Kavli Affiliate: Kohei Inayoshi
| Summary:
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 $z_rm spec=10.361^+0.003_-0.002$ for target, the highest-redshift LRD candidate known to date. The redshift is constrained by a prominent Ly$α$ break and multiple tentative emission lines, including C,sc iv,$λlambda1549,1551$ and [O,sc iii],$lambda4363$ detected at $S/N=3.9$ and $3.6$, respectively. Its extreme NIRCam-MIRI color of $m_rm F444W-m_rm F770W=1.82pm0.1$, characteristic V-shaped continuum with a turnover near the Balmer limit, and continuum slopes of $β_rm UV=-2.26^+0.11_-0.11$ and $β_rm opt=1.17^+0.74_-0.70$ all support its LRD nature. A possible iron complex at $λ_rm restsim0.24$–$0.25,rm μm$ and a possibly high $logleft([rm O,III],lambda4363/rm Hγright)=0.17^+0.24_-0.22$ further support an AGN-like interpretation, as seen in previously known LRDs. Tentative detection of Ly$α$ emission may indicate enhanced Ly$α$ transmission at $zsim10$. 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.
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