Abstract
ABSTRACT With the advent of deep optical-to-near-infrared extragalactic imaging on the degree scale, samples of high-redshift sources are being selected that contain both bright star-forming (SF) galaxies and faint active galactic nuclei (AGN). In this study, we investigate the transition between SF- and AGN-dominated systems at z ≃ 4 in the rest-frame ultraviolet (UV). We find a rapid transition to AGN-dominated sources brightward of MUV ≃ −23.2. The effect is observed in the rest-frame UV morphology and size–luminosity relation, where extended clumpy systems become point-source-dominated, and also in the available spectra for the sample. These results allow us to derive the rest-frame UV luminosity function (LF) for the SF- and AGN-dominated subsamples. We find the SF-dominated LF is best fit with a double power law, with a lensed Schechter function being unable to explain the existence of extremely luminous SF galaxies at MUV ≃ −23.5. If we identify AGN-dominated sources according to a point-source morphology criterion, we recover the relatively flat faint-end slope of the AGN LF determined in previous studies. If we instead separate the LF according to the current spectroscopic AGN fraction, we find a steeper faint-end slope of α = −1.83 ± 0.11. Using a simple model to predict the rest-frame AGN LF from the z = 4 galaxy LF, we find that the increasing impact of host galaxy light on the measured morphology of faint AGN can explain our observations.
Highlights
How supermassive black holes and their host galaxies co-evolve over cosmic time poses many fundamental questions within Astrophysics
As we are primarily concerned with the objects in the ‘transition’ regime where active galactic nuclei (AGN) and LBGs have similar number densities, this analysis focuses on the results at MUV −22
At these bright magnitudes we have a larger proportion of spectroscopic follow-up from magnitude limited surveys and we are able to identify the source morphology at high S/N in the high-resolution HST data
Summary
How supermassive black holes and their host galaxies co-evolve over cosmic time poses many fundamental questions within Astrophysics. The populations of quasars and SF galaxies at redshifts z = 4 − 8 have typically been treated as separate, due primarily to the disparate luminosity space occupied by the current samples. This is despite the majority of galaxies and quasars at very high-redshifts being selected based on the same spectral feature in optical/NIR survey data; the Lyman-continuum and/or the Lyman-α break. The advent of intermediate surveys that probe areas up to a few hundred square degrees on the sky has led to the
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