# High-redshift galaxies and black holes in the eyes of JWST: a population synthesis model from infrared to X-rays

## TL;DR

This paper introduces a population synthesis model for high-redshift galaxies, black holes, and AGNs, validated against observations, predicting their properties across wavelengths and proposing methods to distinguish between stellar and AGN emissions in JWST data.

## Contribution

The study presents a novel empirical population synthesis model for early universe galaxies and black holes, with validation and predictions across multiple wavelengths, aiding in observational identification.

## Key findings

- Disentangling stellar and AGN emission is challenging at UV/optical wavelengths for young high-redshift galaxies.
- High-mass X-ray binaries are significant sources of confusion in X-ray observations.
- A color-color selection method in JWST bands is proposed to distinguish AGN from star-dominated galaxies.

## Abstract

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.