Discovery of red galaxy candidates at z ~ 12: Early dust growth or significant nebular emission with high-temperature stars?

Ikki Mitsuhashi; Katherine A. Suess; Joel Leja; Pratika Dayal; Robert Feldmann; Seiji Fujimoto; Harley Katz; Themiya Nanayakkara; Desika Narayanan; Sedona H. Price; John R. Weaver; Christina C. Williams; Ivo Labbe; Rachel Bezanson; Hakim Atek; Gabriel Brammer; Sam E. Cutler; Lukas J. Furtak; Richard Pan; Bingjie Wang; Katherine E. Whitaker

arXiv · 2510.13240

Discovery of red galaxy candidates at z ~ 12: Early dust growth or significant nebular emission with high-temperature stars?

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Abstract

We report the discovery of two z ~ 12 galaxy candidates with unusually red UV slopes (betaUV ~> -1.5), and probe the origin of such colors at cosmic dawn. From Prospector fits to the UNCOVER/MegaScience dataset -- deep JWST/NIRCam imaging of Abell 2744 in 20 broad- and medium-bands -- we identify several new z > 10 galaxies. Medium-band data improve redshift estimates, revealing two lensed (mu ~ 3.3) z ~ 12 galaxies in a close pair with beta_UV ~> -1.5 at an UV absolute magnitude of M_UV ~ -19 mag, lying away from typical scatter on previously known MUV-betaUV relations. SED fitting with Prospector, Bagpipes, and EAZY support their high-z nature, with probability of low-z interlopers of p(z < 7) < 10%. The potential low-z interlopers are z ~ 3 quiescent galaxies (QGs), but unexpected to be detected at the given field of view unless z ~ 3 QG stellar mass function has a strong turn up at log Mstar/Msun ~ 9. Unlike typical blue high-redshift candidates (beta_UV ~< -2.0), these red slopes require either dust or nebular continuum reddening. The dust scenario implies Av ~ 0.8 mag, which is larger than theoretical predictions, but is consistent with a dust-to-stellar mass ratio (log M_dust/M_star ~ -3). The nebular scenario demands dense gas (log nH /cm^3 ~ 4.0) around hot stars (log Teff [K] ~ 4.9). Spectroscopic follow-up is essential to determine their true nature and reveal missing galaxies at the cosmic dawn.

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Ikki Mitsuhashi, Katherine A. Suess, Joel Leja, Pratika Dayal, Robert Feldmann, Seiji Fujimoto, Harley Katz, Themiya Nanayakkara, Desika Narayanan, Sedona H. Price, John R. Weaver, Christina C. Williams, Ivo Labbe, Rachel Bezanson, Hakim Atek, Gabriel Brammer, Sam E. Cutler, Lukas J. Furtak, Richard Pan, Bingjie Wang, Katherine E. Whitaker. 2025-10-15. Discovery of red galaxy candidates at z ~ 12: Early dust growth or significant nebular emission with high-temperature stars?. https://arxiv.org/abs/2510.13240

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