Chapter 1 Introduction
2.2 X-ray to optical/NIR/MIR Associations
2.2.4 Comparison to previous results in Area 1+2
40 2. The fields: CANDELS/GOODS-S, CDFS, and ECDFS
• Case 1: An X-ray source in both catalogs with one optical/NIR/MIR association.
Case 1 means the same unique association was chosen even though the X-ray catalog positions may differ between X11 and R13 or between L05 and V06. There are 714 of these sources in Areas 1+2+3.
• Case 2: An X-ray source in both catalogs with multiple optical/NIR/MIR asso-ciations. Case 2 can arise from two causes: (1) position differences in the X-ray catalogs may point to different counterparts, or (2) there may be more than one po-tential counterpart near the X-ray position(s), and we cannot tell which is the right one. Some of the latter may be blended sources with more than one galaxy contribut-ing to the X-ray flux. In total, there are 181 case 2 sources in Areas 1+2+3. These sources are identified in the final catalogs, and counterpart photo-z are calculated using both AGN and normal galaxy SED templates (see Section A).
• Case 3: X-ray sources found in one catalog but not the other, having a unique counterpart. There are 235 of these sources in Areas 1+2+3.
• Case 4: X-ray sources found in only one catalog and having multiple possible coun-terparts. There are 77 such sources in Areas 1+2+3. As for Case 2, the catalogs identify all the possible counterparts and provide both AGN and normal galaxy photo-z results for each.
In summary, 1207 out of 1259 (∼96%) of the X-ray sources are associated with multi-wavelength counterparts, and 258 of them (∼21%) have multiple counterparts possible.
There are 26 sources for which the counterpart is detected only in the IRAC bands, and no photo-z computation is possible for these because only two data points (3.6µm and 4.5µm) can be used. All the other sources have at least six photometric points, and a photo-z is provided. The photo-z catalog (see Sec. A) entry for each source indicates the number of photometric points used for the photo-z computation. The remaining 52 sources (∼ 4%) either have no identifications in any of the optical/NIR/MIR catalogs (∼ 1%) or have possible counterparts identified with p < 0.7 (∼ 3%). For these sources, the photo-z are not available as well.
2.2 X-ray to optical/NIR/MIR Associations 41
24636 24636 24636 24636
X-ray R-band H-band IRAC-3.6µm
X-333
X-411
X-434
X-602
X-58
X-568
X-736
Figure 2.10: Multi-wavelength images of the seven sources from X11 for which we found new, secure (p > 0.9) counterparts. Wavelengths are indicated above each set of panels.
The four sources in the upper group are in Area 1 and have CANDELS H-band images.
The three sources in the lower group have no WFC3-H, and TENIS-Ks is shown instead.
X-ray images are full-band from X11. The red dashed circles are centered at the X11 positions with their radii showing the corresponding positional uncertainty. Cyan crosses in the upper panels show all H-band detections, and the solid red circles show the catalog position of the chosen counterpart. All cutouts are 500×500 except that X-736 is 1000×1000.
42 2. The fields: CANDELS/GOODS-S, CDFS, and ECDFS
Table 2.4: Results of X-ray to optical/NIR/MIR associations in ECDFS .
Nx Case1 Case2 Case3 Case4 Nctpsingle Nctpmulti Nctp Nctpmulti/Nctp Nctp/Nx
Area 1 509 272 67 130 29 402 96 498 19% 98%
Area 2 255 170 29 35 12 205 41 246 17% 96%
Area 3 495 272 85 70 36 342 121 463 26% 94%
TOTAL 1259 714 181 235 77 949 258 1207 21% 96%
Note: Nx: Number of X-ray sources; Nctpsingle: Number of sources that have only one possible counterpart;Nctpmulti: Number of sources that have more than one possible counterpart;Nctp: Total number of sources for which at least a counterpart was found.
However, the high-resolution WFC3/H-band image reveals at least four sources close to-gether, and the slightly different coordinates provided by X11 and R13 point to different but equally likely counterparts. This difference is mainly due to the catalogs chosen for cross matching rather than the matching method. The Bayesian method should in prin-ciple give the same result as the maximum likelihood method, but the ability to match several catalogs simultaneously greatly improves the efficiency of the matching.
X-234
X-ray R-band H-band IRAC-3.6µm
Figure 2.11: Negative images of the source R-57 (=X-234). Image wavelengths are indi-cated at the top, and each image is 500×500. Red dashed-line circles are centered at the position provided by X11 and cyan dashed-line circles at the position given by R13. Circle sizes indicate the respective X-ray position uncertainties. Red and cyan solid-line circles are the counterparts we assign to the two X-ray positions, and the blue circle indicates the counterpart assigned by X11.
