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Orbital dynamics and histories of satellite galaxies around Milky Way-mass galaxies in the FIRE simulations
Isaiah B. Santistevan, Andrew Wetzel, Erik Tollerud, Robyn E. Sanderson, Jenna Samuel
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The orbits of satellite galaxies encode rich information about their histories. We investigate the orbital dynamics and histories of satellite galaxies around Milky Way (MW)-mass host galaxies using the FIRE-2 cosmological simulations, which, as previous works have shown, produce satellite mass functions and spatial distributions that broadly agree with observations. We first examine trends in orbital dynamics at z = 0, including total velocity, specific angular momentum, and specific total energy: the time of infall into the MW-mass halo primarily determines these orbital properties. We then examine orbital histories, focusing on the lookback time of first infall into a host halo and pericenter distances, times, and counts. Roughly 37 per cent of galaxies with Mstar < 10^7 Msun were `pre-processed' as a satellite in a lower-mass group, typically ~2.7 Gyr before falling into the MW-mass halo. Half of all satellites at z = 0 experienced multiple pericenters about their MW-mass host. Remarkably, for most (67 per cent) of these satellites, their most recent pericenter was not their minimum pericenter: the minimum typically was ~40 per cent smaller and occurred ~6 Gyr earlier. These satellites with growing pericenters appear to have multiple origins: for about half, their specific angular momentum gradually increased over time, while for the other half, most rapidly increased near their first apocenter, suggesting that a combination of a time-dependent MW-mass halo potential and dynamical perturbations in the outer halo caused these satellites' pericenters to grow. Our results highlight the limitations of idealized, static orbit modeling, especially for pericenter histories.
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