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

TL;DR

This study identifies 28 low-redshift galaxies with significant velocity offsets between emission and absorption lines, suggesting many are dual core systems, which could help understand supermassive black hole mergers.

Contribution

The paper presents a new sample of galaxies with large velocity offsets, indicating potential dual core systems, and analyzes their properties to inform black hole merger studies.

Findings

Sample includes 12 AGNs, 12 composites, 3 HII galaxies, 1 uncertain.

Velocity offset distribution is asymmetric, with more red-shifted objects.

No strong correlation between velocity offset and line width in blue-shifted systems.

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$\beta$ and H$\alpha$ 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$\beta$ and find that the correlation between velocity offset and narrow H$\beta$ 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$\alpha$, 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.