Chemo-dynamics of barred galaxies in the Auriga simulations: The in-situ formation of the Milky Way bulge. (arXiv:1911.06826v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Fragkoudi_F/0/1/0/all/0/1">F. Fragkoudi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Grand_R/0/1/0/all/0/1">R. J. J. Grand</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pakmor_R/0/1/0/all/0/1">R. Pakmor</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Blazquez_Calero_G/0/1/0/all/0/1">G. Bl&#xe1;zquez-Calero</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gargiulo_I/0/1/0/all/0/1">I. Gargiulo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gomez_F/0/1/0/all/0/1">F. Gomez</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Marinacci_F/0/1/0/all/0/1">F. Marinacci</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Monachesi_A/0/1/0/all/0/1">A. Monachesi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ness_M/0/1/0/all/0/1">M. K. Ness</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Perez_I/0/1/0/all/0/1">I. Perez</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tissera_P/0/1/0/all/0/1">P. Tissera</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+White_S/0/1/0/all/0/1">S. D. M. White</a>

We explore a sample of barred galaxies in the Auriga magneto-hydrodynamical
cosmological zoom-in simulations that form boxy/peanut (b/p) bulges. The
morphology of bars and b/p’s vary for different mono-abundance populations,
according to their kinematic properties, which are in turn set by the galaxy’s
assembly history. We find that the Auriga galaxies which best reproduce the
chemo-kinematic properties of the Milky Way bulge have a negligible fraction of
ex-situ stars in the b/p region ($<1%$), with flattened, thick disc-like metal-poor stellar populations, and with their last major merger occurring at $t_{rm lookback}>12,rm Gyrs$. This imposes an upper limit on the stellar
mass ratio of subsequent mergers, which we find is broadly consistent with the
recently proposed Gaia Sausage/Enceladus merger. The average fraction of
ex-situ stars in the central regions of Auriga galaxies that form b/p’s is
$3%$ — significantly lower than in those which do not form bars or b/p’s.
While these central regions contain the oldest populations, they also have
stars younger than 5Gyrs ($>30%$) and exhibit X-shaped age and abundance
distributions. Furthermore, we present a novel method, based on kinematics, for
detecting asymmetric (buckling) b/p’s in face-on galaxies. Examining the inner
discs of galaxies in our sample, we find that in some cases a metal-rich,
star-forming inner ring forms, which surrounds the bar. Further out,
bar-induced resonances form ridges in the $V_{phi}-r$ plane — the longest of
which is due to the Outer Lindblad Resonance — which are younger and more
metal-rich than the surrounding phase-space. Our results suggest an in-situ
origin for the Milky Way bulge and highlight the significant effect the bar can
have on the surrounding disc.

We explore a sample of barred galaxies in the Auriga magneto-hydrodynamical
cosmological zoom-in simulations that form boxy/peanut (b/p) bulges. The
morphology of bars and b/p’s vary for different mono-abundance populations,
according to their kinematic properties, which are in turn set by the galaxy’s
assembly history. We find that the Auriga galaxies which best reproduce the
chemo-kinematic properties of the Milky Way bulge have a negligible fraction of
ex-situ stars in the b/p region ($<1%$), with flattened, thick disc-like
metal-poor stellar populations, and with their last major merger occurring at
$t_{rm lookback}>12,rm Gyrs$. This imposes an upper limit on the stellar
mass ratio of subsequent mergers, which we find is broadly consistent with the
recently proposed Gaia Sausage/Enceladus merger. The average fraction of
ex-situ stars in the central regions of Auriga galaxies that form b/p’s is
$3%$ — significantly lower than in those which do not form bars or b/p’s.
While these central regions contain the oldest populations, they also have
stars younger than 5Gyrs ($>30%$) and exhibit X-shaped age and abundance
distributions. Furthermore, we present a novel method, based on kinematics, for
detecting asymmetric (buckling) b/p’s in face-on galaxies. Examining the inner
discs of galaxies in our sample, we find that in some cases a metal-rich,
star-forming inner ring forms, which surrounds the bar. Further out,
bar-induced resonances form ridges in the $V_{phi}-r$ plane — the longest of
which is due to the Outer Lindblad Resonance — which are younger and more
metal-rich than the surrounding phase-space. Our results suggest an in-situ
origin for the Milky Way bulge and highlight the significant effect the bar can
have on the surrounding disc.

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