Rapid Filamentary Accretion as the Origin of Extended Thin Discs
  Galactic outflows driven by stellar feedback are crucial for explaining the
inefficiency of star formation in galaxies. Although strong feedback can
promote the formation of galactic discs by limiting star formation at early
times and removing low angular momentum gas, it is not understood how the same
feedback can result in diverse objects such as elliptical galaxies or razor
thin spiral galaxies. We investigate this problem using cosmological zoom-in
simulations of two galaxies forming within $10^{12}~\mathrm{M_\odot}$ halos
with almost identical mass accretion histories and halo spin parameters.
However, the two resulting galaxies end up with very different bulge-to-disc
ratios at $z = 0$. At $z>1.5$, the two galaxies feature a surface density of
star formation $\Sigma_{\rm SFR}\simeq 10~\mathrm{M_\odot}~{\rm yr}^{-1}~{\rm
kpc}^{-2}$, leading to strong outflows. After the last starburst episode, both
galaxies feature a dramatic gaseous disc growth from 1~kpc to 5~kpc during
1~Gyr, a decisive event we dub "the Grand Twirl". After this event, the
evolutionary tracks diverge strongly, with one galaxy ending up as a
bulge-dominated galaxy, whereas the other ends up as a disc-dominated galaxy.
The origins of this dichotomy are the angular momentum of the accreted gas, and
whether it adds constructively to the initial disc angular momentum. The
build-up of this extended disc leads to a rapid lowering of $\Sigma_{\rm SFR}$
by over two orders of magnitude with $\Sigma_{\rm SFR} \lesssim
0.1~\mathrm{M_\odot}~{\rm yr}^{-1}~{\rm kpc}^{-2}$, in remarkable agreement
with what is derived from Milky Way stellar populations. As a consequence,
supernovae explosions are spread out and cannot launch galactic outflows
anymore, allowing for the persistence of a thin, gently star forming, extended
disc.
