Bridging Star-Forming Galaxy and AGN Ultraviolet Luminosity Functions at $z=4$ with the SHELA Wide-Field Survey

TL;DR

This study jointly analyzes UV luminosity functions of star-forming galaxies and AGNs at z=4 over a large area, revealing the shape of their bright and faint ends and implications for galaxy evolution and ionizing emissivity.

Contribution

It provides the first simultaneous fit of galaxy and AGN UV luminosity functions at z=4 using a large survey, exploring their shapes and implications for cosmic reionization.

Findings

Both luminosity functions are best fit with a double power-law.

Galaxy bright-end slope is approximately -3.80.

AGN faint-end slope varies depending on the model, from -1.49 to -2.08.

Abstract

We present a joint analysis of the rest-frame ultraviolet (UV) luminosity functions of continuum-selected star-forming galaxies and galaxies dominated by active galactic nuclei (AGNs) at $z \sim$ 4. These 3,740 $z \sim$ 4 galaxies are selected from broad-band imaging in nine photometric bands over 18 deg$^2$ in the \textit{Spitzer}/HETDEX Exploratory Large Area Survey (SHELA) field. The large area and moderate depth of our survey provide a unique view of the intersection between the bright end of the galaxy UV luminosity function (M$_{AB}<-$22) and the faint end of the AGN UV luminosity function. We do not separate AGN-dominated galaxies from star-formation-dominated galaxies, but rather fit both luminosity functions simultaneously. These functions are best fit with a double power-law (DPL) for both the galaxy and AGN components, where the galaxy bright-end slope has a power-law index of $-3.80\pm0.10$, and the corresponding AGN faint-end slope is $\alpha_{AGN} = -1.49^{+0.30}_{-0.21}$. We cannot rule out a Schechter-like exponential decline for the galaxy UV luminosity function, and in this scenario the AGN luminosity function has a steeper faint-end slope of $-2.08^{+0.18}_{-0.11}$. Comparison of our galaxy luminosity function results with a representative cosmological model of galaxy formation suggests that the molecular gas depletion time must be shorter, implying that star formation is more efficient in bright galaxies at $z=4$ than at the present day. If the galaxy luminosity function does indeed have a power-law shape at the bright end, the implied ionizing emissivity from AGNs is not inconsistent with previous observations. However, if the underlying galaxy distribution is Schechter, it implies a significantly higher ionizing emissivity from AGNs at this epoch.