The rest-frame UV luminosity function at $z \simeq 4$: a significant contribution of AGN to the bright-end of the galaxy population

Abstract: We measure the rest-frame UV luminosity function (LF) at $z \sim 4$ self-consistently over a wide range in absolute magnitude ($-27 \lesssim M_{\rm UV} \lesssim -20$). The LF is measured with 46,904 sources selected using a photometric redshift approach over $ \sim 6$ deg$^2$ of the combined COSMOS and XMM-LSS fields. We simultaneously fit for both AGN and galaxy LFs using a combination of Schechter or Double Power Law (DPL) functions alongside a single power law for the faint-end slope of the AGN LF. We find a lack of evolution in the shape of the bright-end of the LBG component when compared to other studies at $z \simeq 5$ and evolutionary recipes for the UV LF. Regardless of whether the LBG LF is fit with a Schechter function or DPL, AGN are found to dominate at $M_{\rm UV} < -23.5$. We measure a steep faint-end slope of the AGN LF with $\alpha_{AGN} = -2.09^{+0.35}_{-0.38}$ ($-1.66^{+0.29}_{-0.58}$) when fit alongside a Schechter function (DPL) for the galaxies. Our results suggest that if AGN are morphologically selected it results in a bias to lower number densities. Only by considering the full galaxy population over the transition region from AGN to LBG domination can an accurate measurement of the total LF be attained.