The Hot and Energetic Universe: The formation and growth of the earliest supermassive black holes

TL;DR

This paper discusses how upcoming X-ray observatories like Athena+ will revolutionize our understanding of the origins and early growth of supermassive black holes in the universe by enabling the detection of obscured and low luminosity AGNs at high redshifts.

Contribution

It highlights the potential of Athena+ to identify and study early SMBHs, including obscured sources, significantly advancing knowledge of their formation and growth in the early universe.

Findings

Athena+ will detect over 400 z>6 AGNs, including obscured ones.

X-ray observations are crucial for identifying early SMBHs unaffected by star formation.

Athena+ will enable first quantitative measurements of SMBH accretion in the early universe.

Abstract

A crucial challenge in astrophysics over the coming decades will be to understand the origins of supermassive black holes (SMBHs) that lie at the centres of most, if not all, galaxies. The processes responsible for the initial formation of these SMBHs and their early growth via accretion - when they are seen as Active Galactic Nuclei (AGN) - remain unknown. To address this challenge, we must identify low luminosity and obscured z>6 AGNs, which represent the bulk of early SMBH growth. Sensitive X-ray observations are a unique signpost of accretion activity, uncontaminated by star formation processes, which prevent reliable AGN identification at other wavelengths (e.g. optical, infrared). The Athena+ Wide Field Imager will enable X-ray surveys to be carried out two orders of magnitude faster than with Chandra or XMM-Newton, opening a new discovery space and identifying over 400 z>6 AGN, including obscured sources. Athena+ will also play a fundamental role to enhance the scientific return of future multiwavelength facilities that will probe the physical conditions within the host galaxies of early SMBHs, which is vital for understanding how SMBHs form, what fuels their subsequent growth, and to assess their impact on the early Universe. Follow-up of samples of z>6 galaxies with the Athena+ X-ray Integral Field Unit could also reveal the presence of highly obscured AGNs, thanks to the detection of strong iron lines. Thus, Athena+ will enable the first quantitative measurements of the extent and distribution of SMBH accretion in the early Universe.