For the remaining 17 sources that are not identified (see Figure 2.12), eight of which were detected with X-ray false-positive threshold less than 1×10−6. Referring to the Luo et al. (2010), one of the eight sources (i.e., X-73) is considered as the extreme X-ray/optical ratio source (EXO; Koekemoer et al., 2004) with F0.5−8keV/Fz ∼ 50. In general, most of the EXOs have clear detections in the IR and NIR, and are suggested be dusty galaxies at moderate redshifts (z ∼ 2−3). Since there is no detection of the source X-73 in the deep IR and NIR, it is likely to be AGN at very high redshift (z > 5). The rest seven sources (X-35, 194, 477, 603, 643, 668, 688) at low-significance levels could be spurious
2.2 X-ray to optical/NIR/MIR Associations 43
X-ray detections. If any of them is a real X-ray detection, then it could have a similar nature as source X-73 (Luo et al., 2010).
For the other nine X-ray sources at threshold greater than 1×10−6 (X-95, 98, 190, 280, 286, 366, 384, 408, 655), they are probably related to a star, galaxy groups and clusters, or off-nuclear sources (Luo et al., 2010; Xue et al., 2011). We visually inspected their optical, IR and X-ray images (as shown in Figure 2.12), and found that only source X-655 is likely to be an off-nuclear object. The rest of eight sources are probably galaxy groups or clusters as described in Luo et al. (2010) and Xue et al. (2011).
44 2. The fields: CANDELS/GOODS-S, CDFS, and ECDFS
35 35
35 35
35
(a) X-35
73 73 73 73 73
(b) X-73
95 95 95 95 95
(c) X-95
98 98 98 98 98
(d) X-98
190 190 190 190 190
(e) X-190
194 194 194 194 194
(f) X-194
Figure 2.12: Cutouts of 17 unidentified X-ray sources from the X11 catalog. From left to right are the images in X-ray, R-band, H-band,Ks-band, and IRAC-3.6µm.
2.2 X-ray to optical/NIR/MIR Associations 45
280 280 280 280 280
(g) X-280
366 366 366 366 366
(h) X-366
384 384 384 384 384
(i) X-384
408 408 408 408 408
(j) X-408
477 483
477
483 477
483 477
483 477
(k) X-477
46 2. The fields: CANDELS/GOODS-S, CDFS, and ECDFS
603 603 603 603
603
(l) X-603
643 643 643 643 643
(m) X-643
655 655 655 655 655
(n) X-655
670
668 670
668
670
668
670
668
670
668
670
668
(o) X-668
688 688 688 688
688
(p) X-688
Chapter 3
Photometric redshift in the Chandra Deep Field South
Photo-z for galaxies and AGNs in the CDFS were computed usingLePhare(Arnouts et al., 1999; Ilbert et al., 2006), which is based on a χ2 template-fitting method (Arnouts et al., 1999, 2002). Applying three free parameters: redshift (z), template (T), and normalization factor (A), the function χ2 is defined as
χ2(z, T, A) =
N
X
i=1
Fobsi −A×Fpredi (z, T)) σiobs
!2
(3.1) where Fpredi (z, T) is the flux predicted for a template T at redshift z, Fobsi is the observed flux, and σobsi is the associated error for the considered filter i. Sum up N filters, the photo-z is obtained by the minimization of χ2.
Depending on whether sources are the detected and non-detected in the X-ray surveys, four libraries of galaxy/AGN hybrids templates are utilized to estimate photo-z. (See the detailed descriptions for each library in the following Section 3.1 and Section 3.2).
In addition to galaxy/AGN hybrids templates, we included a complete library of stellar templates as did following Ilbert et al. (2009) and Salvato et al. (2009). For each source, χ2 is estimated for both galaxy/AGN hybrids and stellar templates. To assess the photo-z quality, we excluded possible stars, using the criterion that they are point-like and have 1.5×χ2star < χ2agn/gal, where χ2star and χ2agn/gal are the minimum χ2 obtained with stellar templates and galaxy/AGN hybrids, respectively.
Four extinction laws (those of Prevot et al. 1984, Calzetti et al. 2000, and two modifi-cations of the latter, depending on the kind of templates) were used withE(B−V) values of 0.00 to 0.50 in steps of 0.05 mag. Photo-z values were allowed to reachz = 7 (in steps of 0.01) as deep photometry allows us to reach high redshifts (see details given by Ilbert et al.
2009). The fitting procedure included a magnitude prior, forcing sources to have an abso-lute magnitude in rest B-band between −8 and −24. Photometric zero-point corrections were incorporated but never exceeded 0.1 mag. Note the we obtained zero-point offsets using normal galaxies photometry, but against this step for the AGNs because optical
48 3. Photometric redshift in the Chandra Deep Field South
variability can be intrinsic to the source and not accounted for in the photometry.
Quantifying the photo-z accuracy (σNMAD), the percentage of the outliers (η), and the mean offset between photo-z and spec-z (biasz) was based on the spectroscopic samples with reliable quality (i.e., Qzs <2, see the description for the value of Qzs in Section A.4).
The measure ofσNMAD is the normalized median absolute deviation (NMAD):
σNMAD = 1.48×median
|∆z|
1 +zs
(3.2) where zs is spec-z, zp is photo-z, and ∆z = (zp −zs). The value of σNMAD is calculated including outliers which are defined as:
|∆z|
1 +zs >0.15 (3.3)
Lastly, biasz (= mean(1+z∆z
s)) is calculated after excluding outliers.