Dual core system candidates: a sample of objects with large velocity offset between absorption and narrow emission lines

TL;DR

This study targets the identification of dual-core SMBH systems by selecting $z<0.3$ galaxies from SDSS DR16 with large velocity offsets between narrow Balmer emission lines and stellar absorption, using pPXF to decompose spectra and measure offsets relative to the Hα absorption frame. A 28-object sample with Δv > 200 km s^{-1} is classified via BPT, and black hole masses are estimated to test the M_BH–σ_* connection, with skewness analyses probing kinematic origins. The results favor a dual-core merger interpretation: offsets are similar across narrow lines, several objects show two-core morphologies or merger features, and the velocity-offset distribution is not easily explained by rotating disks or outflows. This approach identifies robust dual-core candidates and provides a framework for probing merging SMBH pairs at higher redshift, with implications for understanding galaxy and black hole co-evolution.

Abstract

We present a sample of 28 objects at z<0.3 from Data Release 16 of the Sloan Digital Sky Survey (SDSS DR16) with large velocity offset (> 200 km/s) of narrow H$β$ and H$α$ emission lines relative to absorption lines. Diagnostic classification via the Baldwin-Phillips-Terlevich diagram indicates that the sample comprises 12 AGNs, 12 composite galaxies, 3 H{\sc ii} galaxies, and 1 object of uncertain classification. A strong correlation is found between stellar mass and velocity dispersion. We examine the asymmetries of the narrow H$β$ and find that the correlation between velocity offset and narrow H$β$ skewness is negligible in both blue-shifted and red-shifted systems, suggesting that the rotating disk model may not fully explain the observed kinematics. The sample exhibits an asymmetric velocity offset distribution, with more red-shifted (17) than blue-shifted (11) objects. No significant correlation is observed between velocity offset and line width in blue-shifted systems, while red-shifted systems show a weak anti-correlation for narrow H$α$, which is inconsistent with the outflow model. The similarity in velocity offset between narrow emission lines supports the dual core system. Furthermore, the SDSS photometric images reveal eight objects with two cores and two with merger features. Based on the narrow emission line properties, the objects in our sample represent strong candidates for dual core systems exhibiting velocity offset. Extending this property to higher-redshift populations in the near future may facilitate the identification of merging supermassive black hole pairs at earlier cosmic epochs, providing critical constraints on their formation and evolution.

Figures (8)

• Figure 0: An example of the SDSS spectrum in the final sample. The solid black line shows the spectrum with Plate-Mjd-Fiberid shown in the title, the solid red line shows the best fitting results of the starlight determined by the pPXF method with $\chi^2/\rm dof$ shown in the title, and the solid blue line represents the emission-line spectrum calculated by the SDSS spectrum minus the best fitting results. (The complete figure set is available in the online journal.)
• Figure 0: An example of the best fitting results of the emission lines around H$\beta$ (left panel) and H$\alpha$ (right panel). Each of the top panels shows the line spectrum (the solid black line) obtained by subtracting the pPXF determined starlight (the solid blue line) from the SDSS spectrum, the best fitting results of the emission lines as the solid red line with corresponding Plate-MJD-FiberID and $\chi^2/\rm dof$ shown in the title, H$\beta$ (H$\alpha$) emission line as the dashed red line, [O iii] ([N ii], [S ii]) doublet as the dashed purple lines, and the peaks of absorption and emission lines as the vertical dashed blue and red lines, respectively. Each of the bottom panels displays the residuals (solid blue line), obtained by subtracting the best-fitting model from the line spectrum and dividing by the SDSS spectral uncertainties. The solid and dashed green lines indicate residual levels of 0 and $\pm$1, respectively. (The complete figure set is available in the online journal.)
• Figure 3: The relation between the stellar velocity dispersions of the objects in our sample from the SDSS database and from the pPXF method. The solid black line indicates the 1:1 relation. The dashed blue lines show 3$\sigma$ confidence bands derived from F-test.
• Figure 4: The distributions of velocity offset (left panel) and the relationship between redshift and velocity offset (right panel) in both our sample and Co14 sample. In the left panel, the solid black lines show the distributions of $\Delta\upsilon$ of Co14 sample, the histogram filed by blue lines and red lines show the distributions of $\Delta\upsilon$ of narrow H$\beta$ and H$\alpha$ emission lines in our sample, respectively. In the right panel, the solid circles in blue and in red mark the results of narrow H$\beta$ and H$\alpha$ emission lines in our sample, respectively. The solid circles in black mark the results of Balmer emission lines in Co14 sample.
• Figure 5: The BPT diagram for the 27 objects (in red color) with velocity offset in our sample. The blue contour shows the distribution of more than 35,000 narrow emission-line galaxies in SDSS DR15, as shown in Zh20. The solid purple and green lines show the dividing lines reported by Ka03 and Ke01, respectively.