Optical Emission-Line Properties of eROSITA-selected SDSS-V Galaxies

TL;DR

This study analyzes optical emission-line properties of 3684 eROSITA-selected SDSS-V galaxies (0.002 < z < 0.55) using NBursts full-spectrum fitting to decompose emission into narrow and broad components. By isolating the narrow-line fluxes, the authors demonstrate a significant upward shift of galaxies into the AGN region of the BPT diagram and establish robust, multiwavelength connections between X-ray luminosity and optical line luminosities for both broad and narrow components, with LX correlating as $ \\log L_X = 0.68 \\log L_{H\alpha}^{BLR} + 14.26 $ and $ \\log L_X = 0.68 \\log [OIII] + 14.41 $. They also recover a BH-mass–X-ray relation, $ \\log L_X = 0.57 \\log M_{BH} + 38.90 $, and show that higher $M_{BH}$ and narrow-line velocity dispersion $\\sigma_{nlr}$ place galaxies higher on the BPT diagram, reinforcing the link between central black hole properties and ionized gas. The work emphasizes the importance of consistent emission-line decomposition for AGN classification and provides a refined optical–X-ray connection for a large, well-defined X-ray selected galaxy sample, with implications for AGN demographics and co-evolution studies.

Abstract

We present and discuss optical emission line properties obtained from the analysis of Sloan Digital Sky Survey (SDSS) spectra for an X-ray selected sample of 3684 galaxies (0.002 < z < 0.55), drawn from the eRASS1 catalog. We modeled SDSS-V DR19 spectra using the NBursts full spectrum fitting technique with E-MILES simple stellar populations (SSP) models and emission line templates to decompose broad and narrow emission line components for correlation with X-ray properties. We place 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 nuclei (AGN) region on the BPT diagram. On this basis, we confirm the dependence between a galaxys position on the BPT diagram and its (0.2-2.3 keV) X-ray/H$α$ flux ratio. We also verified the correlation between X-ray luminosity and emission line luminosities of the narrow [O\iii]$λ5007$ and broad H$α$ component; as well as the relations between the Supermassive Black Hole (SMBH) mass, the X-ray luminosity, and the velocity dispersion of the stellar component ($σ_{*}$) on the base on 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 AGN classification and refine the connection between X-ray emission and optical emission line properties in galaxies.

Figures (9)

• Figure 1: Parameters of eROSITA and XMM-Newton X-ray selected galaxies. Panel a: angular separation between X-ray and optical sources for eROSITA and XMM-Newton X-ray selected galaxies, bin size $0.5\arcsec$. Panel b: distribution of redshift (z) for eROSITA and XMM-Newton X-ray selected galaxies, bin size $0.02$.
• Figure 2: Step-by-step description of samples selection.
• Figure 3: Some examples of SDSS-V spectra fitted using NBursts full-spectrum fitting technique with E-MILES SSP models and double emission line templates (narrow + broad). Four spectra with high signal-to-noise ratio in continuum (SNR $> 20$), which allows for a reliable identification of both narrow and broad components, were selected for demonstration. The broad-line component is clearly visible and robustly detected in multiple Balmer series lines, confirming the presence of the BLR alongside the NLR. The central panels show the spectrum in the full available wavelengths range, and the side panels show the surrounding ranges of two spectral lines H$\beta$ and H$\alpha$. The black line corresponds to spectrum fluxes, the red line --- a best-fit model, the purple line --- a stellar population model including multiplicative continuum (stellar component), the dark blue line is responsible for an additive component describing the AGN continuum, the light blue and green lines show emission lines templates for narrow and broad lines, respectively. The dark gray line shows the residuals, the light gray areas --- masked and excluded regions from the fit.
• Figure 4: Comparison of fluxes in forbidden optical emission lines obtained with NBursts technique for X-ray selected DR19 galaxies (x-axis, this paper) with previously published results (y-axis). The left panel compares 101 galaxies in common with the RCSED catalog Chilingarian2017. The right panel --- 75 galaxies in common with RCSED SDSS DR17 data release Blanton2017. The dashed black line represents 1:1 relation. The fluxes obtained in this work showed a good agreement with SDSS-IV DR17 data and a correlation with RCSED.
• Figure 5: Change of position on the BPT diagram for 101 galaxies of the matched sample from this work with the RCSED catalog. At the base of the arrow is the position determined by the emission line fluxes from the RCSED without decomposition, and at the top of the arrow is the position determined by the narrow component as a result of decomposition into narrow + broad components. Color code denotes the "distance" between the two measures in the diagram.