A Test of Photometric Redshifts for X-ray Luminous Galaxies

We test the effectiveness of photometric redshifts based upon galaxy spectral template fitting for X-ray lumininous galaxies, using a sample of 29 sources that were detected by Chandra in the Hubble Deep Field and flanking fields. We find that sources with quasar-dominated spectra are easily identified, and that photometric redshifts are robust for the rest of the sources in our sample. Specifically, for the 24 sources that are not quasar-dominated at optical wavelengths, we find that the dispersion in redshift estimates is comparable to the dispersion for the field galaxy population in this region. There is no evidence for a trend of increasing dispersion with X-ray luminosity over the range L_X=10^39 - 5x10^43 erg/s, nor is there a trend with the ratio of X-ray to optical flux, f_X/f_R. The practical result of this work is that photometric redshifts should be robust for the majority (~80%) of the X-ray sources down to f_X~10^-16 erg/s/cm^2 that have optical counterparts brighter than R~24. Further, it is straightforward to use the same photometry to identify the sources for which the photometric redshifts will fail. Photometric redshift estimation can thus be utilized as an efficient tool in analyzing the statistical properties of upcoming large Chandra and XMM-Newton data sets and identifying interesting subsamples for further study