HI vs. H$α$ - Comparing the Kinematic Tracers in Modeling the Initial Conditions of the Mice

S. Alireza Mortazavi, Jennifer M. Lotz, Joshua E. Barnes, George C. Privon, Gregory F. Snyder

Published 2017-01-13Version 1

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 WYIN 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 H$\alpha$ emission line of some of the fibers. We separate the H$\alpha$ emission of the star forming regions from that of shock-heated regions based on the [N II]/H$\alpha$ line ratio and the velocity dispersion of the components. We show that the kinematics of star-forming regions agree with that of the cold gas (HI), particularly in the tidal tails of the system. We put constraints the encounter parameters of the Mice by matching the morphology and kinematics of these tidal tails with an equal-mass galaxy merger simulation. We use an automated modeling method based on the Identikit software package. In our method, we quantify the goodness of the match and the uncertainty of the resulting encounter parameters. Most of the initial conditions reconstructed using H$\alpha$ and HI kinematics are consistent within the estimated uncertainty, and qualitatively agree with the results of previous works based on visual matching techniques. 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.