Quenching and morphological evolution due to circumgalactic gas
  expulsion in a simulated galaxy with a controlled assembly history
  We examine the influence of dark matter halo assembly on the evolution of a
simulated $\sim L^\star$ galaxy. Starting from a zoom-in simulation of a
star-forming galaxy evolved with the EAGLE galaxy formation model, we use the
genetic modification technique to create a pair of complementary assembly
histories: one in which the halo assembles later than in the unmodified case,
and one in which it assembles earlier. Delayed assembly leads to the galaxy
exhibiting a greater present-day star formation rate than its unmodified
counterpart, whilst in the accelerated case the galaxy quenches at $z\simeq 1$,
and becomes spheroidal. We simulate each assembly history nine times, adopting
different seeds for the random number generator used by EAGLE's stochastic
subgrid implementations of star formation and feedback. The systematic changes
driven by differences in assembly history are significantly stronger than the
random scatter induced by this stochasticity. The sensitivity of $\sim L^\star$
galaxy evolution to dark matter halo assembly follows from the close coupling
of the growth histories of the central black hole (BH) and the halo, such that
earlier assembly fosters the formation of a more massive BH, and more efficient
expulsion of circumgalactic gas. In response to this expulsion, the
circumgalactic medium reconfigures at a lower density, extending its cooling
time and thus inhibiting the replenishment of the interstellar medium. Our
results indicate that halo assembly history significantly influences the
evolution of $\sim L^\star$ central galaxies, and that the expulsion of
circumgalactic gas is a crucial step in quenching them.
