Marie Martig, Frederic Bournaud, Darren J. Croton, Avishai Dekel, Romain Teyssier
We analyze a suite of 33 cosmological simulations following the evolution of
Milky Way-mass galaxies in low-density environments. Our sample at z = 0
comprises galaxies with a broad range of Hubble types, from nearly bulgeless
disks to bulge-dominated galaxies. The bulges are typically pseudo-bulges, with
a Sersic index lower than 2, and 70% of the galaxies have bars. Despite the
fact that a large fraction of the bulge is typically in place by z = 1, we find
no significant correlation between the morphology at z = 1 and at z = 0. The
progenitors of disk galaxies span a whole range of morphologies at z = 1,
including smooth disks, unstable disks, interacting galaxies and
bulge-dominated systems. By z = 0.5, the progenitor morphology is correlated
with the z = 0 morphology, with spiral arms and bars largely in place at z =
0.5. From this sample we analyze the formation histories of galaxies with a
bulge-to-total ratio below 0.3 (typically Sb and later types). They do form in
our simulations, but with a lower abundance than observed - a common failure of
cosmological simulations. Amongst these galaxies, we find a correlation between
the bulge fraction at z = 0 and the mass ratio of the largest merger undergone
after z = 2, as well as a correlation with the gas accretion rate at z > 1. We
find that the most disk-dominated galaxies have an extremely quiet baryon input
history; there are typically no major mergers after z = 2, and gas is accreted
at a low and constant rate, with the angular momentum stable at a fixed
direction. By contrast, more violent merger or gas accretion histories give
birth to galaxies with more prominent bulges. The galaxies with the highest
bulge Sersic index at z = 0 are those with intense gas accretion and disk
instabilities, including early bar formation, rather than the galaxies with the
most active merger histories.
View original:
http://arxiv.org/abs/1201.1079
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