James Webb Space Telescope Feed Post

Arxiv: JWST's first glimpses of a z > 2 forming cluster reveals a top-heavy stellar mass function Published: 3/8/2024 2:11:16 PM Updated: 3/8/2024 2:11:16 PM

Paper abstract: Clusters and their progenitors (protoclusters) at z = 2-4, the peak epoch ofstar formation, are ideal laboratories to study the formation process of boththe clusters themselves and their member galaxies. However, a complete censusof their member galaxies has been challenging due to observationaldifficulties. Here we present new JWST/NIRCam observations targeting thedistant cluster CLJ1001 at z = 2.51 from the COSMOS-Webb program, which, incombination with previous narrow-band imaging targeting H-alpha emitters anddeep millimeter surveys of CO-emitters, provide a complete view of massivegalaxy assembly in CLJ1001. In particular, JWST reveals a population ofmassive, extremely red cluster members in the long-wavelength bands that wereinvisible in previous HST/F160W imaging (HST-dark). Based on this highlycomplete spectroscopic sample of member galaxies, we show that the spatialdistribution of galaxies in CLJ1001 exhibits a strong central concentration,with the central galaxy density already resembling that of low-z clusters.Moreover, we reveal a ``top-heavy" stellar mass function for the star-forminggalaxies (SFGs), with an overabundance of massive SFGs piled up in the clustercore. These features strongly suggest that CLJ1001 is caught in a rapidtransition, with many of its massive SFGs likely soon becoming quiescent. Inthe context of cluster formation, these findings suggest that the earliestclusters form from the inside out and top to bottom, with the massive galaxiesin the core assembling first, followed by the less-massive ones in theoutskirts.