Chasing the observational signatures of seed black holes at z > 7: candidate statistics

TL;DR

This study investigates the early growth and observational signatures of seed black holes at high redshifts, using cosmological models to compare low- and high-mass seed formation scenarios and their evolution.

Contribution

It provides a detailed statistical analysis of SMBH progenitors at z~6, highlighting differences in properties based on seed mass and evolution, and explores their observational implications.

Findings

High-mass and low-mass seeds form in similar halos at z=15-18.

Differences in galaxy properties are evident at z>=10 but diminish after z<10.

Mergers erase initial differences in black hole origin signatures.

Abstract

Supermassive black holes (SMBHs) of 10^9-10^10 Msun were already in place ~13 Gyr ago, at z>6. Super-Eddington growth of low-mass BH seeds (~100 Msun) or less extreme accretion onto ~10^5 Msun seeds have been recently considered as the main viable routes to these SMBHs. Here we study the statistics of these SMBH progenitors at z~6. The growth of low- and high-mass seeds and their host galaxies are consistently followed using the cosmological data constrained model GAMETE/QSOdust, which reproduces the observed properties of high-z quasars, like SDSS J1148+5251. We show that both seed formation channels can be in action over a similar redshift range, 15 < z < 18 and are found in dark matter halos with comparable mass, ~5x10^7 Msun. However, as long as the systems evolve in isolation (i.e. no mergers occur), noticeable differences in their properties emerge: at z>= 10 galaxies hosting high-mass seeds have smaller stellar mass and metallicity, the BHs accrete gas at higher rates and star formation proceeds less efficiently than in low-mass seeds hosts. At z<10 these differences are progressively erased, as the systems experience minor or major mergers and every trace of the BH origin gets lost.