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 signs of two substructures in the 3D position-velocity space -- notably a distinctive step-like distribution, from which the nickname is derived. We propose that the Step may be undergoing a merger of two halos, which could also explain the unusually large velocity span (700-850km/s) between galaxies along the densest line of sight. This sightline contains four (seven) galaxies within projected distances of 20 (70) kpc from the quasar. Within a 320 230 kpc2 region, we find ten galaxies (including the quasar), consistent with a remarkably dense protocluster core. We derive a galaxy overdensity of a factor of$>100$in this core and$>15$in the full field (1 Mpc2 $), making it one of the densest structures known at this redshift, and a very massive halo of 13.1 < łog M_ ̊m halo / M_⊙ < 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 appear to show slightly elevated levels of star formation activity. Yet, we find two quiescent galaxies (łog M_⋆ / M_⊙ ∼ 10.2, 10.4 ± 0.8) with strong continuum breaks and no evidence of ongoing star formation. Compared to other protoclusters at z=3, the Step structure exhibits broadly similar or slightly suppressed star-forming activity for its halo mass. We also identify two new active galactic nucleus (AGN) candidates based on optical line ratios and find tentative evidence of an enhanced AGN fraction (10-20 %) with respect to the field. The protocluster constitutes one of an emerging number of maturing structures at $ 2 łeqslant z łeqslant 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.