High-redshift AGN holds the key to understanding early structure formation and to probe the Universe during its infancy. We review the latest searches for high- AGN in the deepest X-ray field so far, the Chandra Deep Field South (CDFS) 4 Msecond exposure. We do not confirm the positive detection of a signal in the stacked Chandra images at the position of galaxies recently reported by Treister and collaborators (2011). We present X-ray sources number counts in the 0.5–2 keV band, obtained joining CDFS faint detections (see Fiore et al. (2011)), with Chandra-COSMOS and XMM-COSMOS detections. We use these number counts to make predictions for surveys with three mission concepts: Athena, WFXT, and a Super-Chandra.

1. Introduction

The study of high-redshift AGN holds the key to understanding early structure formation and probing the Universe during its infancy. There are fundamental issues that can be tackled by studying high- AGN (1) the evolution of the correlations between the black hole mass and the galaxy properties (see, e.g., [1] and references therein); (2) the AGN contribution to the reionization and to the heating of the Intergalactic medium and its effect on structure formation (e.g., [2, 3] and references therein); (3) scenarios for the formation of the black hole (BH) seeds that will eventually grow up to form the supermassive black holes (SMBHs) seen in most galaxy bulges (e.g., [4]); (4) we can investigate the physics of accretion at high-. One open question is whether BH growth is mainly due to relatively few accretion episodes, as predicted in hierarchical scenarios (see, e.g., [5] and references therein) or by the so-called chaotic accretion (hundreds to thousands of small accretion episodes, [6]). The two scenarios predict different BH spin distributions, and thus different distributions for the radiative efficiency; (5) since BHs are the structures with the fastest (exponential) growth rate, they can be used to constrain both the expansion rate of the Universe and the growth rate of the primordial perturbation at high-, that is, competing cosmological scenarios [7, 8]; (6) since the slopes of both the high- AGN luminosity function and the SMBH mass function strongly depend on the AGN duty cycle, their measurements can constrain this critical parameter. In turn, the AGN duty cycle holds information on the AGN triggering mechanisms. The evaluation of the evolution of the AGN duty cycle can thus help us to distinguish the competing scenarios for AGN triggering and feeding [9].

Large area optical and near infrared surveys such as the SDSS, the CFHQS, the NOAO DWFS/DLS, and the UKIDSS surveys have already been able to discover large samples of QSOs (e.g., [10, 11]) and about 50 QSOs at (e.g., [1214]). The majority of these high- AGN are broad line, unobscured, high UV rest-frame luminosity (thus high bolometric luminosity) AGN. They are likely the tips of the iceberg of the high- AGN population. Lower bolometric luminosity and/or moderately obscured AGN can, in principle, be detected directly in current and future X-ray surveys. Dedicated searches for high- AGN using both deep and wide area X-ray surveys and a multiband selection of suitable candidates can increase the number of high- AGN by a factor >10. In particular, it should be possible to find hundreds rare high-, high luminosity QSOs, in both the all sky and deep eROSITA surveys (the 0.5–2 keV flux limit of the all sky survey being the order of 10−14 erg/cm2/s, while that of the deep survey, covering hundreds deg2, should be 2-3 times deeper [15]) with a selection function much less biased than optical surveys. To constrain the faint end of the high- AGN luminosity function and therefore the shape of the luminosity function and of the SMBH mass function, we need to best exploit current and future deep surveys. The Chandra Deep Field South is today the premiere field, with its 4 Msec and 3 Msec exposures obtained by Chandra and XMM, respectively, since 1999 [16, 17]. Three different approaches have been so far applied to this field: (a) direct detection of sources in X-ray maps (e.g., [16]); (b) search for X-ray emission at the position of candidate high- galaxies selected in the red and near infrared bands [9]; (c) stacking of X-ray counts at the position of candidate high- galaxies [18]. Here, we review all three methods and give state of the art number counts of high- AGN at faint fluxes. We use these number counts to predict the number of high- AGN in possible future deep X-ray surveys. A km s−1 Mpc−1, , cosmology is adopted throughout.

