COSMOS-Web: Intrinsically Luminous z$\gtrsim$10 Galaxy Candidates Test Early Stellar Mass Assembly

Abstract

We report the discovery of 15 exceptionally luminous $10\lesssim z\lesssim 14$ candidate galaxies discovered in the first 0.28 deg${}^2$ of JWST/NIRCam imaging from the COSMOS-Web Survey. These sources span rest-frame UV magnitudes of $-20.5>M_{\mathrm{UV}}>-22$, and thus constitute the most intrinsically luminous $z\gtrsim 10$ candidates identified by JWST to-date. Selected via NIRCam imaging with Hubble ACS/F814W, deep ground-based observations corroborate their detection and help significantly constrain their photometric redshifts. We analyze their spectral energy distributions using multiple open-source codes and evaluate the probability of low-redshift solutions; we conclude that 12/15 (80%) are likely genuine $z\gtrsim 10$ sources and 3/15 (20%) likely low-redshift contaminants. Three of our $z\sim 12$ candidates push the limits of early stellar mass assembly: they have estimated stellar masses $\sim 5\times 10^9{M}_{\odot }$, implying an effective stellar baryon fraction of ${\epsilon }_{\star }\sim 0.2-0.5$, where ${\epsilon }_{\star }\equiv M_{\star }/(f_b{M}_{halo})$. The assembly of such stellar reservoirs is made possible due to rapid, burst-driven star formation on timescales $<$100\,Myr where the star-formation rate may far outpace the growth of the underlying dark matter halos. This is supported by the similar volume densities inferred for $M_{\star }\sim 10^{10}{M}_{\odot }$ galaxies relative to $M_{\star }\sim 10^9{M}_{\odot }$ -- both about $10^{-6}$ Mpc${}^{-3}$ -- implying they live in halos of comparable mass. At such high redshifts, the duty cycle for starbursts would be of order unity, which could cause the observed change in the shape of the UVLF from a double powerlaw to Schechter at $z\approx 8$. Spectroscopic redshift confirmation and ensuing constraints of their masses will be critical to understanding how, and if, such early massive galaxies push the limits of galaxy formation in $\Lambda$CDM.

30 pages, 9 figures; ApJ submitted

Cited by in corpus (3)

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• Galaxy Mergers in the Epoch of Reionization I: A JWST Study of Pair Fractions, Merger Rates, and Stellar Mass Accretion Rates at $z=4.5-11.5$
• Galaxy Mergers in the Epoch of Reionization II: Major Merger-Triggered Star Formation and AGN Activities at $z=$ 4.5-8.5