C3PO: A Lyα Emitting Galaxy at $z \sim 9$

Abstract: We present deep, $&gt;10$ hr, JWST NIRSpec G140M and G395M observations of a previously known galaxy C3PO-8447 at $z=8.99781$. Our new NIRSpec data from the Carbon-3 plus Oxygen (C3PO) survey detect Ly$α$ emission with an escape fraction of $1.5\pm0.3\%$ and a velocity offset of $210\pm80$ km s$<sup>{-1}$, making C3PO-8447 the third Ly$α$ emitting galaxy confirmed at $z\gtrsim 9$. The galaxy has a low stellar mass of $2.2\pm0.1\times 10^7\ M_\odot$, a young stellar age of $1.6\pm0.2$ Myr, and a low oxygen abundance of $12+\log(\mathrm{O/H}) = 7.50\pm0.08$. UV diagnostic diagrams incorporating the C IV, O III], and C III] emission lines indicate that the ionization of C3PO-8447 is dominated by the recent starburst. The low Ly$α$ escape fraction of C3PO-8447 implies that C3PO-8447 resides in a small ionized bubble with radius $R \sim 0.07$ to 0.20 pMpc, which can be produced by C3PO-8447 alone. We find no evidence for either a large-scale galaxy overdensity around C3PO-8447 or a significant contribution from nearby galaxies to its ionizing photon budget. Taken together, these results suggest that C3PO-8447 likely represents an ionized bubble powered by a single low-mass, vigorously star-forming galaxy during the early stages of cosmic reionization.