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 $z_{\mathrm{spec}}=8.297\pm 0.002$. Its high magnification factor of $\mu =36_{-12}^{+8}$ 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 $>1.25_{-0.3}^{+0.4}\cdot 10^6{{\mathrm{M}}_{\odot }/pc}^2$, 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 $\log (M_{\star }/M_{\odot })=7.3_{-0.8}^{+0.2}$. Best Friend demonstrates the power of lensing to reveal the morphologies of LRDs.

arXiv commentSubmitted to ApJ Letters. 12 pages including references. It contains 3 figures in the main text + 1 figure in the appendix and 2 tables in the main text