James Webb Space Telescope Feed Post

COSMOS-Web: Intrinsically Luminous z\gtrsim10 Galaxy Candidates Test Early Stellar Mass Assembly

Paper abstract: We report the discovery of 15 exceptionally luminous 10<~ z<~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_{\rm UV}>-22, and thus constitute the most intrinsically luminous z\gtrsim10 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\gtrsim10 sources and 3/15 (20%) likely low-redshift contaminants. Three of our z~12 candidates push the limits of early stellar mass assembly: they have estimated stellar masses ~5\times10^{9}\,M_\odot, implying an effective stellar baryon fraction of \epsilon_{*}~0.2-0.5, where \epsilon_{*}\equiv M_{*}/(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_*~10^{10}\,M_\odot galaxies relative to M_*~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~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 \LambdaCDM.