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

Abstract: In this manuscript, we propose, for the first time, an oversimplified but potentially effective gravitational acceleration model to interpret the double-peaked narrow emission lines (DPNELs) shifted in the same direction. We adopt the framework of a merging kpc-scale dual-core system in an elliptical orbit, which has an emission-line galaxy with clear narrow line regions (NLRs) merging with a companion galaxy lacking 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, 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 kpc-scale dual core systems containing emission-line galaxies. Besides the expected results from our proposed model, we identify a unique galaxy SDSS J001050.52-103246.6, whose apparent DPNELs shifted in the same direction can be plausibly explained by the gravitational acceleration model. This proposed model provides a new path to explain DPNELs shifted in the same direction in the scenario that the two galaxies align along the line-of-sight in kpc-scale dual-core systems.