Photometric redshifts for X-ray-selected active galactic nuclei in the eROSITA era

TL;DR

This paper evaluates machine learning and SED-fitting techniques for estimating photometric redshifts of X-ray-selected AGNs in the eROSITA era, demonstrating comparable accuracy and potential for large-scale application.

Contribution

It compares ML and SED-fitting methods for AGN photo-z estimation using Stripe 82X data, highlighting their effectiveness with available photometry and potential improvements.

Findings

ML and SED fitting perform comparably in accuracy.

Photometric data quality impacts photo-z reliability.

Reliable photo-z can be obtained before spectroscopic follow-up.

Abstract

With the launch of eROSITA (extended Roentgen Survey with an Imaging Telescope Array), successfully occurred on 2019 July 13, we are facing the challenge of computing reliable photometric redshifts for 3 million of active galactic nuclei (AGNs) over the entire sky, having available only patchy and inhomogeneous ancillary data. While we have a good understanding of the photo-z quality obtainable for AGN using spectral energy distribution (SED)-fitting technique, we tested the capability of machine learning (ML), usually reliable in computing photo-z for QSO in wide and shallow areas with rich spectroscopic samples. Using MLPQNA as example of ML, we computed photo-z for the X-ray-selected sources in Stripe 82X, using the publicly available photometric and spectroscopic catalogues. Stripe 82X is at least as deep as eROSITA will be and wide enough to include also rare and bright AGNs. In addition, the availability of ancillary data mimics what can be available in the whole sky. We found that when optical, and near- and mid-infrared data are available, ML and SED fitting perform comparably well in terms of overall accuracy, realistic redshift probability density functions, and fraction of outliers, although they are not the same for the two methods. The results could further improve if the photometry available is accurate and including morphological information. Assuming that we can gather sufficient spectroscopy to build a representative training sample, with the current photometry coverage we can obtain reliable photo-z for a large fraction of sources in the Southern hemisphere well before the spectroscopic follow-up, thus timely enabling the eROSITA science return. The photo-z catalogue is released here.