2. Stacking Analysis of Candidate High- Galaxies

Recently, Treister et al. [18] published a positive detection of X-ray counts in stacked Chandra images obtained adding together the X-ray counts at the position of 197 candidate high- galaxies at in the CDFS and CDFN [19]. They find 5σ and 6.8σ detections in the soft 0.5–2 keV and hard 2–8 keV bands. Since the 2–8 keV flux they detect is about 9 times the 0.5–2 keV flux, they infer that the majority of these faint high- galaxies host highly obscured, Compton thick AGN. The total rest frame 2–10 keV luminosity density implied by the Treister [18] result is 1.6 × 1046 ergs/s/deg2 at . In contrast, Fiore et al. [9] analyzed X-ray counts at the position of the same Bouwens et al. [19] galaxies in the CDFS finding just one marginal detection. Fiore et al. [9] find that the luminosity function can be modeled using the standard double power law shape with  ergs/s, , and (the faint end slope is not truly constrained). By integrating, this luminosity function above a luminosity of 1042 erg/s, one obtains a total rest frame 2–10 keV luminosity density at of 5.6 × 1045 ergs/s/deg2, a value ~3 times smaller than that reported by Treister et al. [18]. We investigated this discrepancy between the Treister [18] and Fiore [9] results. Once again, we considered the Bouwens et al. [19] sample of 371 candidate galaxies in the CDFS. Some of these galaxies lies close to bright X-ray sources, identified with galaxies at a different redshift, and must therefore be excluded from the analysis. We considered two exclusion radii, one similar to that used by Treister et al. [18], that is, 22 arcsec, and another, less conservative, of 10 arcsec. In both cases, we used the new Xue et al. [16] catalog of 740 directly detected X-ray sources (it is not possible to ascertain which X-ray catalog was used by [18]). We considered sources at an offaxis angle <8 arcmin, to avoid the inclusion of sources observed with a too broad PSF. We considered only one galaxy when we find 2 or more within 2 arcsec, not to count twice the contribution from each single object. We finally excluded from the samples galaxies closer than 2 arcsec from lower redshift galaxies brighter than zmag =25, which may emit X-rays and hence contaminate the high redshift stacks. The final samples that we consider include 210 galaxies (10 arcsec exclusion radii) and 77 galaxies (22 arcsec exclusion radii). We performed stacks of Chandra counts at the position of these galaxies in four energy bands: 0.5–2 keV, 2–7 keV, 0.8–4 keV, and 1.5–5.5 keV. The total exposure times for the two sample are ~2.3 × 108 seconds (77 galaxies) and ~6.3 × 108 seconds (210 galaxies). Figure 1 shows the stacked images for the two samples in the four energy bands. We do not find a significant signal at the position of the galaxies in any of these images. Table 1 gives the PSF-corrected 3σ count rate upper limits from the counts collected in boxes of 5 arcsec side (area of 100 original pixels). As a comparison, Treister et al. [18] report a count rate 3.4 ± 0.7 × 10−7 counts/s in the 0.5–2 keV band and 8.8 ± 1.3 × 10−7 counts/s in the 2–8 keV band. Our more stringent upper limits are obtained for the 210 galaxy sample in the 0.5–2 keV and 2–7 keV bands. These are, respectively, comparable and 1.5 times lower than the Treister [18] claimed detections.

We can convert our count rate upper limits to a limit to the rest frame 2–10 keV luminosity density following [18]. We find a 3σ limit of ~1046 ergs/s/deg2, lower than the [18] feature, but about twice the luminosity density estimated by Fiore et al. [9].

We recall that our analysis applies to the CDFS field alone, while the [18] result applies to the joined CDFS and CDFN area. In principle, part of the discrepancy between Treister et al. [18] and our analysis could be due to cosmic variance. However, this seems unlikely because most of the Treister sources are in the CDFS, which has an exposure twice that of the CDFN. We also recall that, for the sake of robustness, our stacking analysis is the simplest possible. First, counts at the position of galaxies are added together. Second, aperture photometry is performed on the stacked images, without any optimization for off-axis-dependent PSF. Third, background is estimated in nearby regions, and, unlike [18], no removal of positive fluctuations is performed. While this simple technique does not probably push the detection to the limit, it nevertheless produced robust result. In particular, it produced positive, valuable results in the past, when applied to samples of candidate, faint, Compton thick AGN [20, 21]. The result of our Chandra analysis of the Bouwens candidate galaxies has been recently confirmed by Willott [22].

