COSMOS-Web: The emergence of the Hubble sequence
Abstract
Context. The first JWST deep surveys have expanded our understanding of the morphological evolution of galaxies across cosmic time. The improved spatial resolution and near-infrared (NIR) coverage have revealed a population of morphologically evolved galaxies at very early epochs. However, all previous works are based on relatively small samples; this has prevented accurate probing of the morphological diversity at cosmic dawn. Aims. Leveraging the wide area coverage of the COSMOS-Web survey, we quantified the abundance of different morphological types from z ∼ 7 with unprecedented statistics and established robust constraints on the epoch of emergence of the Hubble sequence. Methods. We measured the global morphologies (spheroids, disk-dominated, bulge-dominated, peculiar) and resolved morphologies (stellar bars) for about 400 000 galaxies down to F 150 W = 27 using deep learning; this represents an increase of two orders of magnitude over previous studies. We provide reference stellar mass functions (SMFs) of different morphologies between z ∼ 0.2 and z ∼ 7 as well as best-fit parameters to inform models of galaxy formation. All catalogs and data are made publicly available. Results. At redshift z > 4.5, the massive galaxy population (log M * / M ⊙ > 10) is dominated by disturbed morphologies (∼ 70%), even in the optical rest frame, and very compact objects (∼ 30%) with effective radii smaller than ∼ 500 pc. This confirms that a significant fraction of the star formation at cosmic dawn occurs in very dense regions, although the stellar mass for these systems could be overestimated. Galaxies with Hubble-type morphologies, including bulge- and disk-dominated galaxies, arose rapidly around z ∼ 4 and dominate the morphological diversity of massive galaxies as early as z ∼ 3. Using stellar bars as a proxy, we speculate that stellar disks in massive galaxies might have been common (> 50%) among the star-forming population since cosmic noon ( z ∼ 2--2.5) and formed as early as z ∼ 7. Massive quenched galaxies are predominantly bulge-dominated from z ∼ 4 onward, suggesting that morphological transformations briefly precede or are simultaneous to quenching mechanisms at the high-mass end. Low-mass (log M * / M ⊙ < 10) quenched galaxies are typically disk-dominated, which points to different quenching routes at the two ends of the stellar mass spectrum from cosmic dawn.