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The Galaxy Progenitors of Stellar Streams around Milky Way–mass Galaxies in the FIRE Cosmological Simulations

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Stellar streams record the accretion history of their host galaxy. We present a set of simulated streams from disrupted dwarf galaxies in 13 cosmological simulations of Milky Way (MW)-mass galaxies from the FIRE-2 suite at z = 0, including seven isolated MW-mass systems and six hosts resembling the MW-M31 pair. In total, we identify 106 simulated stellar streams, with no significant differences in the number of streams and the masses of their progenitors between the isolated and paired environments. We resolve simulated streams with stellar masses ranging from ~5 105 up to ~109 M o?, similar to the mass range between the Orphan and Sagittarius streams in the MW. We confirm that present-day simulated satellite galaxies are good proxies for stellar stream progenitors, with similar properties including their stellar mass function, velocity dispersion, [Fe/H] and [a/H] evolution tracks, and orbital distribution with respect to the galactic disk plane. Each progenitor's lifetime is marked by several important timescales: its infall, star formation quenching, and stream formation times. We show that the ordering of these timescales is different between progenitors with stellar masses higher and lower than ~2 106 M o?. Finally, we show that the main factor controlling the rate of phase mixing, and therefore the rate of fading, of tidal streams from satellite galaxies in MW-mass hosts is the nonadiabatic evolution of the host potential. Other factors commonly used to predict phase-mixing timescales, such as progenitor mass and orbital circularity, show virtually no correlation with the number of dynamical times required for a stream to become phase-mixed.

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