Velocity Offset Between Emission and Absorption Lines Might Be an Effective Indicator of a Dual Core System

Abstract

This paper presents the detection of a significant velocity offset between the emission and absorption lines of a dual core system in SDSS J155708.82+273518.74 (=SDSS J1557). The photometric image of SDSS J1557 exhibits two clear cores with a projected separation of ∼2.″2 (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 the host starlight. It is found that the velocity offset of emission lines with respect to absorption lines reaches 458 ± 13 km s−1. According to a Baldwin–Phillips–Terlevich diagram, SDSS J1557 is a composite galaxy. In addition, SDSS J1557 can well fit the M BH − σ * relation of bulges, and a galaxy merger would not change this relation. Two reasonable models (an AGN-driven outflow versus a dual core system) are discussed to explain this velocity offset. The model of an AGN-driven outflow fails to interpret the systematic redshift of the emission lines and similar velocity offsets for the various emission lines of SDSS J1557. Instead, the significant velocity offset between the emission and absorption lines might be an effective indicator of a dual core system.