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

Abstract

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 red-shifted narrow emission components from one galaxy and flux ratios $R_{B}$ of blue-shifted 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 red-shifted narrow [O~{\sc iii}] to the red-shifted narrow H$\alpha$ and $R_{B}$ as the flux ratio of blue-shifted narrow [O~{\sc iii}] to the blue-shifted narrow H$\alpha$. Meanwhile, after checking narrow emission line properties of galaxy pairs within 30 (20, 10) arcmins, 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, at least more than 60% of the DPNELs should be not related to the expected DGSs.