The Chandra Cosmological Evolution Survey (C-COSMOS)

Lilly et al. in prep), we refined the high-redshift sample in Chandra-COSMOS (see § 3.1.2) presented by Civano et al. (2011) and found 62 soft-band detected objects at 3 < z < 5.1. Accounting only for the soft-band selected objects, the Civano et al. (2011) sample consisted in 65 z > 3 objects. The difference is the result of the exclusion of 7 sources (ID 293, 666, 871, 1112, 1276, 1304, and 11633) of the Civano et al. (2011) sample with a new, z < 3 redshift in Civano et al. (2012), and the inclusion of 6 objects (ID 529, 688, 1311, 1392, 2059 and 3449), not present in Civano et al. (2011), but identified as high-redshift AGN in Civano et al. (2012).

Chandra ID 125 and 2550 have a redshift z > 5.1 and therefore will not be considered

3.2. THE HIGH-REDSHIFT SAMPLE 55

Figure 3.7: Soft-band (0.5−2 keV) XMM -COSMOS image. Black dashed rectangle represents the original Chandra-COSMOS field, while the green rectangle is the reduced Chandra field used in this work (see§ 3.2.3). Cyan, red and yellow points are the high-redshift sources detected only by Chandra, only by XMM -Newton and by both satellites, respectively. The IDs are the same than in Tab. 3.3.

Almost half of our sample (30 out of 62 AGN) was also detected by the XMM- Newton survey in the COSMOS field, which encompasses the Chandra observations. Since in the outskirts of the Chandra field (black rectangle in Fig. 3.2.3) the sensi- tivity drops below the sensitivity of XMM-COSMOS, there are a number of sources detected by XMM-Newton whose flux is below the Chandra limit at that point. In order 1) to count each source only once and 2) not to lose the AGN detected only in XMM-COSMOS, we did not consider all the C-COSMOS field, but only the central region. The proper way to define it would be the region where Chandra is more sensitive than XMM-Newton. However, for simplicity during the evaluation of the sky-coverage (see § 3.3.1), we defined the new C-COSMOS area by looking at the overlapping XMM and Chandra images and cutting off an external frame of the

Chandra field as narrow as possible, with an area of ∼ 0.20 deg2, to fulfil the two requirements mentioned above.

All the sources detected either only by Chandra (cyan points in Fig. 3.2.3) or by both Chandra and XMM-Newton (yellow points) in the resulting 0.72 deg2 region (green rectangle) will then be counted in the C-COSMOS sample. Three objects (Chandra ID 521, 1730 and 325) detected by both Chandra and XMM-Newton, fall outside the newly defined boundaries of C-COSMOS; therefore, they are counted in the XMM-COSMOS sample (XMM ID 5120, 5259 and 5347, respectively).

redshift range 3 < z ≤ 5.1. A spectroscopic redshift is associated to 30 of them. We performed a spectral analysis assuming the same spectral model described in § 2.3.2 (i.e. an absorbed power-law with Γ = 1.8 and Galactic column density of 2.5× 1020 cm−2; Kalberla et al. 2005) and fitted the spectra of the 59 sources using the Cash statistics (Cash 1979) to estimate the best-fitting parameters (Tab. 3.3). We derived intrinsic luminosities of 1043 . L2−10 keV

erg s−1 . 2 × 10

45 _{and a fraction of}

∼ 32 per cent of very obscured sources (NH > 1023cm−2). The poor spectral quality

of Chandra ID 3449 prevented us from investigating the effect of absorption, hence a simple power-law was assumed as spectral model in order to derive the intrinsic hard band luminosity. This source will be conservatively considered unobscured in the following sections.

3.2.4 The XMM-Newton Cosmological Evolution Survey (XMM- COSMOS)

As for the XMM -COSMOS survey (see § 3.1.1), we considered as parent sample the 1797 sources reported in the Brusa et al. (2010) multi-wavelength catalogue. Redshift were collected from Brusa et al. (2010), Salvato et al. (2011) and Civano et al. (2012) and 53 soft-band detected objects resulted to be at z > 3.

Brusa et al. (2009) presented results on a sample of high-redshift AGN in XMM- COSMOS. In particular, they selected 39 soft-band detected AGN z > 3 with

F0.5−2 keV > 10−15 erg cm−2s−1. Because of the new redshift information we col-

lected, these two samples have 34 sources in common.

Once the central reduced C-COSMOS field is cut out (see Fig. 3.2.3), the re- maining XMM-COSMOS area is∼ 1.45 deg2 and the number of soft-detected object that are placed in that frame are 930, with a redshift completeness of 99.4 per cent (i.e. 6 objects have no redshift information). Twenty-five objects are high-redshift sources (19 of which are included in the Brusa et al. (2009) sample); 15 of them have a spectroscopic redshift. All but one of the remaining 28 high-redshift AGN, which fall on the reduced C-COSMOS area, are detected by Chandra and are counted in the C-COSMOS sample (see § 3.2.3)2.

Mainieri et al. (2007; 2011) performed a detailed spectral analysis on a subsam- ple of X-ray sources in XMM-COSMOS, including 16 objects in our high-redshift sample. They assumed an absorbed power-law with photon index free to vary. Only one of them resulted to be heavily obscured (XID 2518, logNH= 23.07+0.35_−0.37), with

a very steep photon index (Γ > 2). Since the photon counting statistics was too poor to perform a spectral analysis on all these objects and in order to be consistent with the spectral assumptions used for the other surveys and with the procedure adopted in§ 3.3, we conservatively consider all the sources in our z > 3 sample as unabsorbed AGN. This is also justified since, at similar fluxes, the SXDS sample (see § 3.2.2) included only unabsorbed object, with just one exception. We derived the rest-frame hard-band luminosities from the soft-band flux reported in Cappelluti

2

XID 54039 is a high-redshift X-ray source which falls on the central, reduced Chandra field, but is detected only by XMM. This is probably due to statistical fluctuations affecting the flux of the source or the nearby background, or even to intrinsic variability of the source. Since in the reduced C-COSMOS region we considered only sources detected by Chandra, we excluded it from the high-redshift sample.

3.3. THE BINNED HXLF 57

Table 3.2: Main properties of the individual surveys FIELD A [deg2] % compl N (Nabs) % zspec

(1) (2) (3) (4) (5) CDF-S 0.129 95 27 (16) 48 C-COSMOSred 0.717 98 59 (19) 51 XMM-COSMOSred 1.453 99 25 (0) 60 SXDS 1.006 99 30 (1) 67 TOTAL 3.305 98 141 (36) 55

(1) Field; (2) nominal area; (3) redshift completeness of the soft-band detected parent sample; (4) number of soft-band detected AGN at 3 < z < 5.1, the number

of sources included in the A subsample (logNH> 23; see§ 3.3.1) is between

brackets; (5) fraction of high-redshift AGN with a spectroscopic redshift. These numbers refer to the reduced field for Chandra and XMM-COSMOS and to the overlapping region between the X-ray and optical surveys for the SXDS field (see

§ 3.2 and § 3.6).

et al. (2009), assuming a simple power-law with Γ = 1.8 (Tab. 3.3).