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 (2208.01612v3)

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 ($\Delta\chi^2>4$). We thus carefully apply a secure photo-$z$ selection criterion ($\Delta\chi^2>9$) 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.

• The paper identifies 23 dropout galaxies at z~9-16 using early JWST NIRCam data with robust photometric redshift estimates.
• It demonstrates that UV luminosity functions at these epochs prefer a double power-law fit over a Schechter function, indicating a decline in star formation rates from z~9 to 12.
• The findings suggest enhanced star formation efficiencies or low-metallicity stellar populations, prompting a reevaluation of early galaxy evolution models.

Overview of "A Comprehensive Study on Galaxies at $z\sim9-16$ Found in the Early JWST Data: UV Luminosity Functions and Cosmic Star-Formation History at the Pre-Reionization Epoch"

The paper presented by Harikane et al. focuses on the exploration of high-redshift galaxies (specifically at $z \sim 9-16$) using data from the James Webb Space Telescope (JWST). The investigation is centered on understanding the ultraviolet (UV) luminosity functions and the cosmic star-formation rates at these early epochs, just before the epoch of reionization.

Key Highlights and Methodology

1. Data Acquisition and Selection: The paper utilizes the first 90 arcmin$^2$ of JWST/NIRCam images from the early release observations and science programs. The authors implement a stringent selection criterion for dropout galaxies to ensure robust photometric redshift estimates, thus reducing the contamination by foreground interlopers.
2. Sample Identification: A total of 23 dropout galaxies were identified in the redshift range $z \sim 9-16$, based on criteria that require significant continuum breaks and specific color indices. Further, only galaxies that met a secure photometric redshift confidence ($\Delta\chi^2>9$) were included in the paper.
3. Luminosity Function Analysis: The UV luminosity functions derived at $z \sim 9$, $12$, and $16$ were found to align with previous results at high redshifts. The paper suggests that a double power-law fit is more favorable over a Schechter function, especially at $z \sim 9$. The evolution of these luminosity functions supports a decrease in the cosmic star formation rate from $z \sim 9$ to $12$.
4. Cosmic Star Formation Rates: The star formation rate densities (SFRD) calculated indicate a decline from $z \sim 9$ to $12$. The data suggest that SFRD might be higher than predicted by constant star formation efficiency models for redshifts beyond $10$, which resonates with less suppression of star formation due to less intense UV background radiation at these times.
5. Implications of Observed Properties: The findings imply two potential scenarios for the elevated SFRD and bright galaxy candidates observed: (a) Star formation efficiencies at these redshifts are enhanced due to the absence of suppression by the UV background, or (b) The stellar populations at these epochs are of low metallicity, possibly resembling Population III stars with top-heavy initial mass functions, leading to high UV outputs for the same SFR.

Implications and Theoretical Considerations

• The paper critically informs our understanding of galaxy formation and evolution during the Pre-Reionization Epoch. High stellar masses and UV luminosities of galaxies at such early times may necessitate a revision in models of galaxy formation and the role of Population III stars.
• The theoretical implications are profound; the absence of a strong UV background radiation at these epochs could significantly alter the assumed suppression mechanisms in low-mass halos, thereby impacting galaxy evolution theories.
• There is potential for JWST's future observations to significantly refine our understanding of the UV luminosity function evolution and star formation processes in the early universe.

Future Directions

Future observations, particularly spectroscopy with the JWST and further imaging studies, will be crucial to substantiate these findings. Such data could help distinguish between stellar contributions and other sources of luminosity, such as AGNs, and could also help understand the contribution of Population III-like stellar populations to the early cosmic SFRD. The paper lays critical groundwork for these future investigations, highlighting the transformative potential of JWST in unveiling the mysteries of the early universe.