The origin of the atomic and molecular gas contents of early-type
  galaxies. II. Misaligned gas accretion
  We study the origin of the wide distribution of angles between the angular
momenta of the stellar and gas components, $\alpha_{\rm G,S}$, in early-type
galaxies (ETGs). We use the GALFORM model of galaxy formation, set in the
$\Lambda$ cold dark matter framework, and coupled it with a Monte-Carlo
simulation to follow the angular momenta flips driven by matter accretion onto
haloes and galaxies. We consider a gas disk to be misaligned with respect to
the stellar body if $\alpha_{\rm G,S}>30$~degrees. By assuming that the only
sources of misaligments in galaxies are galaxy mergers, we place a lower limit
of $2-5$ per cent on the fraction of ETGs with misaligned gas/stellar
components. These low fractions are inconsistent with the observed value of
$\approx 42\pm 6$ per cent in ATLAS$^{\rm 3D}$. In the more general case, in
which smooth gas accretion in addition to galaxy mergers can drive
misalignments, our calculation predicts that $\approx 46$ per cent of ETGs have
$\alpha_{\rm G,S}>30$~degrees. In this calculation, we find correlations
between $\alpha_{\rm G,S}$ and stellar mass, cold gas fraction and star
formation rate, such that ETGs with high masses, low cold gas fractions and low
star formation rates are more likely to display aligned cold gas and stellar
components. We confirm these trends observationally for the first time using
ATLAS$^{\rm 3D}$ data. We argue that the high fraction of misaligned gas discs
observed in ETGs is mostly due to smooth gas accretion (e.g. cooling from the
hot halo of galaxies) which takes place after most of the stellar mass of the
galaxy is in place and comes misaligned with respect to the stellar component.
Galaxies that have accreted most of their cold gas content prior to the time
where most of the stellar mass was in place show aligned components.
