Dynamics and shocks from H α emission of nearby galaxy mergers

We examine the dynamical properties of interacting galaxies and the properties of shocked gas produced as a result of the interaction. We observed 22 galaxy mergers using the SparsePak IFU at Kitt Peak National Observatory. The goal of the observations was to obtain the H α velocity maps over the entire luminous parts of the galaxies including the faint tidal tails and to find extended shocks and outflows. Our sample consists of major and minor galaxy mergers with mass ratios 1 < μ < 8. We fit multiple kinematic components to the H α and [N II] emission lines, develop an MCMC code to robustly estimate the error of fit parameters, and use the F-test to determine the best number of kinematic components for each fibre. We use [N II]/H α and velocity dispersion of components to separate star-forming (H II) regions from shocks. We use the kinematics of the H α emission from H II regions and an automated modelling method to put the first ever constraints on the encounter parameters of one of the observed systems. Besides, we roughly estimate the fraction of shocked H α emission, f_shocked, without taking extinction into account and examine the spatial distribution of shocks. We find that close galaxy pairs have, on average, a higher shock fraction than wide pairs, and coalesced mergers have the highest average f_shocked. In addition, galaxy pairs with more equal mass ratio tend to have a higher f_shocked. Combining the dynamical models from the literature and this work, we inspect trends between f_shocked and dynamical encounter parameters. Our findings are generally consistent with shocks being produced either by the direct collision of the interstellar medium or by the chain of events provoked by the tidal impulse during the first passage.