A Massive Galaxy at the Edge of Feedback-Free Efficiency

Abstract

The efficiency with which galaxies convert their available baryonic reservoir into stars sets a fundamental ceiling on stellar mass assembly in the early Universe and encodes the cumulative effect of stellar feedback. in this paper, we report the measurement of star-formation efficiency (SFE) of a photometrically and spectroscopically vetted reference sample at $z=9-10$, out of which 31 are spectroscopically confirmed. Among them, we highlight a spectroscopically confirmed galaxy, \texttt{UNCOVER 3686} at $z = 9.31$, which has a stellar mass $M_\star = 10^{9.55}$ M$_\odot$ and physical properties comparable to the predictions of the feedback-free starburst (FFB) scenario. We use this galaxy as an anchor for the first direct observational test of whether galaxy at cosmic dawn with physical properties predicted in the FFB theory reach the maximum baryon conversion efficiencies predicted by the feedback-free starburst scenario. We find the SFE for this galaxy to lie between $\approx 20\%$ and $60\%$, a factor of 2 to 6 above empirical model predictions. The compact morphology ($R_e = 0.45$ kpc), young stellar age ($\sim 160$ Myr), low metallicity $(Z_\star/Z_\odot \approx 0.2)$, and inferred gas density $(n_{\rm gas} \sim 3\times 10^{3}$ cm$^{-3}$) of this galaxy are consistent with feedback-free (FFB) galaxy formation conditions. We conclude that UNCOVER 3686 is an excellent candidate for an FFB galaxy in which the global star formation efficiency approaches the theoretical limits due to weak stellar feedback.

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