Velocity offset between emission and absorption lines might be an effective indicator of dual core system

TL;DR

This study detects a significant velocity offset between emission and absorption lines in a dual core galaxy, suggesting such offsets could indicate dual core systems, with detailed analysis of SDSS J1557's properties and models.

Contribution

It introduces the idea that velocity offsets between emission and absorption lines can serve as indicators of dual core systems, supported by analysis of SDSS J1557.

Findings

Velocity offset of 458 km/s between emission and absorption lines.

SDSS J1557 exhibits two cores separated by 4.9 kpc.

The velocity offset is inconsistent with AGN-driven outflow models.

Abstract

This paper presents a detection of significant velocity offset between emission and absorption lines for a dual core system in SDSS~J155708.82+273518.74 (= SDSS~J1557). The photometric image of SDSS~J1557 exhibits clear two cores with a projected separation of $\sim$2.2 arcseconds (4.9 kpc) determined by GALFIT. Based on the applications of the commonly accepted pPXF code with 636 theoretical SSP templates, the host galaxy contribution can be well determined. Then, the emission line features of SDSS~J1557 can be well measured after subtraction of host starlight. It is found that the velocity offset of emission lines with respect to absorption lines reaches $458 \pm 13$ km/s. According to the Baldwin-Phillips-Terlevich (BPT) diagram, SDSS J1557 is a composite galaxy. In addition, SDSS J1557 can well fit the $M_{\rm BH}-\sigma_{\ast}$ relation of bulges and the galaxy merger would not change this relation. Two reasonable models (say, AGN-driven outflow vs. dual core system) have been discussed to explain this velocity offset. The model of AGN-driven outflow fails to interpret the systematic redshift of emission lines and similar velocity offsets for various emission lines in SDSS~J1557. A significant velocity offset between emission and absorption lines might be an effective indicator of dual core system.