Cosmic Feedback
The Growth of Supermassive Black Holes at Early Times
X-ray luminosity versus redshift for some potential Constellation-X targets. The compiled sources are a heterogeneous combination of different samples in the literature, including wide and deep Chandra blank- field observations and X-ray targeted AGNs. The sources above the solid line can be studied by Constellation-X in the 0.5-10 keV band in < 100 ks exposures. The sources above the dashed line can be detected by Constellation-X in the 10-40 keV band. An X-ray spectral slope of Γ=2.0 was assumed when converting from the 0.5-8.0 keV band to the Constellation-X bands.
Our understanding of the growth and evolution of massive black holes has undergone a revolution over the last few years. It now seems likely that the development of black holes and galaxies are intimately connected, possibly due to accretion-related outflows that regulate star formation (e.g., Silk and Rees 1998, King and Pounds 2003). This growth appears to undergo a curious evolutionary trend whereby black holes are built-up via accretion with the most massive black holes forming first, a process often referred to as cosmic downsizing (e.g., Cowie et al. 2003, Marconi et al. 2004). The number density of these accreting massive black holes [i.e., Active Galactic Nuclei (AGN)] changes dramatically over the history of the Universe; however, there is no clear observational evidence that AGNs of a given luminosity undergo substantial changes in their underlying accretion processes (e.g., Vignali, Brandt, and Schneider 2003). Furthermore, it is apparent that a large (likely luminosity-dependent) fraction of AGNs are obscured at optical wavelengths but are detectable at hard X-ray energies (e.g., Mainieri et al. 2002, Ueda et al. 2003, Barger et al. 2005). These obscured sources are suspected to be the dominant AGN population in the Universe (outnumbering unobscured AGNs by factors of 3-10) and contribute the bulk of the cosmic Xray background (CXRB) radiation. Constellation-X will probe the growth of black holes in the high-redshift Universe with unprecedented precision.
Simulated spectra demonstrating the capabilities of Constellation-X for the three highlighted sources (1) a z = 6.3 quasar, (2) a z = 2.47 Compton-thick AGN in a submillimeter-bright star-forming galaxy, and (3) a z = 1.06 obscured quasar. Interesting spectral features are highlighted.
Constellation-X will study z ~ 1 AGNs with the detail only currently possible for the ~10 brightest local AGN (e.g. NGC 5548); luminous z > 6 quasars will be more accessible to Constellation-X at high spectral resolution than z ~ 1 quasars are to Chandra and XMM-Newton at much lower spectral resolution. Coupled with current and future multiwavelength data, these observations will provide comprehensive probes of the physics of accretion around high-redshift massive black holes. The large-scale AGN outflows that may regulate star formation in massive galaxies will be studied in detail, providing estimates of mass and energy outflow rates and chemical enrichment/heating of the IGM. The energetics and demographics of high-redshift obscured AGNs will be quantified and many luminous Compton-thick AGNs will be revealed with Constellation-X's high-energy sensitivity out to 40 keV.
The basic picture of an AGN is an accretion disk funneling material into a supermassive black hole. AGNs emit X-rays as a result of Compton upscattering of ultraviolet photons from the accretion disk as they pass through a bath of high energy electrons in the accretion disk corona. This X-ray emission provides a unique window on the environment closest to the accreting black hole (to Rg~1.2) which is inaccessible at other wavelengths. Constellation-X will provide X-ray spectroscopy of AGNs down to 0.5-8 keV flux levels of ~10-15 erg s-1 cm-2 permitting constraints on the continuum shape, absorption, recombination emission, fluorescent iron K line emission, Compton reflection, and variability of accreting black holes out to and beyond z = 6. At high redshift this emission gets shifted to lower observed energies, providing measurements at energies that may otherwise be inaccessible for studies of local AGNs.
Typical soft X-ray through to hard X-ray spectral energy distribution of an AGN, showing the variety of spectral features that Constellation-X's resolution, throughput, and energy coverage will reveal. The broad Constellation-X bandpass covers an energy range equivalent to the far-ultraviolet-mid-infrared band at longer wavelengths. At z = 0, Constellation-X will sample 0.25-40 keV; at z = 6, Constellation-X will sample 1.75-280 keV.
Since the launches of Chandra and XMM-Newton and the advent of wide-field optical surveys (see Brandt et al. 2005 for a review), the number of X-ray detections at z > 4 has increased from 6 in 2000 to ~100 in the year 2005. No significant changes in the X-ray emission properties of AGNs at high and low redshift have yet been found, suggesting that the accretion disk environment of AGNs are insensitive to the evolution over the last 12 billion years (e.g., Vignali, Brandt, and Schneider 2003, Strateva et al. 2005). However, the Chandra and XMM-Newton detections of high-z AGN are just that – detections. As a result of comparatively poor photon statistics, we do not have sufficient data to study their detailed properties; existing studies have generally been restricted to broad comparisons of their spectral energy distributions (SEDs). Constellation-X will permit comparisons of X-ray spectral features and emission regions. therefore providing direct astrophysical insight into the evolution of the environment around accreting massive black holes.
A number of local AGN have shown relativistic iron Kα emission, permitting measurements of the black hole spin and environment of the inner accretion disk (e.g., Reynolds & Nowak 2003). By comparison, luminous high-redshift AGN do not typically show these features (the weakness of iron Kα in luminous sources is known as the "X-ray Baldwin effect"); however, the stacked Xray spectra of X-ray faint high-redshift AGN appear to show the signature of relativistically broadened iron Kα emission (Streblyanska et al. 2005). Constellation-X has the sensitivity to identify and study iron Kα emission lines in individual high-redshift AGN, providing insight into the conditions necessary to produce iron Kα emission and its evolution with redshift. Beyond the iron Kα emission line and the reflection component (which peaks at 20-30 keV) the hard X-ray spectra of AGN are dominated by the Compton upscattered component. Since there are currently no reliable measures of the high-energy Compton cutoff in quasars, the sensitive 10-40 keV hard energy coverage of Constellation-X will provide new insights into our understanding of the state (via the cut-off temperature) and structure of the X-ray emitting accretion-disk coronae (e.g., Sobolewska, Siemiginowska, and Zycki 2004, Zdziarski and Gierlinski 2004). For high-redshift quasars where we can probe rest-frame energies of ~100-300 keV (perhaps up to 600 keV), we can expect to see the high-energy cutoff (50-200 keV) in some cases, thus allowing a determination of the electron temperature and optical depth of the Comptonizing plasma. This information is key to understanding of the geometry of the corona, and, in turn, to model the accretion disk and disk-wind emission.
These studies of the central engine of high-redshift AGN benefit significantly from Constellation-X's hard energy coverage, which for the highest-redshift quasars currently known will probe rest frame energies higher than 240 keV. However, the background must be lower than the expected 10-40 keV fluxes of typical z ~ 4 quasars of ~10-14-10-15 erg s-1 cm-2. Response at low energies is also important as crucial spectral features move with redshift; for example, iron Kα is observed at ~1 keV at z ~ 5 and the presence of moderate absorption (i.e., NH ~ 2 × 1022 cm-2) becomes increasingly difficult to identify at high redshift without good low-energy response.
Read more:
- Accretion-disk Outflows and their Role in Galaxy Evolution
- The Energetics and Role of Obscured Accretion
References
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King, A.R., and Pounds, K.A. 2003, MNRAS, 345, 657
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