3. High- AGN Number Counts

The analyses on the CDFS, CDFN, EGS, and COSMOS fields provide samples of individual sources detected; hence, X-ray number counts of faint high- sources can be easily computed from these samples. Figure 2 shows number counts from a compilation of surveys: the Fiore et al. [9] survey of the ERS and GOODS fields, and the Brusa et al. and Civano et al. [23, 24] XMM and Chandra surveys of the COSMOS field. Figure 2 also shows the point from [18] and the upper limit we evaluated in the previous section from a stacking analysis. Black solid lines are model number counts obtained by converting the [9] luminosity functions. The model reproduces reasonably well the number counts at in the full flux range probed by observations. It is consistent with the CDFS points at low fluxes in the other redshift bins, while it is slightly above the Chandra-COSMOS points at intermediate fluxes for the redshift bins and . However, the number of sources in this survey at these redshifts is low, and the corresponding error due to both statistics and cosmic variance is large. Deeper exposures of fields with area of 1-2 deg2  are needed to better explore this region of the redshift-flux parameter space. At the flux limits reached by the deepest Chandra exposure (4 Mseconds), there are >1000   AGN/deg2, several hundreds AGN/deg2, >100  AGN/deg2, 20–100 AGN/deg2 (the uncertainty on the latter number is that large, because the measure is based on just 2 candidate galaxies detected by Chandra in the small ERS field). It is clear that, to obtain a more robust demography of the AGN, a search in a much wider area, such as the CANDELS area [25], is mandatory, and requires spectroscopic confirmation of the X-ray emitting, candidate galaxies. The CANDELS deep and wide surveys cover a total of 130 arcmin2 and 670 arcmin2 to a depth of and , respectively. As a comparison, the ERS survey covers an area of 50 arcmin2 to a depth of . The two candidate ERS galaxies detected by Chandra in the ERS field are faint, and sources. The GOODS source with in the Luo et al. [26] catalog has . The other ERS galaxy with a marginal X-ray detection is brighter, . In summary, we expect 1–5 AGN in the CANDELS deep survey and 4–20 AGN in the CANDELS wide survey. We note that a fraction of these sources will be at the limit, or below, the H band sensitivity threshold of the wide survey. As of today, Chandra has spent of the order of 8 Mseconds on the CANDELS fields, most of them on the CANDELS deep fields. To reach the sensitivity to detect the faint AGN in the wide area, additional 5-6 Mseconds are needed. This is within reach of the Chandra observatory in the next few years. To make further progresses with Chandra, that is, quantitatively probe the first generation of accreting SMBH, which would allow putting stringent constraints on SMBH formation models [2730] and accretion scenarios [46, 31], would require to at least triple the exposure times, that is, 30–40 Mseconds dedicated to deep surveys. While this is certainly extremely expensive, it is not technically unfeasible.

4. Predictions for Future Deep Surveys

The Chandra limiting problem is that its sensitivity is very good on axis but degrades quickly at off-axis angles higher than a few arcmin, making difficult and expensive in terms of exposure time to cover with good sensitivity an area larger than a few hundred arcmin2. A significant leap forward in the field would then be obtained by an instrument capable of reaching the Chandra Msecond on axis sensitivity (i.e., flux limit of 2–4 × 10−17 erg/cm2/s), but on a factor of >10 wider field of view (FOV). We consider here three possible mission concepts, and we make predictions on the number of , and faint X-ray sources based on our best knowledge of the X-ray the number counts at high redshift (Figure 2) and of the X-ray AGN luminosity functions [9].

(1)  The first mission concept that we consider is that of Athena. This is a proposal for L class mission in the framework of the ESA Cosmic Vision program. The baseline mission concept foresees an effective area for imaging of the order of half square meter at 1-2 keV, a mirror PSF with half power diameter arcsec (requirement, 5 arcsec goal), focal length 11 m, FOV = 0.17 deg2 (25 × 25 arcmin). The observatory should be launched on a high earth orbit (HEO) or a L2 orbit, and therefore a rather high internal backgrond is predicted, similar to the internal background measured by the instruments on board XMM and Chandra, which are flying on HEO.

(2)  The second mission concept that we consider is a wide field X-ray telescope, WFXT, fully dedicated to X-ray surveys. This idea is quite old, the first proposal dating mid 90, and the mission concept evolved considerably over the years. We assume a configuration similar to that in [32], that is an effective area of >half square meter at 1-2 keV, split in three mirror units, with arcsec (requirement, goal 5 arcsec) and 5.5 m focal length. Each mirror unit is feeding a focal plane camera with FOV ~ 1 deg2. We assume that the observatory is in a low earth orbit (LEO), ensuring a low internal background (similar to that of the instrument on board Swift and Suzaku, which are flying on a LEO).

(3)  We finally consider the concept of a Super-Chandra. This is a straw-man design for a mission with imaging capabilities comparable to Chandra (i.e., arcsec HPD), but using high throughput, light-weight mirrors (a concept pioneered by Elvis and Fabbiano some 15 years ago [33]). Good imaging capabilities using thin glass or nickel shells may be obtained by correcting the shell shape with actuators. Studies of active X-ray mirrors have been performed in the past with good results (see http://www.mssl.ucl.ac.uk/sxoptics/). A SPIE conference had been devoted to active X-ray mirrors in 2010 (proceedings of SPIE 7803). Active X-ray optics have been foreseen for extremely large throughput, subarcsec future missions like Generetion-X [34, 35] or, more recently, for a square meter, subarcsec mission (Vikhlinin et al. 2011, HEAD meeting). Here, we assume more modest throughput (~3000 cm2 at 1-2 keV) and PSF (1-2 arcsec HPD). We also assume a limited FOV (0.1 deg2) and a LEO, which ensures a low internal background.

