What excites the optical emission in X-ray-selected galaxies?

Abstract

We present a study of 1347 galaxies at z < 0.35 with detected nuclear X-ray emission and optical emission line diagnostics in the Baldwin-Phillips-Terlevich (BPT) diagram. This sample was obtained by cross-matching the XMM-Newton DR10 catalogue with Sloan Digital Sky Survey (SDSS) DR17 galaxies with well-measured line ratios. The distribution of these sources in the BPT diagram covers all three excitation regimes: Ionized Hydrogen (HII) regions (23%), `composites' (30%), and Seyfert galaxies with the low ionization nuclear emission line regions (47%). In contrast, the fraction of objects classified as active galactic nuclei (AGN) in the SDSS subsample selected for cross-match with XMM-Newton is only 13%. This fact illustrates that X-ray emission from galaxies commonly points towards the presence of AGN. Our data show, for the first time, a clear dependence of the BPT position on the ratio of the X-ray to Hα fluxes. Sources dominated by X-ray emission lie in the Seyfert and LINER regimes of the BPT diagram. Most sources with a low X-ray-to-Hα-luminosity ratio, log₁₀(L_X/L_Hα) < 1.0, lie in the HII regime. In our sample, there are even 45 galaxies that have L_XR^Star/L_Xray^Total > 0.5. In contrast, the positions of the sample members in the BPT diagram exhibit no dependence on the X-ray hardness ratio. Our finding suggests that the X-ray-to-Hα ratio can help us to differentiate galaxies whose X-ray flux is dominated by an AGN from galaxies with central X-ray binaries and other stellar X-ray sources.

The catalogue of X-ray galaxies is available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/686/A223