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

TL;DR

The study uses early JWST CEERS NIRCam imaging to identify a robust z~12 galaxy candidate, Maisie’s Galaxy, via conservative color cuts and photometric redshifts. By combining PSF-matched multi-band photometry with diverse SED-fitting templates and rigorous non-detection in blue bands, the authors obtain a strong z>11 solution supported by independent codes. The galaxy exhibits a sharp Lyman-break, blue rest-UV slope, and physical properties consistent with an actively star-forming system in the first few hundred million years of the Universe. If spectroscopic redshift is confirmed, this object provides a key data point for early galaxy assembly and informs tests of UV luminosity function evolution at z>11.

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.

Figures (3)

• Figure 1: Top) 1.8$^{\prime\prime}$$\times$ 1.8$^{\prime\prime}$ cutout images centered on the position of Maisie's Galaxy in the non-PSF-matched images. This source exhibits the hallmark colors of a distant galaxy -- no discernible flux in a dropout band (we show stacked F606W$+$F814W as well as F115W images; the circle has a radius of 0.3$^{\prime\prime}$) and a significant detection in the bluest detection band (F200W in this case). Very faint flux is visible (at $\sim$2.8$\sigma$ significance when measured in a 0.2$^{\prime\prime}$ diameter aperture) in F150W, which drives the redshift to $z \sim$ 12. The wide wavelength range of NIRCam allows this source to be well-detected in multiple filters, and in the imaging alone it is clear this source exhibits a blue spectral shape. Bottom) Same ordering as the top, for sky-uncertainty maps constructed from the variance of the readout noise, all using a linear scale from 0.33 to 3$\times$ the robustly-measured sky standard deviation in each band. The patchiness of the uncertainties is due to loss of exposure time when cosmic-rays are detected and rejected in the multiple readouts or in outlier rejection when combining the dithered exposures (the 2$\times$ larger original pixel scale of the long-wavelength channels results in larger patches than the short-wavelength channels). The uncertainty arrays show no excess in rejected pixels near the candidate galaxy.
• Figure 2: Three-color images of Maisie's Galaxy. The left image is a composite of HST/ACS F606W and F814W in blue, F115W and F150W in green, and F200W in red. This shows the galaxy candidate as red due to the very high redshift resulting in no detected flux in the filters assigned to the blue and green colors. The right image shows an approximated "true" rest-UV color image, composed of F200W$+$F277W in blue, F356W in green, and F410M+F444W in red. As we discuss further in § 5, intrinsically this galaxy is quite blue. The scale bar corresponds to 1 (physical) kpc assuming $z = 12$ at a scale of 0.37$^{\prime\prime}$ per kpc.
• Figure 3: Left) The circles denote our fiducial photometry, with green and red denoting HST/ACS$+$WFC3 and NIRCam instruments, respectively. This SED exhibits the hallmark shape of a high-redshift galaxy, with several non-detections in blue filters, followed by significant detections with a blue spectral slope. The arrows denote 1$\sigma$ upper limits. The F115W-F200W break color is $>$1.9 mag (2$\sigma)$, which is sufficient to rule out all low-redshift solutions. The blue curve shows the best-fitting eazy model at $z =$ 11.8. The orange curve shows the result if we force eazy to find a solution at $z <$ 7. This model is unable to match the amplitude of the break as well as the slope redward of the break, and is correspondingly ruled out at high confidence ($\chi^2_{low-z} =$ 54.5, compared to 15.8 for the $z =$ 11.8 solution). Right) Photometric redshift probability distribution functions for Maisie's Galaxy. The thick blue curve shows the fiducial PDF from eazy, which exhibits no low-redshift solution and a peak at $z =$ 11.8$^{+0.2}_{-0.3}$ (the purple curve shows the eazy result without HST photometry, which prefers $z =$ 12.0 and has a tail to $z =$ 14). The remaining curves show the results from independent runs with Prospector, Bagpipes, Cigale and Dense Basis (see § 5). All results significantly prefer a $z >$ 12 solution, with all four codes finding best-fit redshifts nearly identical to eazy ($z =$ 11.8$^{+0.2}_{-0.3}$, $z =$ 11.6$^{+0.2}_{-0.2}$, $z =$ 11.8$^{+0.4}_{-0.4}$, and 11.9$^{+0.4}_{-0.4}$, respectively).