All Figures

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig1.ps} \end{figure}$ Figure 1: Adaptively smoothed Chandra image of the CDF-S in the [0.5-7] keV band. The circles show the position of OFS. The big dashed rectangle is the area of the GOODS survey ( "GOODS area''); the polygon indicates the region of the deep public VLT/ISAAC observations currently available and the small square indicates the area of the Hubble Ultra Deep Field (UDF).
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{1212fig2.ps} \end{figure}$ Figure 2: The R-band magnitude distribution for the optical counterparts of the X-ray sample. The hatched box are the OFS, while the shaded part refers to the sources without a R band counterparts (down to R=26.7).
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{1212fig3.ps} \end{figure}$ Figure 3: The distributions of X-ray fluxes in the 0.5-2 keV ( left panel) and 2-10 keV band ( right panel) for the OFS (hatched histogram) and optically bright sources (open histogram).
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In the text

$\begin{figure} \par\includegraphics[width=7.cm,clip]{1212fig4.ps} \end{figure}$ Figure 4: The distribution of hardness ratios (HR) for the optically faint sample (hatched histogram) and the optically bright sample (open histogram).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig5.ps}\par\vspace*{2... ...\par\vspace*{2mm} \includegraphics[width=8.8cm,clip]{1212fig7.ps} \end{figure}$ Figure 5: Comparison of the photometric and spectroscopic redshifts of the spectroscopically identified OFS in the "GOODS area''. Left side: best fit template (continuum line) with the observed photometry (filled circles with error bars) and the best fit model photometry (filled squares) overplotted. Right side: confidence contours ( $1 \sigma$ , $2 \sigma$ and $3 \sigma$ ) for the photometric redshift determination in the SED template vs. redshift plane. The dashed line indicates the spectroscopic redshift and the cross shows the best fit solution. The shaded area refers to a low-probability region in the solution space (see Sect. 4.2). XID are from Giacconi et al. (2002).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig8.ps}\par\includegraphics[width=8.8cm,clip]{1212fig9.ps} \end{figure}$ Figure 6: As in Fig. 5.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig10.ps}\par\vspace*{... ...}\par\vspace*{2mm} \includegraphics[width=8.8cm,clip]{1212fig13.ps} \end{figure}$ Figure 7: Photometric redshifts for OFS in the "GOODS area'' with $\rm odds\ge 0.3$ . Left side: best fit galaxy template (solid line) with overplotted the observed photometry (filled circles with error bars), and the best fit model photometry (filled squares). Right side: confidence contours ( $1 \sigma$ , $2 \sigma$ and $3 \sigma$ ) for the photometric redshift determination in the SED template vs. redshift plane. The cross indicates the best-fit solution. The shaded area refers to a low-probability region in the solution space (see text). XIDs are from Giacconi et al. (2002).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig14.ps}\par\vspace*{... ...}\par\vspace*{2mm} \includegraphics[width=8.8cm,clip]{1212fig17.ps} \end{figure}$ Figure 7: continued.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig18.ps}\par\vspace*{... ...}\par\vspace*{2mm} \includegraphics[width=8.8cm,clip]{1212fig21.ps} \end{figure}$ Figure 7: continued.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig22.ps}\par\vspace*{... ...}\par\vspace*{2mm} \includegraphics[width=8.8cm,clip]{1212fig25.ps} \end{figure}$ Figure 7: continued.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig26.ps}\par\vspace*{... ...}\par\vspace*{2mm} \includegraphics[width=8.8cm,clip]{1212fig29.ps} \end{figure}$ Figure 7: continued.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig30.ps}\par\vspace*{... ...}\par\vspace*{2mm} \includegraphics[width=8.8cm,clip]{1212fig33.ps} \end{figure}$ Figure 7: continued.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig34.ps}\par\vspace*{... ...}\par\vspace*{2mm} \includegraphics[width=8.8cm,clip]{1212fig37.ps} \end{figure}$ Figure 7: continued.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig38.ps}\par\vspace*{... ...}\par\vspace*{2mm} \includegraphics[width=8.8cm,clip]{1212fig41.ps} \end{figure}$ Figure 7: continued.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig42.ps}\par\vspace*{... ...}\par\vspace*{2mm} \includegraphics[width=8.8cm,clip]{1212fig45.ps} \end{figure}$ Figure 7: continued.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig46.ps}\par\vspace*{... ...}\par\vspace*{2mm} \includegraphics[width=8.8cm,clip]{1212fig48.ps} \end{figure}$ Figure 7: continued.
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In the text

