  The first billion years of the Universe is a pivotal time: stars, black holes
(BHs) and galaxies form and assemble, sowing the seeds of galaxies as we know
them today. Detecting, identifying and understand- ing the first galaxies and
BHs is one of the current observational and theoretical challenges in galaxy
formation. In this paper we present a population synthesis model aimed at
galaxies, BHs and Active Galactic Nuclei (AGNs) at high redshift. The model
builds a population based on empirical relations. Galaxies are characterized by
a spectral energy distribution determined by age and metallicity, and AGNs by a
spectral energy distribution determined by BH mass and accretion rate. We
validate the model against observational constraints, and then predict
properties of galaxies and AGN in other wavelength and/or luminosity ranges,
estimating the contamination of stellar populations (normal stars and high-mass
X-ray binaries) for AGN searches from the infrared to X-rays, and vice-versa
for galaxy searches. For high-redshift galaxies, with stellar ages < 1 Gyr, we
find that disentangling stellar and AGN emission is challenging at restframe
UV/optical wavelengths, while high-mass X-ray binaries become more important
sources of confusion in X-rays. We propose a color-color selection in JWST
bands to separate AGN vs star-dominated galaxies in photometric observations.
We also esti- mate the AGN contribution, with respect to massive, hot,
metal-poor stars, at driving high ionization lines, such as C IV and He II.
Finally, we test the influence of the minimum BH mass and occupa- tion fraction
of BHs in low mass galaxies on the restframe UV/near-IR and X-ray AGN
luminosity function.
