Early Results from GLASS-JWST. X. Rest-frame UV-optical Properties of Galaxies at 7 < z < 9

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 mu m as part of the GLASS-JWST Early Release Science program. This is the deepest data set acquired at these wavelengths to date, with an angular resolution less than or similar to 0.'' 14. We robustly identify 13 galaxies with signal-to-noise ratio greater than or similar to 8 in F444W from 8 arcmin(2) of data at m (AB) <= 28 from a combination of dropout and photometric redshift selection. From simulated data modeling, we estimate the dropout sample purity to be greater than or similar to 90%. 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 a median stellar mass of 10(8.5) M (circle dot) 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 of galaxy formation in the first billion years.

Automated summary

This study presents the first determination of galaxy properties at 7 < z < 9 based on broadband imaging data from the James Webb Space Telescope/NIRCam. Thirteen galaxies with high signal-to-noise ratio were identified using dropout and photometric redshift selection. The physical properties of these galaxies, including their mass and age, were characterized using a Bayesian spectral energy distribution fitting method. The results demonstrate the potential of JWST in unveiling unprecedented details of galaxy formation in the early universe.