Rapid disc settling and the transition from bursty to steady star
  formation in Milky Way-mass galaxies
  Recent observations and simulations indicate substantial evolution in the
properties of galaxies with time, wherein rotationally-supported and steady
thin discs (like those frequently observed in the local universe) emerge from
galaxies that are clumpy, irregular, and have bursty star formation rates
(SFRs). To better understand the progenitors of local disc galaxies we carry
out an analysis of three FIRE-2 simulated galaxies with a mass similar to the
Milky Way at redshift z=0. We show that all three galaxies transition from
bursty to steady SFRs at a redshift between z=0.5 and z=0.8, and that this
transition coincides with a rapid (< ~1 Gyr) emergence of a
rotationally-supported interstellar medium (ISM).In the late phase with steady
SFR, the rotational energy comprises > ~90% of the total kinetic + thermal
energy in the ISM, and is roughly half the gravitational energy. By contrast,
during the early phase with bursty star formation, the ISM has a
quasi-spheroidal morphology and its energy budget is dominated by
quasi-isotropic flows including turbulence and coherent inflows/outflows. This
result, that rotational support is subdominant at early times, challenges the
common application of equilibrium disc models to the high-redshift progenitors
of Milky Way-like galaxies. We further find that the formation of a
rotation-supported ISM coincides with the formation of a thermal
energy-supported inner circumgalactic medium (CGM). Before this transition, the
inner CGM is also supported by turbulence and coherent flows, indicating that
at early times there is no clear boundary between the ISM and inner CGM.
