[Paper Review] CEERS Key Paper I: An Early Look into the First 500 Myr of Galaxy Formation with JWST

Steven L. Finkelstein, Micaela B. Bagley+65 more|arXiv (Cornell University)|Nov 10, 2022

Astronomy and Astrophysical Research25 citations

TL;DR

The paper analyzes the first JWST CEERS data to identify z~9–16 galaxy candidates, builds a robust HST+JWST photometric catalog, and presents an early z~11 UV luminosity function with 26 high-redshift candidates.

ABSTRACT

We present an investigation into the first 500 Myr of galaxy evolution from the Cosmic Evolution Early Release Science (CEERS) survey. CEERS, one of 13 JWST ERS programs, targets galaxy formation from z~0.5 to z>10 using several imaging and spectroscopic modes. We make use of the first epoch of CEERS NIRCam imaging, spanning 35.5 sq. arcmin, to search for candidate galaxies at z>9. Following a detailed data reduction process implementing several custom steps to produce high-quality reduced images, we perform multi-band photometry across seven NIRCam broad and medium-band (and six Hubble broadband) filters focusing on robust colors and accurate total fluxes. We measure photometric redshifts and devise a robust set of selection criteria to identify a sample of 26 galaxy candidates at z~9-16. These objects are compact with a median half-light radius of ~0.5 kpc. We present an early estimate of the z~11 rest-frame ultraviolet (UV) luminosity function, finding that the number density of galaxies at M_UV ~ -20 appears to evolve very little from z~9 to z~11. We also find that the abundance (surface density [arcmin^-2]) of our candidates exceeds nearly all theoretical predictions. We explore potential implications, including that at z>10 star formation may be dominated by top-heavy initial mass functions, which would result in an increased ratio of UV light per unit halo mass, though a complete lack of dust attenuation and/or changing star-formation physics may also play a role. While spectroscopic confirmation of these sources is urgently required, our results suggest that the deeper views to come with JWST should yield prolific samples of ultra-high-redshift galaxies with which to further explore these conclusions.

Motivation & Objective

• Motivate studying the first 500 Myr of galaxy formation with JWST to constrain early star formation and IMF evolution.
• Develop a robust photometric catalog combining JWST NIRCam and HST data for faint, compact high-z galaxies.
• Estimate photometric redshifts and apply stringent, simulation-backed selection criteria to construct a credible z>9 candidate sample.
• Provide an initial assessment of the z~11 rest-frame UV luminosity function and galaxy surface densities at these epochs.

Proposed method

• Reduce and calibrate JWST NIRCam imaging with custom corrections for artifacts (snowballs, wisps, 1/f noise).
• Align JWST images to an absolute WCS frame using a Gaia-tied HST reference mosaic.
• Create PSF-matched images by building empirical PSFs and convolving to the F444W (largest, 0.161'' FWHM).
• Perform two-image photometry with Source Extractor in PSF-matched, multi-band data using a joint detection image (F277W+F356W).
• Use small elliptical apertures for accurate colors, and derive total flux corrections via an aperture-correction scheme informed by simulations and comparisons to templates.
• Estimate photometric uncertainties with data-driven noise modeling as a function of aperture size, and apply local-noise corrections.

Experimental results

Research questions

• RQ1 What is the abundance and nature of z>9 galaxy candidates in the first epoch of CEERS data?
• RQ2 How does the rest-frame UV luminosity function evolve at z~11 and what does this imply for early star formation and IMF?
• RQ3 Do JWST NIRCam+HST colors and photometric redshifts robustly select ultra-high-redshift galaxies in this early data release?
• RQ4 How do observed high-z galaxy surface densities compare to theoretical predictions?

Key findings

• Identified a robust sample of 26 galaxy candidates at z~9–16 from CEERS epoch 1 data.
• The z~11 rest-frame UV luminosity function suggests little evolution in number density at MUV ~ -20 between z~9 and z~11.
• The abundance (surface density) of z>10 candidates exceeds nearly all theoretical predictions.
• Candidates are compact, with a median half-light radius of ~0.5 kpc.
• Spectroscopic confirmation is urgently needed to validate the high-redshift identifications and interpretations of star formation physics at these epochs.
• The results hint that JWST deeper views will yield prolific ultra-high-redshift galaxy samples for further analysis.