[Paper Review] The Hot and Energetic Universe: Understanding the build-up of supermassive black holes and galaxies at the heyday of the Universe

A. Georgakakis, F. J. Carrera+20 more|arXiv (Cornell University)|Jun 10, 2013

Astrophysical Phenomena and Observations1 references16 citations

TL;DR

This paper proposes that the Athena+ X-ray observatory will enable transformative studies of supermassive black hole and galaxy co-evolution during the universe's 'heyday' (z=1–4). By leveraging high-throughput X-ray spectroscopy and wide-field imaging, Athena+ will detect blue-shifted absorption lines to probe AGN feedback and identify up to 100× more obscured AGN—especially Compton-thick sources—thereby resolving key uncertainties in black hole growth and galaxy evolution.

ABSTRACT

Observations in the last decade have provided strong evidence that the growth of supermassive black holes at the centres of galaxies is among the most influential processes in galaxy evolution. Open questions that relate to our current understanding of black hole growth and its relation to the build-up of galaxies at redshifts z=1-4, when most black holes and stars we see in present-day galaxies were put in place, include: what is the nature of AGN feedback and whether it plays a significant role in the evolution of galaxies? what is the dominant population of accreting AGN at that critical epoch? is it dominated by obscured objects as required by many current observations and models? The Athena+ mission concept will provide the technological leap required for a breakthrough in our understanding of AGN and galaxy evolution at the heyday of the Universe. The high throughput of Athena+ will allow the systematic study of the incidence, nature and energetics of AGN feedback processes to z~4 via the identification and measurement of blue-shifted X-ray absorption lines. The excellent survey and spectral capabilities of the Athena+ Wide Field Imager will complete the census of black hole growth by yielding samples of up to 100 times larger than is currently possible of the most heavily obscured, including Compton thick, AGN to redshifts z~4. The demographics of this population relative to their hosts is fundamental for understanding how major black hole growth events relate to the build-up of galaxies.

Motivation & Objective

• Address open questions about AGN feedback and its role in shaping galaxy evolution during the peak epoch of black hole and star formation (z=1–4).
• Determine the dominant population of accreting AGN at high redshift, particularly whether obscured, Compton-thick AGN dominate the accretion budget.
• Complete the census of black hole growth by detecting faint, heavily obscured AGN beyond current survey limits.
• Establish the demographic link between AGN and their host galaxies to understand co-evolutionary processes.
• Enable systematic study of AGN feedback energetics via blue-shifted X-ray absorption lines across cosmic time.

Proposed method

• Utilize the high-throughput X-ray spectroscopy of Athena+ to detect and measure blue-shifted X-ray absorption lines, a direct tracer of outflowing ionized gas in AGN.
• Employ the Wide Field Imager (WFI) on Athena+ to conduct deep, wide-area surveys of the X-ray sky, enabling detection of rare, heavily obscured AGN up to z~4.
• Combine spectral resolution with high sensitivity to identify and characterize Compton-thick AGN, which are currently missed in most surveys.
• Use statistical analysis of AGN samples to determine the fraction of obscured versus unobscured AGN and their evolution with redshift.
• Correlate AGN properties (luminosity, obscuration, outflow velocity) with host galaxy characteristics to assess co-evolutionary trends.
• Leverage existing deep X-ray surveys (e.g., XMM-COSMOS, Chandra Deep Fields) as reference points for Athena+ survey design and expected yield.

Experimental results

Research questions

• RQ1 What is the nature and impact of AGN feedback on galaxy evolution at z=1–4, and does it regulate star formation?
• RQ2 Is the accretion of supermassive black holes at z=1–4 dominated by obscured or unobscured AGN, particularly Compton-thick sources?
• RQ3 How do the demographics of AGN (luminosity, obscuration, outflow properties) correlate with those of their host galaxies at high redshift?
• RQ4 To what extent do current models of black hole growth and galaxy evolution require revision in light of the full census of obscured AGN?
• RQ5 Can blue-shifted X-ray absorption lines be used as a reliable probe of AGN outflows and feedback energetics across cosmic time?

Key findings

• Athena+ will enable the detection of blue-shifted X-ray absorption lines in AGN outflows, providing direct evidence of AGN feedback at work.
• The Wide Field Imager on Athena+ will increase the number of detectable AGN by up to 100× compared to current surveys, especially for heavily obscured and Compton-thick sources.
• The mission will achieve a complete census of black hole growth at z~4, resolving the long-standing issue of missing obscured AGN in high-redshift surveys.
• By measuring the incidence and energetics of AGN feedback via X-ray spectroscopy, Athena+ will quantify the role of feedback in quenching star formation in massive galaxies.
• The combination of high sensitivity and spectral resolution will allow precise determination of the obscuration function and luminosity function of AGN up to z~4.
• The data from Athena+ will enable statistical studies linking AGN properties to host galaxy mass, star formation rate, and morphology, advancing co-evolution models.