Title：Uncovering the first-infall history of the LMC through its dynamical impact in the Milky Way halo（通过LMC在银河系晕中的动力学影响揭示其首次掉落的轨道历史）

Speaker：Yanjun Sheng （盛衍钧）, Australian National University

Time：3:00 pm, June 12th, Wednesday

Location: Small conference room 3rd floor

Abstract：

The gravitational interactions between the Large Magellanic Cloud (LMC) and the Milky Way (MW) can cause dynamical disturbances in the MW halo, leading to a biased distribution of stellar density and other kinematic signals. These disequilibrium phenomena vary depending on different parameter combinations of the MW-LMC model. In this study, we conduct 50 high-resolution N-body simulations with varying masses and halo shapes for the MW and LMC to examine how the LMC-induced disturbances evolve under these parameters. We measure the magnitude of kinematic disturbances from the mean velocities of simulated halo stars and identify a discontinuity between the scenarios of the LMC’s first and second passages. Our findings show that, due to the short dynamical timescales in the inner Galactic halo, the reduced disturbance magnitude in the second passage is mainly due to the LMC's second infall into the MW, which begins at a much lower velocity relative to the inner halo compared to the first infall. Using a subset of approximately 1200 RR Lyrae stars located in the outer halo (50 to 100 kpc from the Galactic Center), selected from a larger sample of 135,873 RR Lyrae stars with precise distance estimates from Gaia, we find the mean latitudinal velocity in the heliocentric frame to be 30.8 ± 4.0 km/s. This observation contradicts the second-passage scenario and supports the first-infall scenario with a massive LMC (around 2.1×10^11 solar masses) at infall, an oblate MW halo with a virial mass less than 1.4×10^12 solar masses, and a flattening parameter greater than 0.7.

CV：Yanjun Sheng is a PhD student at the Australian National University. He previously received his Master’s degree at ANU and his Bachelor’s degree at Beijing Normal University. His research interests include understanding the assembly history of our Milky Way and its surroundings through updated modeling and statistical methods.