Orbital dynamics and histories of satellite galaxies around Milky Way-mass galaxies in the FIRE simulations

TL;DR

This study uses FIRE-2 cosmological simulations to analyze the orbital histories of satellite galaxies around Milky Way-like galaxies, revealing complex infall and pericenter patterns influenced by evolving halo potentials.

Contribution

It provides detailed insights into satellite orbital dynamics and histories, emphasizing the importance of time-dependent halo potentials over static models.

Findings

37% of low-mass satellites were pre-processed in smaller groups.

Half of satellites experienced multiple pericenters; most recent pericenter was not the smallest.

Satellite pericenters often grew over time due to halo evolution and perturbations.

Abstract

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.