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

¹ Max-Planck-Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany
² Sternberg Astronomical Institute, M.V. Lomonosov Moscow State University, Universitetskiy prosp. 13, Moscow 119234, Russia
³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St. MS09, Cambridge, MA 02138, USA
⁴ Department for Physics and Astronomy, Heidelberg University, Im Neuenheimer Feld 226, 69120 Heidelberg, Germany
⁵ Universite Paris Cite, CNRS, Astroparticule et Cosmologie, F-75013 Paris, France
⁶ New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE
⁷ Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
⁸ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
⁹ Exzellenzcluster ORIGINS, Boltzmannstr. 2, 85748 Garching, Germany
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹¹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
¹² Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, D-69120 Heidelberg, Germany
¹³ Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
¹⁴ Instituto de Astronomía, Universidad Nacional Autónoma de México, A.P. 70-264, 04510 Mexico, D.F., Mexico
¹⁵ Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
¹⁶ Department of Astronomy, University of Washington, Box 351580 Seattle, WA 98195, USA
¹⁷ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
¹⁸ Institute for Gravitation and the Cosmos, the Pennsylvania State University, University Park, PA 16802, USA

^⋆ Corresponding author: pulatova@mpia.de

Received: 11 April 2025
Accepted: 21 July 2025

Abstract

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 NBURSTS 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/Hα flux ratio. We also verified the correlation between the X-ray luminosity and the emission line luminosities of the narrow [O III]λ5007 and broad Hα 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.