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

Scott C. Chapman, Kedar A. Phadke, Nikolaus Sulzenauer, Dazhi Zhou, Hollis Akins, Manuel Aravena, James R. Burgoyne, Caitlin M. Casey, Erin Foley, Leah Fryer, Ryley Hill, Nicholas LeVar, Monica Natalia Isla Llave, Matt Malkan, Vismaya Pillai, William Rasakanya, Manuel Solimano, Justin S. Spilker, Aurelie Torti, Joaquin D. Vieira, Fabio Vito, David Vizgan, George Wang

First listed 2026-09-21 | Last updated 2026-09-21

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 not yet spectroscopically confirmed, their extremely red colours (m200-m444 > 2.0 mag), strong spatial concentration, and number density - at least 2000 times that of comparable field populations - make a chance projected association unlikely. Their colours overlap those used to select the widely studied ``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 put strong limits on obscured star formation, and the ERD population mostly lies significantly below the star-forming main sequence. One object, ERD1/Q1, is substantially brighter and more massive, lying far below the main sequence, making it a strong candidate for a massive quiescent galaxy. These JWST-selected red galaxies reveal a previously inaccessible population extending to lower luminosities and stellar masses than the Dusty Star Forming Galaxy (DSFG) population which characterizes the dense protocluster core. SPT2349-56 may therefore capture an early phase of cluster assembly in which extreme starbursts coexist with galaxies already transitioning toward the passive galaxies characteristic of mature cluster cores.

Short digest

Chapman et al. identify 11 ultra-red JWST/NIRCam sources in the compact core of the z=4.30 SPT2349-56 protocluster, selected by m200−m444 > 2.0 and reaching a projected number density at least 2000 times that of comparable field populations. Their colors overlap little red dot selections, but all except ERD5/LRD1 are resolved, favoring substantial stellar emission over continuum dominated by an unresolved AGN; ALMA Band 7 limits further place most of the extended red dots below the star-forming main sequence. ERD1/Q1 stands out as a bright, massive candidate quiescent or recently quenched member, making this core a striking coexistence of dusty starbursts and galaxies already transitioning toward the passive population, although spectroscopic confirmation of the red sources remains outstanding.

Key figures to inspect

• Figure 1. This JWST overview establishes the extraordinary spatial context: all 11 red sources sit in the SPT2349-56 core, including a compact subgroup of six ERDs near SMG-A amid known ALMA members and radio AGN.
• Figure 2. This is the key selection and morphology diagnostic, showing the m200−m444 > 2.0 cut relative to cluster SMGs and field sources while separating the mostly resolved ERDs from the single source satisfying the compact LRD criterion.
• Figure 5. The full ERD SED comparison connects the red colors to predominantly old stellar populations and low obscured-AGN contributions, while highlighting ERD1/Q1 as a massive system comparable in stellar mass to previously known high-SFR cluster members.
• Figure 6. This figure carries the paper's central evolutionary claim by placing ERDs below the star-forming main sequence relative to the protocluster SMGs; the ALMA stack indicates that the population is quenched on average, with the high-mass end most clearly quiescent.