$\begin{figure} \par\includegraphics[angle=-90,width=8.2cm,clip]{1212fig49.ps} \end{figure}$ Figure 8: X-ray spectrum of source CDF-S/508 with a significant feature that we identify with the Fe line at rest-frame 6.4 keV.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{1212fig50.ps} \end{figure}$ Figure 9: Distribution of redshifts (spectroscopic and photometric) for the OFS (shaded histogram), optically bright sources (hatched histogram) and the total X-ray sample (open histogram). Sources belonging to the large scale structures in the CDF-S have been excluded (see text). The dashed line is the redshift distribution predicted by the model B of Gilli et al. (2001) normalized to the total number of sources in the "GOODS area'' for which we have either a spectroscopic or photometric redshift.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1212fig51.ps} \end{figure}$ Figure 10: R-K colour versus redshift. Following the X-ray classification (Szokoly et al. 2004) described in Sect. 4.1: circles are X-ray unabsorbed AGN/QSOs, squares X-ray absorbed AGN/QSOs, triangles galaxies. Filled symbols refer to OFS, empty symbols to optically bright sources. The numbers are the XIDs of OFS spectroscopically identified in the CDF-S (see Table 3). The four evolutionary tracks correspond to an unreddened QSO (solid line; Vanden Berk et al. 2001), and to unreddened elliptical, Sbc and irregular galaxies from the Coleman et al. (1980) template library (dashed lines). The bars indicate 1 $\sigma$ errors in the photometric redshift estimates.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{1212fig52.ps} \end{figure}$ Figure 11: The Hardness ratios (HR) versus X-ray-to-optical flux ratios (X/O). Open symbols indicate optically bright (R<25) sources and filled symbols indicate OFS. Symbols are as in Fig. 10. For clarity, we have omitted optically bright objects that are spectroscopically unidientified. Vertical lines are for $\rm X/O=0.1$ and $\rm X/O=10$ respectively.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{1212fig53.ps} \end{figure}$ Figure 12: Intrinsic $N_{\rm H}$ distributions for the OFS (hatched histogram) and optically bright sources (open histogram).
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In the text

$\begin{figure} \par\includegraphics[width=17.8cm,clip]{1212fig54.ps}\par\include... ...{1212fig55.ps}\par\includegraphics[width=17.8cm,clip]{1212fig56.ps} \end{figure}$ Figure 13: Cutouts in the four UDF filters ( F435W, F606W, F775W, F850LP) with X-ray contours of the three OFS inside this area. Images are 3 $^{\prime \prime }$ across. The $3 \sigma$ Chandra positional error is indicated by the solid circle.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig57.ps}\par\includeg... ...]{1212fig59.ps}\par\includegraphics[width=8.8cm,clip]{1212fig60.ps} \end{figure}$ Figure 14: Photometric redshifts for the counterparts of OFS in the UDF area. Left side: best fit template (solid line), the observed photometry (filled circles with error bars), and the best fit model photometry (filled squares). Right side: confidence contours ( $1 \sigma$ , $2 \sigma$ and $3 \sigma$ ) for the photometric redshift determination in the SED template vs. redshift plane.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig1.ps} \end{figure}$	Figure 1: Adaptively smoothed Chandra image of the CDF-S in the [0.5-7] keV band. The circles show the position of OFS. The big dashed rectangle is the area of the GOODS survey ( "GOODS area''); the polygon indicates the region of the deep public VLT/ISAAC observations currently available and the small square indicates the area of the Hubble Ultra Deep Field (UDF).
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$\begin{figure} \par\includegraphics[width=7.2cm,clip]{1212fig2.ps} \end{figure}$	Figure 2: The R-band magnitude distribution for the optical counterparts of the X-ray sample. The hatched box are the OFS, while the shaded part refers to the sources without a R band counterparts (down to R=26.7).
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{1212fig3.ps} \end{figure}$	Figure 3: The distributions of X-ray fluxes in the 0.5-2 keV ( left panel) and 2-10 keV band ( right panel) for the OFS (hatched histogram) and optically bright sources (open histogram).
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$\begin{figure} \par\includegraphics[width=7.cm,clip]{1212fig4.ps} \end{figure}$	Figure 4: The distribution of hardness ratios (HR) for the optically faint sample (hatched histogram) and the optically bright sample (open histogram).
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig8.ps}\par\includegraphics[width=8.8cm,clip]{1212fig9.ps} \end{figure}$	Figure 6: As in Fig. 5.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig14.ps}\par\vspace{... ...}\par\vspace{2mm} \includegraphics[width=8.8cm,clip]{1212fig17.ps} \end{figure}$	Figure 7: continued.
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$\begin{figure} \par\includegraphics[angle=-90,width=8.2cm,clip]{1212fig49.ps} \end{figure}$	Figure 8: X-ray spectrum of source CDF-S/508 with a significant feature that we identify with the Fe line at rest-frame 6.4 keV.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{1212fig52.ps} \end{figure}$	Figure 11: The Hardness ratios (HR) versus X-ray-to-optical flux ratios (X/O). Open symbols indicate optically bright (R<25) sources and filled symbols indicate OFS. Symbols are as in Fig. 10. For clarity, we have omitted optically bright objects that are spectroscopically unidientified. Vertical lines are for $\rm X/O=0.1$ and $\rm X/O=10$ respectively.
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$\begin{figure} \par\includegraphics[width=8.4cm,clip]{1212fig53.ps} \end{figure}$	Figure 12: Intrinsic $N_{\rm H}$ distributions for the OFS (hatched histogram) and optically bright sources (open histogram).
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$\begin{figure} \par\includegraphics[width=17.8cm,clip]{1212fig54.ps}\par\include... ...{1212fig55.ps}\par\includegraphics[width=17.8cm,clip]{1212fig56.ps} \end{figure}$	Figure 13: Cutouts in the four UDF filters ( F435W, F606W, F775W, F850LP) with X-ray contours of the three OFS inside this area. Images are 3 $^{\prime \prime }$ across. The $3 \sigma$ Chandra positional error is indicated by the solid circle.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1212fig57.ps}\par\includeg... ...]{1212fig59.ps}\par\includegraphics[width=8.8cm,clip]{1212fig60.ps} \end{figure}$	Figure 14: Photometric redshifts for the counterparts of OFS in the UDF area. Left side: best fit template (solid line), the observed photometry (filled circles with error bars), and the best fit model photometry (filled squares). Right side: confidence contours ( $1 \sigma$ , $2 \sigma$ and $3 \sigma$ ) for the photometric redshift determination in the SED template vs. redshift plane.
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