The Xgal Catalog of X-Ray Galaxies

One of the mainstreams in modern X-ray astronomy is research into extragalactic X-ray sources on the basis of the data acquired at the X-ray Multi-Mirror Newton (XMM-Newton) space observatory. According to observations, X-rays coming from galaxies are mainly radiated from their central regions, i.e., active galactic nuclei and groups of X-ray sources in the galactic disks. In this paper, we consider the cross-correlation between the 4XMM-DR9 catalog and the Hyper-Linked Extragalactic Databases and Archives (HyperLeda) of galaxies. The 4XMM-DR9 catalog is a large, up-to-date catalog of observations, which contains 550 124 unique sources and covers 2.85% of the sky, while the HyperLeda database comprises 1.5 million galaxies. Our analysis resulted in a sample of more than 5000 X-ray galaxies, most of which are active galactic nuclei of low luminosity. From this sample, we selected galaxies whose the X-ray flux exceeds F = 10^–20 J/cm²s. The sources of this kind are of particular interest since it is easier to construct an informative spectrum for them. The identified and classified catalog of 1172 manually verified galaxies—the X-ray galaxy catalog named Xgal—was created. In the Xgal catalog, most galaxies have an active X-ray nucleus; Seyfert galaxies predominate among them at short distances, while quasars are prevalent at large distances. We revealed 169 galaxies that exhibit extended nuclei with a visible surface brightness distribution and 173 galaxies with more than one X-ray source. Based on the Xgal catalog, we created a catalog of elongated X-ray galaxies (the optical angular sizes of which are a > 60″) that have X-ray sources outside the nucleus. Both catalogs are freely accessible. The Xgal catalog may serve to construct the spectra of objects of a certain class in different ranges, to develop or improve the theory of their emission, and to survey bright and extended quasars. Moreover, the entire cross-sample may be used to study active galactic nuclei with low luminosity and a large-scale structure of the universe in the X-ray range.