[Paper Review] A Long Time Ago in a Galaxy Far, Far Away: A Candidate z ~ 12 Galaxy in Early JWST CEERS Imaging

Steven L. Finkelstein, Micaela B. Bagley+118 more|arXiv (Cornell University)|Jul 25, 2022

Gamma-ray bursts and supernovae20 citations

TL;DR

This paper reports the identification of a robust candidate galaxy at z ~ 12 (Maisie’s Galaxy) in early JWST CEERS imaging, with strong multi-band detections and a high-redshift photometric redshift, suggesting star-forming activity less than 400 Myr after the Big Bang.

ABSTRACT

We report the discovery of a candidate galaxy with a photo-z of z~12 in the first epoch of the JWST Cosmic Evolution Early Release Science (CEERS) Survey. Following conservative selection criteria we identify a source with a robust z_phot = 11.8^+0.3_-0.2 (1-sigma uncertainty) with m_F200W=27.3, and >7-sigma detections in five filters. The source is not detected at lambda < 1.4um in deep imaging from both HST and JWST, and has faint ~3-sigma detections in JWST F150W and HST F160W, which signal a Ly-alpha break near the red edge of both filters, implying z~12. This object (Maisie's Galaxy) exhibits F115W-F200W > 1.9 mag (2-sigma lower limit) with a blue continuum slope, resulting in 99.6% of the photo-z PDF favoring z > 11. All data quality images show no artifacts at the candidate's position, and independent analyses consistently find a strong preference for z > 11. Its colors are inconsistent with Galactic stars, and it is resolved (r_h = 340 +/- 14 pc). Maisie's Galaxy has log M*/Msol ~ 8.5 and is highly star-forming (log sSFR ~ -8.2 yr^-1), with a blue rest-UV color (beta ~ -2.5) indicating little dust though not extremely low metallicity. While the presence of this source is in tension with most predictions, it agrees with empirical extrapolations assuming UV luminosity functions which smoothly decline with increasing redshift. Should followup spectroscopy validate this redshift, our Universe was already aglow with galaxies less than 400 Myr after the Big Bang.

Motivation & Objective

• Investigate whether JWST CEERS imaging reveals galaxies at redshifts z > 11 to constrain early star formation and cosmic SFRD.
• Identify and vet robust high-redshift galaxy candidates using conservative photometric criteria and multiple consistency checks.
• Characterize the physical properties (stellar mass, sSFR, rest-UV color) of the z > 11 candidate to assess its place in early galaxy evolution.

Proposed method

• Process four CEERS NIRCam pointings with revised JWST pipeline and custom steps to remove artifacts and 1/f noise.
• Construct PSF-matched photometry across seven bands using small Kron apertures with aperture corrections derived from simulations.
• Estimate photometric redshifts with EAZY using an expanded template set including blue high-z templates from BPASS with nebular emission.
• Apply conservative, multi-criterion color and SNR cuts based on the full z PDFs to select robust z > 11 candidates.

Experimental results

Research questions

• RQ1 Can JWST CEERS imaging identify robust galaxy candidates at z > 11, approaching the z ~ 12 regime?
• RQ2 What are the physical properties (mass, sSFR, UV slope) of a z > 11 candidate and do they align with extrapolations of UV luminosity functions?
• RQ3 Do the photometric redshift PDFs and color selections effectively discriminate high-z sources from low-z interlopers and stars?
• RQ4 How does the observed population at z > 11 compare with predictions from cosmic star-formation rate density evolution and simulations?

Key findings

• A source with photometric redshift zphot = 11.8+0.3−0.2 and mF200W = 27.3 is robustly detected (≥7σ in five filters).
• The object is not detected at wavelengths shorter than 1.4 μm in deep HST and JWST imaging, with faint ~3σ detections in F150W and F160W indicating a Lyα break near z ~ 12.
• The galaxy (Maisie’s Galaxy) has log(M*/M⊙) ~ 8.5 and is highly star-forming with log(sSFR) ~ −8.2 yr−1, and a blue rest-UV color with β ~ −2.5."
• The z > 11 interpretation is favored with 99.6% of the photo-z PDF supporting z > 11, and colors are inconsistent with Galactic stars while the source is resolved (rh ≈ 340 pc).
• The findings are broadly consistent with empirical extrapolations of UV luminosity functions that smoothly decline with increasing redshift, though they tension with some theoretical predictions.