First HETDEX Spectroscopic Determinations of Ly$\alpha$ and UV Luminosity Functions at $z=2-3$: Bridging a Gap Between Faint AGN and Bright Galaxies

Abstract

We present Ly$\alpha$ and ultraviolet-continuum (UV) luminosity functions (LFs) of galaxies and active galactic nuclei (AGN) at $z=2.0-3.5$ determined by the un-targetted optical spectroscopic survey of the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). We combine deep Subaru imaging with HETDEX spectra resulting in $11.4$ deg${}^2$ of fiber-spectra sky coverage, obtaining $18320$ galaxies spectroscopically identified with Ly$\alpha$ emission, $2126$ of which host type 1 AGN showing broad (FWHM$>1000$ km s${}^{-1}$) Ly$\alpha$ emission lines. We derive the Ly$\alpha$ (UV) LF over 2 orders of magnitude covering bright galaxies and AGN in $\log L_{\mathrm{Ly}\alpha }/[\mathrm{erg}{\mathrm{s}}^{-1}]=43.3-45.5$ ($-27<M_{\mathrm{UV}}<-20$) by the $1/V_{\max }$ estimator. Our results reveal the bright-end hump of the Ly$\alpha$ LF is composed of type 1 AGN. In conjunction with previous spectroscopic results at the faint end, we measure a slope of the best-fit Schechter function to be ${\alpha }_{\mathrm{Sch}}=-1.70_{-0.14}^{+0.13}$, which indicates ${\alpha }_{\mathrm{Sch}}$ steepens from $z=2-3$ towards high redshift. Our UV LF agrees well with previous AGN UV LFs, and extends to faint-AGN and bright-galaxy regimes. The number fraction of Ly$\alpha$-emitting objects ($X_{\mathrm{LAE}}$) increases from $M_{\mathrm{UV}}^{\ast }\sim -21$ to bright magnitude due to the contribution of type 1 AGN, while previous studies claim that $X_{\mathrm{Ly}\alpha }$ decreases from faint magnitude to $M_{\mathrm{UV}}^{\ast }$, suggesting a valley in the $X_{\mathrm{Ly}\alpha }-$magnitude relation at $M_{\mathrm{UV}}^{\ast }$. Comparing our UV LF of type 1 AGN at $z=2-3$ with those at $z=0$, we find that the number density of faint ($M_{\mathrm{UV}}>-21$) type 1 AGN increases from $z\sim 2$ to $z\sim 0$ as opposed to the evolution of bright ($M_{\mathrm{UV}}<-21$) type 1 AGN, suggesting the AGN downsizing in the rest-frame UV luminosity.

arXiv comment27 pages, 19 figures, accepted for publication in ApJ

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