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

Using JWST/NIRISS Wide-Field Slitless Spectroscopy, astronomers discovered a dense protocluster at z=3 in the field of the luminous quasar and starburst SDSSJ165202.64+172852.3, nicknamed 'The Step' due to a distinctive step-like distribution of galaxies in 3D position-velocity space. The structure contains at least 18 member galaxies and appears to be undergoing a merger of two dark matter haloes, which could explain the large velocity span of 700–850 km/s between galaxies along the densest line of sight.

The protocluster core is exceptionally dense: seven galaxies lie within 70 projected kpc of the quasar, and the galaxy overdensity exceeds a factor of 100 in the core and 15 within 1 Mpc². The inferred halo mass is log M_halo/M_sun between 13.1 and 13.8, suggesting the system could evolve into a massive, Coma-like cluster by z=0. Compared to the field, star-forming galaxies in the Step show slightly elevated star formation rates, yet two quiescent galaxies with strong D4000 breaks and no ongoing star formation are also present. Relative to other z=3 protoclusters, the Step exhibits similar or somewhat suppressed star-forming activity. Additionally, two new AGN candidates were identified, giving a tentative AGN fraction of 10–20%, higher than in the field.

The Step is one of only a handful of maturing structures at 2 < z < 4 that host both active star-forming and quenched galaxies. This redshift regime may represent a critical transitional epoch, where overdense regions shift from being the most active star-forming sites in the early Universe to the quiescent, massive elliptical populations found in the cores of present-day clusters.