Photometric redshifts of faint X-ray sources

Active Galactic Nuclei (AGN) are the accreting super massive black hole (SMBH) at the center of massive galaxies. The tight M-σ and M_BH-M_bulge correlations reveal that the host galaxies are affected by the effects of the SMBHs. In addition, many works studying on the rest-frame color-magnitude relation have shown that AGN host galaxies have intermediate colors, which are considered as a transition from the blue cloud to red sequence in host-galaxy color evolution. Some works interpreted this result as an evidence for the AGN feedback, in the sense that the accretion process not only builds up the massive black holes, but also has a powerful influence on the surrounding environment, triggering or suppressing the star-forming activity in the host galaxy. These correlations make obvious the need to investigate AGN-host coevolution. One way to understand this coevolution is to study the AGN duty cycle (i.e., the time scale that the SMBH is active), which can be obtained by estimating AGN population among all the galaxies through cosmic times. Such demographic studies require a well-sampled census and accurate redshift information. In particular at high redshift, objects are extremely faint and sample numbers are very small. This could cause large statistical errors. For example, Aird et al. (2010) argued that luminosity-dependent density evolution with a flattening faint-end slope of the X-ray luminosity function (XLF) at z > 1.2 may result from catastrophic photo-z failures caused by observational limitations and improper templates used for photo-z computation. However, spectroscopic redshifts are time-consuming and difficult to be obtained for faint sources at high redshifts. Therefore we have to rely on photometric redshifts (photo-z) techniques which need to be tuned specifically to be reliable for AGNs (i.e., proper magnitude priors, appropriate AGN-galaxy hybrid template for SED-fitting, and correct multi-wavelength counterparts). In recent years, many deep and high-resolution observations become available in multiple wavebands, specially at near/mid-infrared. This allows us to reach higher redshift, and make more accurate analysis on the multi-wavelength properties of AGNs. In this thesis, we focus on the ECDFS area which comprise also the GOODS-S and CDFS regions. This is the portion of the sky with the deepest and most complete photometric information from X-ray to radio, including intermediate bands from the Subaru telescope, and optical/near-infrared data from the Hubble space telescope. To compute accurate photo-z using these data, first we combined multi-wavelength catalogs from UV to infrared after the astrometric calibration and correction for the different methods of flux extraction (e.g., total fluxes, flux apertures and PSF- fitted photometry). Second we identified the best multi-wavelength counterparts for X-ray sources from the 4Ms-CDFS and 250ks-ECDFS surveys, taking into account the positional errors and multiple magnitude distributions as priors simultaneously. We found that more then 96% of X-ray sources have multi-wavelength counterparts. Thirdly we built a new library of active galactic nuclei/galaxy hybrid templates appropriate for the faint X-ray population in the CDFS to simulate the AGN spectral energy distribution from low to high redshift. For X-ray-selected AGNs, we achieved a photo-z accuracy of 0.013 with an outliers fraction of 5.3%, while for non-X-ray galaxies, the photo-z accuracy is 0.010 with an outlier fraction of 4.6%. With the SED-fitting results of our well-trained AGN-galaxy hybrids, we further studied the galaxy and AGN host properties via the rest-frame color-magnitude diagram (CMD) which is an useful probe to trace the stellar populations. We made corrections for dust extinction and/or AGN contamination for the galaxy/AGN host colors in the CANDELS/GOODS-S region. We found that the AGN host colors also present bimodality in the CMD up to z~2.5 as found in normal galaxies, and the position of the blue peaks in the AGN samples are almost constant with cosmic time. This implies a weak connection between AGN activity and star formation in the host galaxy. For the X-ray sources in the 4Ms-CDFS survey, we found that for most of the sources, the correction for dust extinction is larger than the correction for the AGN contribution. This is because the AGN population in this field is dominated by low-luminosity AGNs which have host-dominated SEDs. However for few bright sources, their host colors are strongly effected by AGN contribution rather than by the dust extinction. For these sources, the correction for AGN contribution is about two times larger than the correction for dust extinction in general. Therefore AGN/galaxy decomposition becomes more important in a shallower and wider X-ray surveys, e.g., XMM-COSMOS and eROSITA, which contains a larger fraction of bright AGNs. Furthermore, with our accurate redshifts for galaxies and AGNs, we defined a high-redshift (high-z) sample using the redshift probability distribution function P(z) rather than relying on the best-fit value of photo-z. We integrated P(z) within a given redshift range to obtain the photo-z probability in that range and selected high-z sources above a given threshold. When computing the number of sources in a given redshift range, each source will not be counted as "1" but as the proportion of it. We compared this P(z) technique with traditional color techniques adopted for galaxy evolutionary stages, like the Lyman break galaxy and the BzK color-color selection via sample completeness and purity. We found that the P(z) technique is the most efficient and reliable method for selecting high-z sources. This is not surprising as it makes use of photometric information from the entire SED rather than using only three photometric points. Lastly, we built a high-z (z > 3) sources list for X-ray sources in ECDFS region, and compared our list with previous work. In our work, we made better X-ray-to-optical/NIR associations considering the positional errors and magnitude distribution. In addition, we obtained accurate photo-z using well-established AGN-galaxy hybrids for X-ray selected AGNs and applied P(z) for each source. These procedures help improving on our high-z sample selection