[Paper Review] The growth and evolution of super massive black holes

S. S. Murray, R. Gilli+20 more|ArXiv.org|Mar 30, 2009

Astrophysical Phenomena and Observations7 references3 citations

TL;DR

This paper advocates for the Wide-Field X-ray Telescope (WFXT) to conduct a large-area, deep X-ray survey with high sensitivity and large grasp, enabling the statistical study of supermassive black hole growth and evolution across cosmic time. By detecting thousands of AGNs, including Compton-thick and high-redshift sources, and enabling variability monitoring, WFXT would resolve key questions on black hole formation, feedback, and duty cycles.

ABSTRACT

We discuss the central role played by X-ray studies to reconstruct the past history of formation and evolution of supermassive Black Holes (BHs), and the role they played in shaping the properties of their host galaxies. We shortly review the progress in this field contributed by the current X-ray and multiwavelength surveys. Then, we focus on the outstanding scientific questions that have been opened by observations carried out in the last years and that represent the legacy of Chandra and XMM, as for X-ray observations, and the legacy of the SDSS, as for wide area surveys: 1) When and how did the first supermassive black holes form? 2) How does cosmic environment regulate nuclear activity (and star formation) across cosmic time? 3) What is the history of nuclear activity in a galaxy lifetime? We show that the most efficient observational strategy to address these questions is to carry out a large-area X-ray survey, reaching a sensitivity comparable to that of deep Chandra and XMM pointings, but extending over several thousands of square degrees. Such a survey can only be carried out with a Wide-Field X-ray Telescope (WFXT) with a high survey speed, due to the combination of large field of view and large effective area, i.e., grasp, and sharp PSF. We emphasize the important synergies that WFXT will have with a number of future groundbased and space telescopes, covering from the radio to the X-ray bands and discuss the immense legacy value that such a mission will have for extragalactic astronomy at large.

Motivation & Objective

• Address the unresolved cosmic history of supermassive black hole (SMBH) formation and growth, particularly during early epochs.
• Overcome limitations of current surveys by achieving both deep sensitivity and wide sky coverage to sample the full AGN population, including obscured and high-redshift sources.
• Investigate the role of galaxy environment and mergers in triggering nuclear activity and regulating star formation.
• Characterize the duty cycle of AGN activity, including the transition between obscured and unobscured phases.
• Enable statistical studies of X-ray variability to infer black hole mass and accretion rates across cosmic time.

Proposed method

• Propose a wide-field X-ray survey using a telescope with large effective area (grasp), wide field of view, and sharp point spread function (PSF) to achieve deep sensitivity over thousands of square degrees.
• Leverage repeated observations to enable monitoring of X-ray variability in AGNs, with sensitivity to flux changes of ~20% on timescales from hours to years.
• Use the iron Kα line at ~6.4 keV (shifted to ~3 keV at z≥1) as a spectroscopic diagnostic to identify Compton-thick AGNs and measure redshifts independently of optical data.
• Combine X-ray data with multiwavelength follow-up (optical, IR, radio) to classify AGN types, determine obscuration, and study host galaxy properties.
• Utilize the synergy with future optical surveys (e.g., LSST, Pan-STARRS) to cross-match X-ray variability with optical transients and time-domain phenomena.
• Model the expected source counts and redshift distributions to demonstrate the statistical power of WFXT compared to existing and proposed missions (e.g., NuSTAR, Simbol-X, EXIST).

Experimental results

Research questions

• RQ1 When and how did the first supermassive black holes form, and what mechanisms enabled their rapid growth in the first billion years after the Big Bang?
• RQ2 How does the cosmic environment—especially galaxy mergers and interactions—regulate nuclear activity and star formation across cosmic time?
• RQ3 What is the full history of nuclear activity in a galaxy’s lifetime, including the duty cycle and the relative phases of obscured and unobscured accretion?
• RQ4 To what extent is the X-ray variability of AGNs linked to black hole mass and accretion rate, and can this be used to infer physical properties at high redshift?
• RQ5 What is the true population of Compton-thick AGNs at z≥1, and how do they contribute to the cosmic X-ray background?

Key findings

• WFXT would detect approximately 10^5 AGNs in a 100 deg² survey down to a flux sensitivity allowing 20% flux variability detection, enabling robust statistical studies.
• The mission would provide a statistically significant sample of bona-fide Compton-thick AGNs at z≥1—previously inaccessible—offering direct insight into the obscured phase of SMBH growth.
• WFXT would detect ~10^5 AGNs with sufficient photon statistics to measure X-ray variability, allowing inference of black hole mass and accretion rate via scaling relations observed in local AGNs.
• The survey would identify transient X-ray outbursts from tidal disruption events, with a predicted detection rate of ~10^-5 per year per galaxy, enabling studies of stellar dynamics in galactic centers.
• Compared to existing and proposed missions (e.g., NuSTAR, EXIST, Simbol-X), WFXT offers a unique combination of large grasp and wide field of view, enabling orders-of-magnitude improvement in source statistics for high-redshift and obscured AGNs.
• The mission would resolve the discrepancy between the observed cosmic X-ray background and known AGN populations by detecting the missing population of high-redshift, heavily obscured AGNs.