Chemodynamics of barred galaxies in cosmological simulations: On the
  Milky Way's quiescent merger history and in-situ bulge
  We explore the chemodynamical properties of a sample of barred galaxies in
the Auriga magneto-hydrodynamical cosmological zoom-in simulations, which form
boxy/peanut (b/p) bulges, and compare these to the Milky Way (MW). We show that
the Auriga galaxies which best reproduce the chemodynamical properties of
stellar populations in the MW bulge have quiescent merger histories since
redshift $z\sim3.5$: their last major merger occurs at $t_{\rm
lookback}>12\,\rm Gyrs$, while subsequent mergers have a stellar mass ratio of
$\leq$1:20, suggesting an upper limit of a few percent for the mass ratio of
the recently proposed Gaia Sausage/Enceladus merger. These Auriga MW-analogues
have a negligible fraction of ex-situ stars in the b/p region ($<1\%$), with
flattened, thick disc-like metal-poor stellar populations. The average fraction
of ex-situ stars in the central regions of all Auriga galaxies with b/p's is 3%
-- significantly lower than in those which do not host a b/p or a bar. While
the central regions of these barred galaxies contain the oldest populations,
they also have stars younger than 5Gyrs (>30%) and exhibit X-shaped age and
abundance distributions. Examining the discs in our sample, we find that in
some cases a star-forming ring forms around the bar, which alters the
metallicity of the inner regions of the galaxy. Further out in the disc,
bar-induced resonances lead to metal-rich ridges in the $V_{\phi}-r$ plane --
the longest of which is due to the Outer Lindblad Resonance. Our results
suggest the Milky Way has an uncommonly quiet merger history, which leads to an
essentially in-situ bulge, and highlight the significant effects the bar can
have on the surrounding disc.
