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Long-term orbital evolution of Galactic satellites and the effects on their star formation histories

We investigate the orbital motions of dwarf spheroidal galaxies (dSphs) in the halo of the Milky Way (MW) to understand their possible effects on the diversity of the star formation histories seen in these MW satellites. In this work, we explicitly consider a time-varying gravitational potential due to the growth of the MW's dark halo mass to calculate the long-term orbital evolutions of the dSphs, guided with {\it Gaia} DR2 proper motions, over the past 13.5 billion years. We find that the infall time of a satellite, defined at which the galaxy first crosses within the growing virial radius of the MW's halo, coincides well with the time when the star formation rate (SFR) is peaked for the sample of classical dSphs. On the other hand, ultra-faint dSphs already finished their SF activity prior to their infall times as already suggested in previous works, but there is a signature that their earlier SF histories are affected by interaction with the growing MW's halo to some extent. We also find, for classical dSphs, that the relative fraction of stars formed after the peak of the SFR to the current stellar mass is smaller for the smaller pericentric radius of the galaxy at its first infall. These results suggest that the infalling properties of the dSphs into the MW and the resultant environmental effects such as ram-pressure stripping and/or tidal disturbance in the MW's dark halo containing hot gas play important roles in their star formation histories.