EMBER: A Little Red Dot Candidate at $z_{\mathrm{spec}}=10.36$ in the Seed Black Hole Era

Abstract

Little red dots (LRDs) are a newly discovered population that may trace rapidly growing black holes (BHs) and provide a link between seed BHs and luminous active galactic nuclei (AGNs). However, no LRD had previously been spectroscopically confirmed at $z>10$, leaving this earliest phase observationally unexplored. Using JWST/NIRSpec spectroscopy from the EMBER program, we measure a spectroscopic redshift of $z_{\mathrm{spec}}=10.361_{-0.002}^{+0.003}$ for \target, the highest-redshift LRD candidate known to date. The redshift is constrained by a prominent Ly$\alpha$ break and multiple tentative emission lines, including C\,{\sc iv}\,$\lambda \lambda 1549,1551$ and [O\,{\sc iii}]\,$\lambda 4363$ detected at $S/N=3.9$ and $3.6$, respectively. Its extreme NIRCam-MIRI color of $m_{F444W}-m_{F770W}=1.82\pm 0.1$, characteristic V-shaped continuum with a turnover near the Balmer limit, and continuum slopes of ${\beta }_{\mathrm{UV}}=-2.26_{-0.11}^{+0.11}$ and ${\beta }_{\mathrm{opt}}=1.17_{-0.70}^{+0.74}$ all support its LRD nature. A possible iron complex at ${\lambda }_{\mathrm{rest}}\sim 0.24$--$0.25\mu \mathrm{m}$ and a possibly high $\log \left([\mathrm{O}\mathrm{III}]\lambda 4363/\mathrm{H}\gamma \right)=0.17_{-0.22}^{+0.24}$ further support an AGN-like interpretation, as seen in previously known LRDs. Tentative detection of Ly$\alpha$ emission may indicate enhanced Ly$\alpha$ transmission at $z\sim 10$. Our findings extend the observational frontier of BH growth into the seed-BH formation era, and upcoming MIRI spectroscopy will further establish the physical nature of the source.

arXiv comment17 pages, 5 figures, 2 tables