The Galaxy Progenitors of Stellar Streams around Milky Way-mass Galaxies
  in the FIRE Cosmological Simulations
  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 7 isolated Milky Way-mass systems and 6 hosts resembling the MW-M31
pair (full dataset at: https://flathub.flatironinstitute.org/sapfire). In
total, we identify 106 simulated stellar streams, with no significant
differences in the number of streams and masses of their progenitors between
the isolated and paired environments. We resolve simulated streams with stellar
masses ranging from $\sim 5\times10^5$ up to $\sim 10^{9} M_\odot$, 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 [$\alpha$/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 $\sim 2\times10^6 M_\odot$. Finally, we show that the main factor
controlling the rate of phase-mixing, and therefore fading, of tidal streams
from satellite galaxies in MW-mass hosts is non-adiabatic 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.
