A large overdensity of LRD-like galaxies discovered by JWST in the SPT2349-56 protocluster at z=4.30

Abstract

We present 11 newly identified ultra-red galaxies selected with JWST/NIRCam in the core of the z=4.3 protocluster SPT2349-56. Although these sources are yet spectroscopically confirmed, their extremely red colours (m200-m444 > 2.0 mag), strong spatial concentration, and number density - at least 2000x that of comparable field populations - make a chance projected association unlikely. Their colours overlap those used to select the "little red dot" (LRD) population, although most of the sources are spatially resolved and are therefore better described as extended red dots (ERDs). The resolved morphologies disfavour continuum emission dominated by an unresolved active nucleus and imply a substantial stellar contribution. Deep ALMA observations yield stringent limits on obscured star formation, and the ERD population mostly lies significantly below the star-forming main sequence. One object, ERD1/Q1, is exceptional, being substantially brighter and more massive than the other ERDs and lying far below the main sequence, making it a strong candidate for a massive quiescent or recently quenched galaxy associated with the protocluster. The coexistence within the same compact core of extreme dusty starbursts, galaxies with suppressed star formation, and a candidate massive quiescent system suggests that the dense environment of SPT2349-56 may already be driving rapid and diverse evolutionary pathways at z>4. These JWST-selected red galaxies reveal a previously inaccessible population extending the protocluster census to lower luminosities and stellar masses, and may represent an early stage in the emergence of the passive galaxy population characteristic of mature cluster cores. SPT2349-56 may therefore capture an early phase of cluster assembly in which extreme starbursts coexist with galaxies already transitioning toward the passive population that will dominate mature cluster cores.