36^{+8}_{-12}$ makes it the highest magnified LRD reported to date and unveils both the resolved structure of its host and a central engine less than 7 pc in size. The spectrum of the central engine, in which broad lines are not detected, possibly owing to the limited S/N and spectral resolution, is better interpreted as black hole activity rather than a dense star cluster. The latter cannot be fully excluded, but if true, the resulting lower limit on stellar density would be $>1.25^{+0.4}_{-0.3}\cdot10^6 \mathrm{M_{\odot}/pc}^2$, which is around 2 orders of magnitude higher than stellar densities of typical clusters at high-z. By performing a spatially resolved analysis of NIRCam photometry and NIRSpec/PRISM spectroscopy, we find that the host stellar mass is $\log(M_\star/M_{\odot}) = 7.3^{+0.2}_{-0.8}$. Best Friend demonstrates the power of lensing to reveal the morphologies of LRDs.