High-redshift Galaxies and Black Holes Detectable with the JWST: A Population Synthesis Model from Infrared to X-Rays

Abstract

The first billion years of the Universe has been a pivotal time: stars, black holes (BHs), and galaxies formed and assembled, sowing the seeds of galaxies as we know them today. Detecting, identifying, and understanding 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. The spectral energy distribution of galaxies is determined by age and metallicity, and that of AGNs by BH mass and accretion rate. We validate the model against observations, and 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 the James Webb Space Telescope bands to separate AGN versus star-dominated galaxies in photometric observations. We also estimate the AGN contribution, with respect to massive, hot, and 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 occupation fraction of BHs in low-mass galaxies on the restframe UV/near-IR and X-ray AGN luminosity function.

High-redshift AGN in the Chandra Deep Fields: the obscured fraction and space density of the sub-L* population

“…The spread of the XLF shapes in this faint regime is expected to increase at higher redshifts (see Fig. 12 of Volonteri et al 2017). A tremendous step forward toward understanding the early growth of SMBH will be provided by Lynx (also known as X-ray Surveyor, Weisskopf constrained at lower redshifts, separately for obscured and unobscured AGN, we found an excess of obscured sources, likely caused by the positive evolution of F obsc from the local universe to high redshift.…”

“…At logLX 42.5, the luminosity limit we applied to our luminosity functions, as the 42 logLX 42.5 regime was strongly affected by incompleteness, the density of star-forming galaxies is a factor Figure 19. X-ray luminosity functions of AGN at z = 3 − 3.6 (left panel) and z = 3.6 − 6 (right panel), compared with results from previous observational (Aird et al 2010;Ueda et al 2014;Vito et al 2014;Buchner et al 2015;Georgakakis et al 2015;Giallongo et al 2015; extrapolated to lower luminosities than those probed by the original works) and theoretical (Habouzit et al 2016;Volonteri et al 2017) works at similar redshifts. In the right panel, the downward-pointing arrow is the upper limit derived by Vito et al (2016) through a stacking analysis of the 7 Ms CDF-S dataset, and the dashed green line is the expected XLF of HMXBs at z = 4 (from Volonteri et al 2017).…”

“…The green dashed line in Fig. 19 (right panel) is the XLF of star-forming galaxies at z = 4, where the emission is mostly due to high-mass XRB, from the model by Volonteri et al (2017, see in particular their section 2.2 for assumptions and caveats). Briefly, they mapped the stellar-mass functions of Song et al (2016) into an XLF through the mainsequence of star-forming galaxies (Salmon et al 2015) and the SFR-LX scaling relation by Fragos et al (2013, model 269), which Lehmer et al (2016) demonstrated to be the best representation at z = 0 − 2.5.…”

“…19 (right panel) is the locus of the XLF models of Volonteri et al (2017) at z = 4, under different assumptions. In particular, the upper bound is derived using a minimum initial BH mass of 10 2 M and occupation fraction equal to unity, while for the lower bound a minimum initial mass of 10 5 M and a modified occupation fraction from Habouzit et al (2016), flagged as "H17low1" in Volonteri et al (2017). Different combinations of parameters are mostly encompassed by these boundaries.…”

Warm dark matter constraints from high-z direct collapse black holes using the JWST

“…A number of works (Pacucci et al 2016;Natarajan et al 2017;Volonteri et al 2017) have developed intricate photometric selection criteria for DCBHs, especially in context of the JWST. In this work, we use the models developed in which, in brief, use the radiation-hydrodynamic code GEMS that models the accretion process onto the DCBH seed.…”