We computed on axis sensitivities as a function of the observing time using the above parameters and assuming a signal to noise ratio of 3 for source detection. We assume that the effective area decreases linearly from the center to the limit of the FOV by 50% to make realistic predictions for the number of detected high- AGN in each FOV. The background includes particle-induced internal background, as measured on HEO and LEO by Swift, Suzaku, XMM, and Chandra, cosmic X-ray background (CXB), and low temperature thermal X-ray background due to the local superbubble. To simulate the background expected for the three mission concepts, we have modified background scripts and files prepared to produce simulations for the IXO and NHXM missions. We find that the internal background dominates over the X-ray background (CXB and the local superbubble) above 0.5 keV in a HEO. Conversely, in LEO, the local superbubble dominates below 1 keV. We finally assumed a total net observing time of 12 Mseconds devoted to surveys, split in several shorther observations, to cope with source confusion and optimize the detection of sources with 2–10 keV luminosity 1042 ergs/s. The standard criterion for source confusion (40 beams per source) translates in a flux limit for source confusion of ~6 × 10−17 erg/cm2/s in the 0.5–2 keV band for PSF arcsec and just above 10−17 erg/cm2/s for arcsec. Source confusion is not an issue for any realistic exposure time for a PSF with arcsec or below.

To estimate the faint X-ray sources number density, we used the model number counts in Figure 2, based on the luminosity functions presented in [9]. We conservatively assumed a flat faint-end slope of the X-ray luminosity functions, . Table 2 gives the predicted number of , , and sources, along with their minimum 2–10 keV luminosity, for several indicative mosaics for the three mission concepts briefly described above.

It must be noted that the uncertainties on the number of sources in Table 2 is large. It is at least a factor of two at and even larger at (factor of 3 lower limit and a factor of 2 upper limit). The obvious message of Table 2 is that a wide field greatly helps in searching for faint high- AGN (also see [36]). This is probably the only solution to collect samples of thousands X-ray AGN at . However, even a PSF as good as 5 arcsec HPD does not allow searching for sources fainter than 1043 ergs/s at . This means that only a mission with Chandra-like PSF but much higher throughput (>5× Chandra effective area at 1-2 keV) would be able to target normal star-forming galaxies and miniquasars at . On one hand, a 2–10 keV luminosity of 1042 ergs/s at , reachable by extradeep exposures with a 1 arcsec PSF Super-Chandra, would be produced by a 7 × 105 SMBH emitting at its Eddington luminosity (assuming a bolometric correction of 10). Even smaller masses may be probed, if the accretion is super-Eddington. A Super-Chandra would then be able to directly search for the first generation of SMBH produced by monolithic collapse of 105 M⊙ gas clouds to BH [2830, 37]. On the other hand,  ergs/s is also produced by galaxies that form stars at a rate of 200 M/yr ([38]). Since, at such high redshifts, the X-ray emission should mainly be due to high mass X-ray binaries, X-ray high-z galaxies could then be used to constrain the initial mass function at the epoch of galaxy formation. A Super-Chandra would then be able to open two brand-new fields in structure formation. Of course, it is not casual that the considered configuration for a Super-Chandra is able to reach these goals. Going back from scientific requirements to mission parameters, the goal of detecting sources with a 2–10 keV luminosity of ~1042 at , in feasible exposure times, requires an effective area ~3000 cm2, given a PSF arcsec and assuming a LEO internal background.

Unfortunately a Super-Chandra is beyond the horizon of the present decade, both because of technological and programmatic issues. Furthermore, it is not clear whether a WFXT is truly feasible with such huge 1 deg2 FOV and large throughput, and, in any case, it does not appear to be a priority in the latest US Decadal Survey (http://sites.nationalacademies.org/bpa/BPA_049810), nor in the ESA Cosmic Vision program. Conversely, Athena is a study mission for an L class mission in the framework of the ESA Cosmic Vision program. A decision on Cosmic Vision L class missions should be taken in February 2012. If positive, Athena could be implemented for the first years of the next decade. Although not reaching exquisite, Chandra-like image quality, nor extralarge field of view, Athena would be able to give a substantial contribution to the knowledge of the high- Universe, with hundreds to a thousand faint X-ray AGN (an improvement by a factor 10–100 with respect to today situation) and tens to hundreds faint X-ray selected AGN (today, there are only 3-4 candidate X-ray selected AGN in the literature [9, 26, 39], none of them spectroscopically confirmed so far).


This work was supported by ASI/INAF contracts I/024/05/0 and I/009/10/0. This work is based on observations made with NASA X-ray observatory Chandra. The authors thank the Chandra Director’s office for allocating the time for these observations. X-ray data were obtained from the archive of the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory.