Early results from GLASS-JWST. X: Rest-frame UV-optical properties of galaxies at 7 < z < 9

TL;DR

This paper presents early JWST/NIRCam observations of galaxies at redshifts 7-9, revealing their physical properties, number densities, and star formation activities, demonstrating JWST's potential to study early galaxy formation and reionization.

Contribution

First JWST/NIRCam-based analysis of galaxies at 7<z<9, providing detailed physical properties and a new framework for understanding galaxy evolution at these epochs.

Findings

Identified 13 galaxies at 7<z<9 with high confidence.

Galaxy properties suggest early star formation and ionization starting at z>9.5.

Results align with predictions from cosmological simulations.

Abstract

We present the first James Webb Space Telescope/NIRCam-led determination of $7<z<9$ galaxy properties based on broadband imaging from 0.8 to 5~$\mathrm{\mu m}$ as part of the GLASS-JWST Early Release Science program. This is the deepest dataset acquired at these wavelengths to date, with an angular resolution $\lesssim0.14$ arcsec. We robustly identify 13 galaxies with $S/N\gtrsim8$ in F444W from 8 arcmin$^2$ of data at $m_{AB}\leq 28$ from a combination of dropout and photometric redshift selection. From simulated data modeling, we estimate the dropout sample purity to be $\gtrsim90\%$. We find that the number density of these F444W-selected sources is broadly consistent with expectations from the UV luminosity function determined from Hubble Space Telescope data. We characterize galaxy physical properties using a Bayesian Spectral Energy Distribution fitting method, finding median stellar mass $10^{8.5}M_\odot$ and age 140 Myr, indicating they started ionizing their surroundings at redshift $z>9.5$. Their star formation main sequence is consistent with predictions from simulations. Lastly, we introduce an analytical framework to constrain main-sequence evolution at $z>7$ based on galaxy ages and basic assumptions, through which we find results consistent with expectations from cosmological simulations. While this work only gives a glimpse of the properties of typical galaxies that are thought to drive the reionization of the universe, it clearly shows the potential of JWST to unveil unprecedented details on galaxy formation in the first billion years.