Browsing by Author "Choi, Yumi"

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The Recent LMC–SMC Collision: Timing and Impact Parameter Constraints from Comparison of Gaia LMC Disk Kinematics and N-body Simulations
(American Astronomical Society, 2022-03) Choi, Yumi; Olsen, Knut A. G.; Besla, Gurtina; Van Der Marel, Roeland P.; Zivick, Paul; Kallivayalil, Nitya; Nidever, David L.
We present analysis of the proper-motion (PM) field of the red clump stars in the Large Magellanic Cloud (LMC) disk using the Gaia Early Data Release 3 catalog. Using a kinematic model based on old stars with 3D velocity measurements, we construct the residual PM field by subtracting the center-of-mass motion and internal rotation motion components. The residual PM field reveals asymmetric patterns, including larger residual PMs in the southern disk. Comparisons of the observed residual PM field with those of five numerical simulations of an LMC analog that is subject to the tidal fields of the Milky Way and the Small Magellanic Cloud (SMC) show that the present-day LMC is not in dynamical equilibrium. We find that both the observed level of disk heating (PM residual rms of 0.057 ± 0.002 mas yr−1) and kinematic asymmetry are not reproduced by Milky Way tides or if the SMC impact parameter is larger than the size of the LMC disk. This measured level of disk heating provides a novel and important method to validate numerical simulations of the LMC–SMC interaction history. Our results alone put constraints on an impact parameter ≲10 kpc and impact timing <250 Myr. When adopting the impact timing constraint of ∼140–160 Myr ago from previous studies, our results suggest that the most recent SMC encounter must have occurred with an impact parameter of ∼5 kpc. We also find consistent radial trends in the kinematically and geometrically derived disk inclination and line-of-node position angles, indicating a common origin.
Unveiling the purely young star formation history of the SMC’s northeastern shell from colour–magnitude diagram fitting
(Oxford University Press, 2024-07) Sakowska, J.D.; Noël, Noëlia E. D.; Ruiz-Lara, T.; Gallart, Carme; Massana, Pol; Nidever, David L.; Cassisi, Santi; Correa-Amaro, Patricio; Choi, Yumi; Besla, Gurtina; Erkal, Denis; Martínez‐Delgado, David; Monelli, M.; Olsen, Knut; Stringfellow, Guy S.
We obtain a quantitative star formation history (SFH) of a shell-like structure (‘shell’) located in the northeastern part of the Small Magellanic Cloud (SMC). We use the Survey of the MAgellanic Stellar History to derive colour–magnitude diagrams (CMDs), reaching below the oldest main-sequence turnoff, from which we compute the SFHs with CMD-fitting techniques. We present, for the first time, a novel technique that uses red clump (RC) stars from the CMDs to assess and account for the SMC’s line-of-sight depth effect present during the SFH derivation. We find that accounting for this effect recovers a more accurate SFH. We quantify an 7 kpc line-of-sight depth present in the CMDs, in good agreement with depth estimates from RC stars in the northeastern SMC. By isolating the stellar content of the northeastern shell and incorporating the line-of-sight depth into our calculations, we obtain an unprecedentedly detailed SFH. We find that the northeastern shell is primarily composed of stars younger than 500 Myr, with significant star formation enhancements around 250 and 450 Myr. These young stars are the main contributors to the shell’s structure. We show synchronicity between the northeastern shell’s SFH with the Large Magellanic Cloud’s (LMC) northern arm, which we attribute to the interaction history of the SMC with the LMC and the Milky Way (MW) over the past 500 Myr. Our results highlight the complex interplay of ram pressure stripping and the influence of the MW’s circumgalactic medium in shaping the SMC’s northeastern shell.