Bridging Star-forming Galaxy and AGN Ultraviolet Luminosity Functions at z = 4 with the SHELA Wide-field Survey

Abstract

We present a joint analysis of the rest-frame ultraviolet (UV) luminosity functions of continuumselected star-forming galaxies and galaxies dominated by active galactic nuclei (AGNs) at z ∼ 4. These 3,740 z ∼ 4 galaxies are selected from broad-band imaging in nine photometric bands over 18 deg 2 in the 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 ± 0.10, and the corresponding AGN faint-end slope is α 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.

Modelling a bright z = 6 galaxy at the faint end of the AGN luminosity function

“…The number density of faint AGN is close to that of the brightest Lyman-break galaxies (LBGs) observed, with the faint end of the AGN LF overlapping with the bright end of the galaxy LF as found by the SHELLQs (e.g. Matsuoka et al 2018), GOLDRUSH or SHELA survey (Stevans et al 2018). This overlap happens just around the luminosity regime probed by the objects investigated by Giallongo et al (2015) at z ∼ 4, who found in the COSMOS field a larger than expected number of AGN candidates of this magnitude or fainter.…”

“…Bouwens et al 2017;Livermore et al 2017;Ishigaki et al 2018, but see also Atek et al 2018 for a detailed analysis of the model uncertainties), and therefore that the number of faint galaxies able to produce ionizing radiation is large In the recent years, a lot of work has been dedicated to identify faint active galactic nuclei (AGN) in the highredshift Universe and determine their contribution to the reionization of the Universe (e.g. Giallongo et al 2015;Ricci et al 2017;Boutsia et al 2018;Parsa et al 2018;Matsuoka et al 2018;Stevans et al 2018). These AGN, while less luminous than bright quasars, could in principle be far more numerous.…”

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

“…Bouwens et al 2015;Finkelstein et al 2015), whereas other studies have suggested a rapid change in this parameter at z > 5 (e.g. McLure et al 2009McLure et al , 2013Bunker et al 2010;Wilkins et al 2011;Oesch et al 2012;Schenker et al 2013;Schmidt et al 2014;Bowler et al 2015;Stevans et al 2018;Viironen et al 2018).…”

“…To compensate we elect to use the data points from Bouwens et al (2015) for M UV ≥ −20 to allow for stronger constraints on the LBG faint-end slope. Furthermore, although our sample contains objects as bright as M UV ∼ −26, we do not have sufficient survey volume at this redshift to determine the position of the knee of the AGN LF at M UV −26.5 (Akiyama et al 2018;Stevans et al 2018). We therefore fit the AGN component of the LF with a single power law with the normalisation calculated at M UV = −25.7.…”

“…While previous LBG and AGN studies have been undertaken separately, the advent of wide-area LBG studies and deeper, smaller-area AGN studies, has resulted in a convergence of the samples. Around M UV −23.5 the number density of the sources of the two populations at z 4 are found to be comparable (Stevans et al 2018;Ono et al 2018). As the two populations can be selected in similar ways, contamination makes the derivation of the faint-end AGN slope and bright-end LBG slope difficult.…”