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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

TL;DR

This work tackles the question of why some galaxies at cosmic dawn exhibit red rest-frame UV colors, exploring whether early dust production or intense nebular continuum around hot, possibly metal-poor stars is responsible. It leverages deep JWST/NIRCam imaging from the UNCOVER/MegaScience program with 20 bands and applies multiple SED-fitting tools to assess redshifts and physical scenarios, highlighting the critical role of medium-band data in reducing low-$z$ interlopers. The authors report two plausible $z\sim12$ red galaxy candidates (likely lensed, $\mu\sim3.3$, $M_{\rm UV}\sim-19$) that lie off the standard $M_{\rm UV}$–$\beta_{\rm UV}$ relation, favoring either $A_V\sim0.8$ dust reddening or extreme nebular conditions around $T_{\rm eff}\gtrsim5\times10^4$ K. They find $p(z<7)<0.10$ for these sources, underscoring the potential presence of missed galaxies at the earliest epochs and emphasizing spectroscopic follow-up to distinguish between the proposed scenarios and to refine our understanding of dust growth and stellar populations at $z>10$.

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.

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

TL;DR

This work tackles the question of why some galaxies at cosmic dawn exhibit red rest-frame UV colors, exploring whether early dust production or intense nebular continuum around hot, possibly metal-poor stars is responsible. It leverages deep JWST/NIRCam imaging from the UNCOVER/MegaScience program with 20 bands and applies multiple SED-fitting tools to assess redshifts and physical scenarios, highlighting the critical role of medium-band data in reducing low- interlopers. The authors report two plausible red galaxy candidates (likely lensed, , ) that lie off the standard – relation, favoring either dust reddening or extreme nebular conditions around K. They find for these sources, underscoring the potential presence of missed galaxies at the earliest epochs and emphasizing spectroscopic follow-up to distinguish between the proposed scenarios and to refine our understanding of dust growth and stellar populations at .

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.
Paper Structure (3 sections, 1 figure)

This paper contains 3 sections, 1 figure.

Table of Contents

  1. Introduction
  2. Two $z\sim12$ red galaxy candidates
  3. Data

Figures (1)

  • Figure 1: Photometry of one of the target galaxies in UNCOVER DR2 (gray) and DR3 (red, with MegaScience data) with the best-fit Bagpipes SEDs. The inserted panel shows $p(z)$ based on the photometry in each catalog. The best-fit SED and $p(z)$ changes significantly before and after adding the photometric points of the medium bands.