A cosmic collision in the making: JWST/NIRISS reveals a merging and maturing protocluster core at z~3 around a red quasar

C. Bertemes, D. Wylezalek, G. Noirot, R. Hviding, D. S. N. Rupke, N. L. Zakamska, S. Veilleux, L. Gatto, W. Liu, A. Vayner, Y. Ishikawa, Y. Chen, S. Sankar

First listed 2026-08-12 | Last updated 2026-08-10

Abstract

We report the discovery of ``The Step'', a dense protocluster at z=3 revealed by JWST/NIRISS Wide-Field Slitless Spectroscopy (WFSS) of the field around the luminous quasar and starburst SDSSJ165202.64+172852.3. The protocluster contains at least 18 member galaxies and exhibits two substructures in the 3D position-velocity space - notably a distinctive step-like distribution, from which the nickname derives. We propose that the Step may be undergoing a merger of two haloes, which could also explain the large velocity span (700-850km/s) between galaxies along the densest line of sight. This sightline contains seven galaxies within 70 projected kpc from the quasar, consistent with a remarkably dense protocluster core. We derive a galaxy overdensity of a factor >100 in the core and >15 within 1 Mpc$^2$, making it one of the densest structures known at this redshift, and a very massive halo of $13.1 < log M_{\rm halo}/ M_\odot < 13.8$, which suggests the structure could potentially evolve into a massive, Coma-like cluster at z=0. Compared to the field, the star-forming galaxies in the Step show slightly elevated levels of star formation. Yet, we find two quiescent galaxies with strong D4000 breaks and no evidence for ongoing star formation. Compared to other z=3 protoclusters, the Step exhibits similar or slightly suppressed star-forming activity. We also identify two new AGN candidates and find tentative evidence for an enhanced AGN fraction (10-20%) compared to the field. The protocluster constitutes one of the emerging handful of maturing structures at 2 < z < 4 that host both star-forming and quenched galaxies. This redshift regime may thus be a key transitional epoch for cluster studies, where overdensities transition from being the most active sites of star formation in the early Universe towards shaping the massive passive ellipticals seen in the cores of today's clusters.

Short digest

Using JWST/NIRISS wide-field slitless spectroscopy around the luminous red quasar SDSSJ165202.64+172852.3, Bertemes et al. identify “The Step,” a z~3 protocluster with at least 18 spectroscopic members. Its exceptionally dense core, >100-fold galaxy overdensity, 700–850 km/s line-of-sight span, and step-like position–velocity structure point to a merger between two massive haloes, potentially assembling a Coma-like descendant. The system is already environmentally mature: alongside mildly elevated star formation and two new optical-line-ratio AGN candidates, it hosts two line-less, D4000-break quiescent galaxies, making it a compelling snapshot of the transition toward passive cluster cores.

Key figures to inspect

• Figure 3. Use this field overview to show the confirmed membership, the concentration around J1652, and the relationship between the NIRISS-wide protocluster map and the much smaller NIRSpec quasar-core footprint.
• Figure 5. This is the central structural diagnostic: the 3D position–velocity rendering makes the proposed northern and southern substructures and the distinctive step-like signature of a two-halo merger immediately visible.
• Figure 1. Highlight the representative spectra, especially the two passive members with strong continuum breaks, because their line-less nature provides the direct evidence that this z~3 overdensity already contains quenched galaxies.
• Figure 6. This SFR–stellar-mass comparison separates the putative infalling northern subgroup from the main halo and shows that member star formation is only slightly elevated relative to the field main sequence.
• Figure 7. Use this literature-scale synthesis to place the Step’s halo-normalized total star formation on the lower envelope of comparable protoclusters, supporting the paper’s conclusion that its activity is typical to mildly suppressed for its halo mass.