Apparent Effect of Dust Extinction on the Observed Outflow Velocity of Ionized Gas in Galaxy Mergers

Abstract

In this study, we examine photoionization outflows during the late stages of galaxy mergers, with a specific focus on the relation between the observed velocity of outflowing gas and the apparent effects of dust extinction. We used the N-body/smoothed particle hydrodynamics code ASURA for galaxy merger simulations. These simulations concentrated on identical galaxy mergers featuring supermassive black holes of 108 M ⊙ and gas fractions of 30% and 10%. From the simulation data, we derived velocity and velocity dispersion diagrams for the active galactic nuclei (AGN)-driven ionized outflowing gas. Our findings show that high-velocity outflows with velocity dispersions of 500 km s−1 or greater can be observed in the late stages of galactic mergers. Particularly, in buried AGNs, both the luminosity-weighted outflow velocity and velocity dispersion increase owing to the apparent effects of dust extinction. Owing to these effects, velocity–velocity dispersion diagrams display a noticeable blue-shifted tilt in models with higher gas fractions. Crucially, this tilt is not influenced by the AGN luminosity but emerges from the observational impacts of dust extinction. Our results imply that the observed high-velocity [O iii] λ5007 outflow exceeding 1000 km s−1 in buried AGNs may be linked to the dust extinction that occurs during the late stages of gas-rich galaxy mergers.