Letter to the Editor

A gravitational acceleration model to explain the double-peaked narrow emission lines shifted in the same direction

School of Physical Science and Technology, Guangxi University, No. 100, Daxue East Road, Nanning 530004, PR China

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

Received: 17 May 2025
Accepted: 5 July 2025

Abstract

In this Letter, we propose a new, oversimplified, but potentially effective gravitational acceleration model to interpret the double-peaked narrow emission lines (DPNELs) shifted in the same direction. We adopted the framework of a merging kpc-scale dual-core system in an elliptical orbit, which includes an emission-line galaxy (ELG) with clear narrow line regions (NLRs) merging with a companion galaxy that clearly lacks emission line features. Due to gravitational forces induced by both galaxies on the NLRs, the accelerations of the far-side and near-side NLR components may share the same vector direction when projected along the line of sight (LoS), leading the velocities of the observed DPNELs to shift in the same direction. Our simulations indicate that the probability of producing double-peaked features shifted in the same direction reaches 5.81% in merging kiloparsec-scale (kpc-scale) dual-core systems containing ELGs. In addition to the expected results from our proposed model, we also identified a unique galaxy, SDSS J001050.52−103246.6, whose apparent DPNELs, shifted in the same direction, could plausibly be explained by the gravitational acceleration model. This proposed model provides a new path to explain DPNELs shifted in the same direction in a scenario where the two galaxies would align along LoS in kpc-scale dual-core systems.