[Paper Review] A population of red candidate massive galaxies ~600 Myr after the Big Bang

Ivo Labbé, Pieter van Dokkum+9 more|arXiv (Cornell University)|Jul 25, 2022

Gamma-ray bursts and supernovae36 citations

TL;DR

Using JWST CEERS NIRCam data, the study identifies six candidate massive (M* > 1e10 Msun) red galaxies at z ~ 7.4–9.1 (potentially up to ~1e11 Msun), suggesting higher-than-expected mass densities in the early universe.

ABSTRACT

Galaxies with stellar masses as high as $\sim 10^{11}$ solar masses have been identified out to redshifts $z \sim 6$, approximately one billion years after the Big Bang. It has been difficult to find massive galaxies at even earlier times, as the Balmer break region, which is needed for accurate mass estimates, is redshifted to wavelengths beyond $2.5\mum$. Here we make use of the $1-5\mum$ coverage of the JWST early release observations to search for intrinsically red galaxies in the first ~750 million years of cosmic history. In the survey area, we find six candidate massive galaxies (stellar mass $>10^{10}$ solar masses) at $7.4 < z < 9.1$, 500 - 700 Myr after the Big Bang, including one galaxy with a possible stellar mass of $\sim 10^{11}$ solar masses. If verified with spectroscopy, the stellar mass density in massive galaxies would be much higher than anticipated from previous studies based on rest-frame ultraviolet-selected samples.

Motivation & Objective

• Motivate the search for intrinsically red, massive galaxies in the first ~750 million years after the Big Bang using JWST's 1–5 μm coverage.
• Identify robust high-z candidates via double-break SED selection spanning Lyman and Balmer breaks to constrain redshifts and M*/L ratios.
• Quantify redshifts and stellar masses with multiple SED-fitting approaches that account for strong nebular emission.
• Assess implications for the high-redshift stellar-mass function and mass density in light of JWST results.

Proposed method

• Use JWST/NIRCam CEERS imaging (1–5 μm) with six broad bands and a 410M medium band to detect red, double-break SEDs.
• Empirically select high-z candidates with non-detection in optical, blue F150W–F277W < 0.7, red F277W–F444W > 1.0, and F444W < 27 AB.
• Derive redshifts and masses with EAZY (emission-line templates), Prospector-α, and Bagpipes across multiple modeling assumptions to gauge systematics.
• Adopt fiducial masses by median of seven measurements per galaxy and consider potential emission-line contributions to photometry.
• Evaluate dependence of results on attenuation laws, SFHs, age priors, and photometric uncertainties to test mass robustness.

Experimental results

Research questions

• RQ1 Can intrinsically red, massive galaxies exist at z ≳ 7 identified via JWST NIRCam photometry?
• RQ2 What are the photometric redshifts and stellar masses of these double-break selected galaxies when accounting for strong emission lines?
• RQ3 How do different SED-fitting frameworks and modeling assumptions affect inferred masses and redshifts?
• RQ4 What are the implications of these candidate galaxies for the high-redshift stellar mass function and total stellar-mass density?

Key findings

• Thirteen double-break selected galaxies were found; six have fiducial masses > 1e10 Msun and photometric redshifts 6.5 < z < 9.1.
• The brightest galaxy (38094) lies at z = 7.5 with possible M* ≈ 1e11 Msun, and may be in a small group.
• SED fits indicate rest-frame optical emission lines can significantly affect broad-band fluxes, especially redward of the Balmer break.
• Compared to UV-selected samples, these JWST-red galaxies are redder in their entire SED, implying higher mass-to-light ratios (M/L) and potentially higher masses.
• Mass estimates are sensitive to dust attenuation law, age prior, and treatment of photometric uncertainties, with possible mass reductions by up to factors of 100 in some models.
• If spectroscopically confirmed, the implied stellar-mass density at z ~ 8–9 would be much higher than previous UV-based estimates, challenging standard LCDM-based assembly.