Stellar Streams and Their Progenitors in MW-like Simulations

Stellar streams record the accretion history of their host galaxy since they are direct imprints left behind by past and ongoing mergers. We present a set of simulated streams in the FIRE-2 cosmological hydrodynamical simulations with 7 isolated MW-like systems and 3 paired MW-Andromeda-like systems. We show that the median object local velocity dispersion is an effective criterion to separate simulated stellar streams from phase-mixed objects. In total, there are 111 stellar stream candidates, with no significant differences in the number of streams and masses of their progenitors between the isolated and paired environments. These simulated streams have stellar masses ranging from ~5×10⁵ up to ~10⁹ solar masses, similar to the Sagittarius and Orphan streams. We confirm that present-day simulated satellite galaxies are good models of stellar stream progenitors. They have similar properties including their stellar mass function, velocity dispersion, [Fe/H] and [alpha/H] evolution tracks, and their orbital distribution with respect to the disk plane. They have slightly preferential orbits that align with the galactic disk plane. Each progenitor lifetime involves important timescales of infall time, quenching time, and stream-formation time. We show that the ordering of these timescales is different between progenitors with stellar masses higher and lower than ~2.25×10⁶ solar masses. Low-mass progenitors are likely to have their star formation quenched before infall, while most high-mass progenitors have their star formation quenched by the host environment, with many continuing to form star particles and reaching peak stellar mass after stream-formation time.