Filling the Gap in Cluster Evolution: JWST's Glimpse into a Young, Star-Forming Cluster at Cosmic Noon

Abstract

We present a detailed study of HUDFJ0332.4-2746.6 (HUDF46), a $z \approx 1.84$ overdensity in the Hubble Ultra Deep Field, previously identified with HST as a proto-cluster. JWST/NIRISS spectroscopy expands its confirmed membership from 18 to 37 galaxies, while deep HST/ACS, JWST/NIRCam, and JWST/MIRI imaging provide a comprehensive multiwavelength view from the rest-frame UV to the mid-infrared. This dataset probes the population across three dex in stellar mass ($M_\bigstar \approx 10^{7.5\text{--}10.5}\,M_\odot$), delivering the first direct view of a young cluster down to such low-$M_\bigstar$ at $z\gtrsim1$. Assuming virialization, we derive a velocity dispersion of $σ\approx 670\pm 91\,\mathrm{km\,s^{-1}}$ and a halo mass of $M_{200} \approx (1.2\pm0.2) \times 10^{14}\,M_\odot$, in agreement with X-ray constraints from deep {\it Chandra} data. Despite residing in a massive halo likely in the hot-halo regime, the population is overwhelmingly star-forming, with no established red sequence and no extended X-ray emission from a hot intracluster medium. HUDF46 members have stellar and structural properties nearly indistinguishable from coeval field galaxies, and the structure hosts only one AGN candidate, found in its brightest galaxy, which lies at the cluster center. Overall, HUDF46 appears to be in a transitional phase prior to the onset of environmental quenching, making its galaxy population a key benchmark for tracing the processes that will later build a passive population and shape the assembly of massive clusters at later cosmic times.

Figures (3)

• Figure 1: Stellar mass distributions of galaxies grouped by quality flag of their redshift (QF1, QF2, QF3), shown as cumulative fractions to enable direct comparison between the samples. For QF1 sources, stellar masses are derived in this work by fixing the redshift to the spectroscopic value (see Section 5.4.1), while for QF2 and QF3 sources stellar masses are taken from existing field catalogs and are based on photometric redshifts (rinaldi_emergence_2024navarro-carrera_burstiness_2024). The vast majority of galaxies in QF3 ($\approx80\text{--}85\%$) have stellar masses below $\approx10^{8}\,M_{\odot}$.
• Figure 2: We perform a benchmark comparison between our spectroscopic redshifts from NIRISS/WFSS and those available in the HUDF from 3D-HST, MUSE, FRESCO, ASPECS, and recent NIRSpec observations from JADES and SMILES (only their QF=1). The left panel shows the comparison between the redshifts measured here with previous results. Overall, we find good agreement with the existing literature (black box, top left). The right panel highlights the few outliers (denoted by numbers on the left panel) which we visually inspected. We find that the sources previously spectroscopically classified as high-redshift objects are likely affected by unreliable quality flags in the earlier datasets. Their photometric redshifts (from JADES DR2) agree well with our NIRISS-based spectroscopic redshifts, further supporting the revised lower-redshift interpretation. After removing these sources, the agreement between $z_{\text{NIRISS}}$ and $z_{\text{HUDF}}$ improves further (red box, top right).
• Figure 3: EAC co-association matrix for the HUDF spectroscopic sample. Galaxies are ordered by their final EAC consensus group for visualization only. The color scale shows the co-association value $C_{ij}$, i.e., the fraction of FoF realizations (across a broad grid of projected separations and velocity thresholds) in which galaxies $i$ and $j$ are grouped together. Diagonal elements are unity by construction. Most pairs have low $C_{ij}$, indicating weak association across parameter space. The highlighted region and inset zoom into HUDF46 at $z\approx1.84$, revealing a compact set of galaxies consistently recovered as a group. While the visual appearance of the matrix depends on the chosen ordering, all pairwise co-association values and derived stability metrics are invariant under permutations of the axes. We note that, beyond HUDF46, other overdensities are present within HUDF, which are however less rich in member galaxies.