The $z \gtrsim 9$ galaxy UV luminosity function from the JWST Advanced Deep Extragalactic Survey: insights into early galaxy evolution and reionization

TL;DR

Using deep JWST/NIRCam imaging from the JWST Advanced Deep Extragalactic Survey, the study directly constrains the rest-UV luminosity function at $z \gtrsim 9$ down to $M_\mathrm{UV} \sim -17$, extending previous work to fainter galaxies. They identify about 309 candidates at $z \sim 9-12$ and 63 at $z \sim 12-16$ (with no detections at $z \sim 16-22.5$) and derive the luminosity function in two redshift bins, including an estimate for $z \sim 14-16$. The results show a bright-end excess relative to many models at $z \gtrsim 12$, together with a high number density at $-18 \lesssim M_\mathrm{UV} \lesssim -17$, implying a substantial faint-galaxy population. Parametric fits yield faint-end slopes of $-2.5 \lesssim \alpha \lesssim -2.3$, indicating a steep faint-end and a large population of faint galaxies that could drive the UV luminosity density. Taken together, the elevated normalization and steep faint-end slope suggest that the reionization process was already well underway by $z \approx 10$ and place strong constraints on galaxy-formation physics in the first 500 Myr.

Abstract

The high-redshift UV luminosity function provides important insights into the evolution of early galaxies. JWST has revealed an unexpectedly large population of bright ($M_\mathrm{UV} \lesssim -20$) galaxies at $z\gtrsim10$, implying fundamental changes in the star forming properties of galaxies at increasingly early times. However, constraining the fainter population ($M_\mathrm{UV} \gtrsim -18$) has been more challenging. In this work, we present the $z\gtrsim9$ UV luminosity function from the JWST Advanced Deep Extragalactic Survey. We calculate the UV luminosity function from several hundred $z\gtrsim9$ galaxy candidates that reach UV luminosities of $M_\mathrm{UV}\sim-17$ in redshift bins of $z\sim9-12$ (309 candidates) and $z\sim12-16$ (63 candidates). We search for candidates at $z\sim16-22.5$ and find none. We also estimate the $z\sim14-16$ luminosity function from the $z\geq14$ subset of the $z\sim12-16$ sample. Consistent with other measurements, we find an excess of bright galaxies that is in tension with many theoretical models, especially at $z\gtrsim12$. However, we also find high number densities at $-18\lesssim M_\mathrm{UV} \lesssim-17$, suggesting that there is a larger population of faint galaxies than expected, as well as bright ones. From our parametric fits for the luminosity function, we find steep faint end slopes of $-2.5\lesssimα\lesssim-2.3$, suggesting a large population of faint ($M_\mathrm{UV} \gtrsim -17$) galaxies. Combined, the high normalization and steep faint end slope of the luminosity function could imply that the reionization process is appreciably underway as early as $z=10$.