COSMOS-Web: Intrinsically Luminous z ? 10 Galaxy Candidates Test Early Stellar Mass Assembly

We report the discovery of 15 exceptionally luminous 10 less than or similar to z less than or similar to 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-UV > -22, and thus constitute the most intrinsically luminous z greater than or similar to 10 candidates identified by JWST to date. Selected via NIRCam imaging, 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 greater than or similar to 10 sources and 3/15 (20%) likely low-redshift contaminants. Three of our z similar to 12 candidates push the limits of early stellar mass assembly: they have estimated stellar masses similar to 5 x 10(9) M-circle dot, implying an effective stellar baryon fraction of epsilon(star) similar to 0.2-0.5, where epsilon(star) equivalent to 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 similar to 10(10) M-circle dot galaxies relative to M-star similar to 10(9) M-circle dot-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 UV luminosity function from a double power law to a Schechter function at z approximate to 8. Spectroscopic redshift confirmation and ensuing constraints of their masses will be critical to understand how, and if, such early massive galaxies push the limits of galaxy formation in the Lambda cold dark matter paradigm.