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

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.

• The paper reports the detection of a candidate galaxy at z ~11.8 using multi-band NIRCam imaging from the JWST CEERS survey.
• The study employs robust photometric criteria to reveal a low-mass, high star formation rate galaxy with a clear Lyman-alpha break.
• This early discovery challenges existing models of cosmic star formation and highlights JWST’s potential to redefine our understanding of the cosmic dawn.

A Candidate $z \sim 12$ Galaxy in JWST CEERS Imaging

The paper "A Long Time Ago in a Galaxy Far, Far Away: A Candidate $z \sim 12$ Galaxy in Early JWST CEERS Imaging" by Finkelstein et al. presents the identification of a candidate high-redshift galaxy from the first epoch of Cosmic Evolution Early Release Science Survey (CEERS) using the James Webb Space Telescope (JWST). The researchers report a strikingly high-redshift galaxy, suggested to be from around 370 million years post-Big Bang, at a photometric redshift of $z \sim 11.8$.

Key Findings

The candidate galaxy, referred to as "Maisie’s Galaxy," was detected leveraging the unprecedented sensitivity of JWST. The CEERS survey data allowed for the identification of this galaxy through a robust selection criterion, marking it as one of the earliest galaxies detected to date. The galaxy has a strong photometric signal in several infrared filters, indicating its significant emission in the rest-frame ultraviolet, which is critical for early galaxy recognition.

Photometric Redshift and Properties:

• The candidate exhibits a photometric redshift of $z=11.8^{+0.2}_{-0.3}$.
• It is characterized by a relatively low stellar mass of $\log(M_\ast / M_\odot) \sim 8.5$ and an intensely high specific star formation rate (sSFR $\sim -8.2 \text{ yr}^{-1}$).
• The rest-UV color of the galaxy indicates a low dust content and possibly moderately low metallicity.

Methodology and Detection:

• The identification was carried out using the NIRCam of JWST, utilizing imaging from seven different bands.
• The galaxy does not show flux in the shorter wavelengths, confirming a significant Lyman-alpha break, indicative of a high-redshift nature.
• The galaxy's morphology was sufficiently resolved to suggest it is not a stellar object, with its half-light radius measuring approximately 340 $\pm$ 14 pc.

Implications and Comparison with Models

The discovery of Maisie’s Galaxy poses compelling implications for existing models of cosmic star formation rates (SFRD) and galaxy formation at early times. The presence of such a galaxy at this redshift presents a challenge to several theoretical models, which typically predict significantly fewer bright galaxies at $z > 10$. However, models which predict a smooth decline in the SFRD, as opposed to an accelerated decline, seem less contradicted by this finding.

The results and methodology highlight the potential of JWST in advancing our understanding of cosmic dawn. Maisie’s Galaxy, if spectroscopically confirmed, would affirm that the universe was already harboring luminous galaxies within 400 million years of the Big Bang, bolstering hypotheses about early star formation activity and galaxy evolution.

Future Prospects

• Spectroscopic Confirmation: While the photometric redshift provides strong evidence for the high-redshift nature of the galaxy, spectroscopic follow-up would be invaluable to confirm its redshift and further detailed study.
• Comparative Surveys: Expanding the search with JWST data through other observational programs will help populate the number density of galaxies at these early epochs, further informing models of galaxy formation and cosmic evolution.
• Theoretical Refinement: This finding invites a reevaluation of galaxy formation scenarios and their constraints at high redshift, particularly those involving dark matter models and the nature of star formation in the first billion years of cosmic history.

In conclusion, the discovery of Maisie's Galaxy within the initial cycles of JWST's operational life foreshadows an era of profound insights into the universe's formative epochs, reinforcing the telescope's anticipated role as a cornerstone of observational cosmology.