H I versus H α – comparing the kinematic tracers in modelling the initial conditions of the Mice

We explore the effect of using different kinematic tracers (HI and H$\alpha$) on reconstructing the encounter parameters of the Mice major galaxy merger (NGC 4676A/B). We observed the Mice using the SparsePak Integral Field Unit (IFU) on the WIYN telescope, and compared the H$\alpha$ velocity map with VLA HI observations. The relatively high spectral resolution of our data (R $\approx$ 5000) allows us to resolve more than one kinematic component in the emission lines of some fibers. We separate the H$\alpha$-[N II] emission of the star-forming regions from shocks using their [N II]/H$\alpha$ line ratio and velocity dispersion. We show that the velocity of star-forming regions agree with that of the cold gas (HI), particularly, in the tidal tails of the system. We reconstruct the morphology and kinematics of these tidal tails utilizing an automated modeling method based on the Identikit software package. We quantify the goodness of fit and the uncertainties of the derived encounter parameters. Most of the initial conditions reconstructed using H$\alpha$ and HI are consistent with each other, and qualitatively agree with the results of previous works. For example, we find 210$\pm^{50}_{40}$ Myrs, and 180$\pm^{50}_{40}$ Myrs for the time since pericenter, when modeling H$\alpha$ and HI kinematics, respectively. This confirms that in some cases, H$\alpha$ kinematics can be used instead of HI kinematics for reconstructing the initial conditions of galaxy mergers, and our automated modeling method is applicable to some merging systems.