Letter to the Editor

More than 60% of double-peaked narrow emission lines possibly not related to dual galaxy systems

Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, GuangXi University, No. 100, Daxue Road, Nanning 530004, PR China

^⋆ Corresponding author: xgzhang@gxu.edu.cn

Received: 28 April 2025
Accepted: 15 July 2025

Abstract

A dual galaxy system (DGS) is one of the widely accepted scenarios to explain the double-peaked narrow emission lines (DPNELs) due to orbital motions of the two galaxies in a merging system. After considering no physical connections between two independent narrow emission line regions in two galaxies in one DGS, there should be no correlations between flux ratios R_R of redshifted narrow emission components from one galaxy and flux ratios R_B of blueshifted narrow emission components from the other galaxy in the DGS. However, after checking the large sample of DPNELs in the SDSS, there are strong linear correlations in different groups between R_R as the flux ratio of redshifted narrow [O III] to the redshifted narrow Hα and R_B as the flux ratio of blueshifted narrow [O III] to the blueshifted narrow Hα. Meanwhile, after checking narrow emission line properties of galaxy pairs within 30 (20, and 10) arcmin, there are no connections between narrow emission line fluxes in the galaxy pairs, to support the detected linear correlations being robust enough between R_R and R_B in the DPNELs in SDSS. Furthermore, through oversimplified simulations, more than 60% of the DPNELs should be not related to the expected DGSs.