[Paper Review] A Comprehensive Study on Galaxies at z~9-16 Found in the Early JWST Data: UV Luminosity Functions and Cosmic Star-Formation History at the Pre-Reionization Epoch

Yuichi Harikane, Masami Ouchi+7 more|arXiv (Cornell University)|Aug 2, 2022

Galaxies: Formation, Evolution, Phenomena22 citations

TL;DR

The paper extracts a secure sample of 23 dropout galaxies at z~9-16 from early JWST NIRCam data, derives UV luminosity functions and cosmic star-formation history, and discusses implications for pre-reionization star formation.

ABSTRACT

We conduct a comprehensive study on dropout galaxy candidates at $z\sim9-16$ using the first 90 arcmin$^2$ JWST/NIRCam images taken by the early release observations (ERO) and early release science (ERS) programs. With the JWST simulation images, we find that a number of foreground interlopers are selected with a weak photo-$z$ determination ($Δχ^2>4$). We thus carefully apply a secure photo-$z$ selection criterion ($Δχ^2>9$) and conventional color criteria with confirmations of the ERO NIRSpec spectroscopic redshifts, and obtain a total of 23 dropout galaxies at $z\sim9-16$, including two candidates at $z_\mathrm{phot}=16.25_{-0.46}^{+0.24}$ and $16.41_{-0.55}^{+0.66}$. We perform thorough comparisons of dropout galaxies found in our work with recent JWST studies, and conclude that our galaxy sample is reliable enough for statistical analyses. We derive the UV luminosity functions at $z\sim9-16$, and confirm that our UV luminosity functions at $z\sim 9$ and $12$ agree with those determined by other HST and JWST studies. The cosmic star-formation rate density decreases from $z\sim9$ to $12$, and perhaps to $16$, but the densities at $z\sim12-16$ are higher than the constant star formation efficiency model. Interestingly, there are six bright galaxy candidates at $z\sim10-16$ with $M_\mathrm{UV}<-19.5$ mag and $M_*\sim10^{8-9} M_\odot$. Because a majority ($\sim80$\%) of these galaxies show no signatures of AGNs in their morphologies, the high cosmic star-formation rate densities and the existence of these UV-luminous galaxies are explained by no suppression of star-formation by the UV background radiation at the pre-reionization epoch and/or an efficient UV radiation production by a top-heavy IMF with Population III-like star formation.

Motivation & Objective

• Characterize dropout galaxy candidates at z~9-16 using early JWST NIRCam data.
• Derive UV luminosity functions for z~9-16 and compare with prior HST/JWST results.
• Estimate cosmic star-formation rate density and compare with models.
• Assess physical properties and potential star-formation efficiency in the pre-reionization era.

Proposed method

• Construct multi-band catalogs from JWST NIRCam data across ERO and ERS fields totaling ~90 arcmin^2.
• Apply secure dropout selection with color criteria and PSF-matched photometry to identify F115W, F150W, and F200W dropouts.
• Perform SED fitting with Prospector to obtain photometric redshifts and enforce a strict Delta chi^2 > 9 to separate high-z solutions from low-z interlopers.
• Cross-check with available NIRSpec redshifts and with simulated JWST data to validate the Delta chi^2 threshold and sample reliability.

Experimental results

Research questions

• RQ1 What is the rest-frame UV luminosity function at z~9, 12, and 16 as inferred from JWST/NIRCam dropouts?
• RQ2 How does the cosmic star-formation rate density evolve from z~9 to z~16 in the pre-reionization epoch?
• RQ3 Are there UV-bright, massive galaxies at z~10-16 consistent with efficient star formation in a UV background?
• RQ4 Do the JWST-derived results align with or challenge constant star-formation efficiency models?

Key findings

• A secure sample of 23 dropout galaxies at z~9-16 is identified, including two candidates at z_phot ≈ 16.25 and 16.41.
• The UV luminosity functions at z~9 and z~12 agree with those determined by other HST and JWST studies.
• The cosmic star-formation rate density decreases from z~9 to z~12 and perhaps to z~16, with densities at z~12-16 higher than a constant star-formation efficiency model.
• There are six bright candidates at z~10-16 with M_UV < -19.5 mag and M_* ~ 10^8-10^9 Msun, most showing no AGN signatures in their morphologies.
• The results suggest no suppression of star formation by UV background radiation at the pre-reionization epoch and/or efficient UV production by a top-heavy IMF with Population III-like star formation.
• The study demonstrates reliability through comparisons with other JWST studies and spectroscopic redshifts where available.