Star-Gas Misalignment in Galaxies: II. Origins Found from the
  Horizon-AGN Simulation
  There have been many studies aiming to reveal the origins of the star-gas
misalignment found in galaxies, but there still is a lack of understanding of
the contribution from each formation channel candidate. We aim to answer the
question by investigating the misaligned galaxies in Horizon-AGN, a
cosmological large-volume simulation of galaxy formation. There are 27,903
galaxies of stellar mass $M_* > 10^{10} M_\odot$ in our sample, of which 5,984
are in a group of the halo mass of $M_{200} > 10^{12} M_\odot$. We have
identified four main formation channels of misalignment and quantified their
level of contribution: mergers (35%), interaction with nearby galaxies (23%),
interaction with dense environments or their central galaxies (21%), and
secular evolution including smooth accretion from neighboring filaments (21%).
We found in the simulation that the gas, rather than stars, is typically more
vulnerable to dynamical disturbances; hence, misalignment formation is mainly
due to the change in the rotational axis of the gas rather than stars,
regardless of the origin. We have also inspected the lifetime (duration) of the
misalignment. The decay timescale of the misalignment shows a strong
anti-correlation with the kinematic morphology ($V/{\sigma}$) and the cold gas
fraction of the galaxy. The misalignment has a longer lifetime in denser
regions, which is linked with the environmental impact on the host galaxy.
There is a substantial difference in the length of the misalignment lifetime
depending on the origin, and it can be explained by the magnitude of the
initial position angle offset and the physical properties of the galaxies.
