The rest-frame UV luminosity function at z ≃ 4: a significant contribution of AGNs to the bright end of the galaxy population

Abstract

We measure the rest-frame UV luminosity function (LF) at z ∼ 4 self-consistently over a wide range in absolute magnitude (−27 M UV −20). The LF is measured with 46,904 sources selected using a photometric redshift approach over ∼ 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 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 UV < −23.5. We measure a steep faint-end slope of the AGN LF with α AG N = −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.

The rapid transition from star formation to AGN-dominated rest-frame ultraviolet light at z ≃ 4

“…This allowed us to identify any potential systematics in the size measurement from using data of different depth and seeing. The HSC -band is available across both fields, with varying depth, while the CFHT -band is uniform in depth but is available for only a 1 deg 2 subsection of each field (Bowler et al 2020 Adams et al (2020). The samples in the COSMOS and XMM-LSS fields are shown in the upper and lower plots respectively.…”

“…first samples that bridge both faint AGN as well as bright galaxies, filling in a previously unachievable parameter space in volume and luminosity (Matute et al 2013;Kashikawa et al 2015;Matsuoka et al 2018b;Stevans et al 2018;Ono et al 2018;Adams et al 2020). The properties of these intermediate luminosity sources are important for several reasons.…”

“…These studies have been able to constrain the faint-end 2 of the AGN LF for the first time at 4, however there remain large discrepancies in the derived faint-end slope which ranges from −1.3 (Matsuoka et al 2018b;Akiyama et al 2018) to as steep as −2 (McGreer et al 2018;Giallongo et al 2019;Shin et al 2020). A key challenge in the robust determination of the number density of the lowest luminosity AGN is that faint-ward of a certain absolute UV magnitude ( UV −23; Stevans et al 2018;Ono et al 2018;Adams et al 2020), LBGs become overwhelmingly more numerous. In response, AGN selection methodologies have typically included a condition that the source must be unresolved in imaging data, as expected for a source dominated by the central AGN.…”

“…In Adams et al (2020) we selected a sample of LBGs and AGN at 4 from the COSMOS and XMM-LSS deep extragalactic fields using a photometric redshift analysis based on the groundbased optical to NIR photometry. The advantages of this sample over previous studies are i) we do not impose any condition on source size or morphology and hence we are complete to both point-sources and extended galaxies, ii) we exploit the NIR data which results in a very clean selection of 4 sources and iii) we have used two of the most widely studied deep fields where there is wealth of deep multi-wavelength data and spectroscopy available.…”

UV bright red-sequence galaxies: a comparative study between UV upturn and UV weak systems Maria

“…• AGN have been pointed to influence the UV emission of galaxies, could those be meddling in the sample (see e.g. Adams et al, 2020)?…”