Optical emission line properties of eROSITA-selected SDSS-V galaxies

We present and discuss optical emission line properties obtained from the analysis of spectra obtained in the Sloan Digital Sky Survey (SDSS) for an X-ray-selected sample of 3684 galaxies (0.002 < z < 0.55) that were drawn from the eRASS1 catalog. We modeled the SDSS-V DR19 spectra using the NB URSTS full spectrum-fitting technique with E-MILES simple stellar population models and emission line templates to decompose the broad and narrow emission line components for a correlation with the X-ray properties. We placed the galaxies on the Baldwin-Phillips-Terlevich (BPT) diagram to diagnose their dominant excitation mechanism. We show that the consistent use of the narrow component fluxes shifts most galaxies systematically and significantly upward to the active galactic nucleus (AGN) region in the BPT diagram. On this basis, we confirm the dependence of the position of a galaxy in the BPT diagram on its (0.2 − 2.3 keV) X-ray/H α flux ratio. We also verified the correlation between the X-ray luminosity and the emission line luminosities of the narrow [O III ] λ 5007 and broad H α component and the relations between the supermassive black hole mass, the X-ray luminosity, and the velocity dispersion of the stellar component ( σ * ) on the base of the unique sample of optical spectroscopic follow-up of X-ray sources detected by eROSITA. These results highlight the importance of emission line decomposition in the AGN classification and refine the connection between X-ray emission and optical emission line properties in galaxies.