Dissecting Photometric redshift for Active Galactic Nuclei using XMM- and Chandra-COSMOS samples

Abstract: With this paper, we release accurate photometric redshifts for 1692 counterparts to Chandra sources in the central square degree of the COSMOS field. The availability of a large training set of spectroscopic redshifts that extends to faint magnitudes enabled photometric redshifts comparable to the highest quality results presently available for normal galaxies. We demonstrate that morphologically extended, faint X-ray sources without optical variability are more accurately described by a library of normal galaxies (corrected for emission lines) than by AGN-dominated templates, even if these sources have AGN-like X-ray luminosities. Preselecting the library on the bases of the source properties allowed us to reach an accuracy sigma_(Delta z/(1+z_spec)) \sim0.015 with a fraction of outliers of 5.8% for the entire Chandra-COSMOS sample. In addition, we release revised photometric redshifts for the 1735 optical counterparts of the XMM-detected sources over the entire 2 sq. deg.of COSMOS. For 248 sources, our updated photometric redshift differs from the previous release by Delta z>0.2. These changes are predominantly due to the inclusion of newly available deep H-band photometry H_AB=24 mag. We illustrate once again the importance of a spectroscopic training sample and how an assumption about the nature of a source together with the number and the depth of the available bands influence the accuracy of the photometric redshifts determined for AGN. These considerations should be kept in mind when defining the observational strategies of upcoming large surveys targeting AGN, such as eROSITA at X-ray energies and ASKAP/EMU in the radio band.

Photometric Redshifts for AGN using XMM- and Chandra-COSMOS Samples

This paper addresses the challenge of obtaining accurate photometric redshifts (photo-z) for active galactic nuclei (AGN) by utilizing XMM-Newton and Chandra data from the COSMOS field. The accuracy of photometric redshifts is critical, given that spectroscopic redshift measurements, while providing high accuracy, are impracticable for large surveys due to significant time and resource demands. Thus, developing reliable methods for estimating photo-z for AGNs becomes imperative, especially for deep cosmological fields like COSMOS.

Methodology

The study employs a dataset comprising 1692 counterparts to Chandra sources and provides updates to 1683 optical counterparts from XMM-detected sources in the COSMOS field. The extensive use of a large training set of spectroscopic redshifts allows the authors to develop and refine a methodology that ensures high accuracy of the photometric redshifts, comparable to those of earlier works on normal galaxies.

To achieve an accuracy of (\sigma_{\Delta z/(1+z_{spec})}\approx 0.015) with an outlier fraction of 5.8%, the study categorizes sources based on morphological and variability properties into extended/non-varying and point-like/varying groups. Importantly, they demonstrate that faint, morphologically extended X-ray sources without optical variability are more accurately described by templates of normal galaxies rather than AGN-dominated templates. This nuanced approach, which preselects the library based on the source properties, allows for improved accuracy across the dataset.

Numerical Results and Implications

The paper highlights significant improvements in the determined photo-z, finding discrepancies ((\Delta z>0.2)) for 248 sources from previous releases due to the inclusion of newly available deep H-band photometry. These results underscore the efficacy of spectroscopic training samples in fine-tuning photometric analysis models and address the critical interplay between template choice, source characteristics, and observational parameters influencing the photo-z accuracy.

The improved photometric redshifts have practical implications for upcoming AGN-oriented surveys like eROSITA and ASKAP/EMU. Surveys can leverage such methodologies to preemptively counter the challenges inherent in photometric observations, namely flux variability and morphological ambiguity, ensuring the reliability of the photometric redshifts.

Future Directions

The research suggests implementing comprehensive multi-wavelength coverage and utilizing spectroscopic data to enhance the robustness and reliability of photometric redshifts for AGNs. The introduced methodology can be adapted and applied across various deep survey fields, essential for extragalactic studies targeting AGN and their co-evolution with host galaxies. Future developments may focus on integrating machine learning techniques for better modeling the complexities of AGN photometry, possibly incorporating real-time variability corrections and data from forthcoming telescopic surveys.

In conclusion, the paper contributes significantly to advancing the field by refining photometric techniques for AGN study, emphasizing the importance of template selection, and considering variability corrections. The methodologies have far-reaching potential applications in large-scale sky surveys, crucial for unraveling galaxy evolution and the universe's structural fabric.