The evolution of galaxy intrinsic alignments in the MassiveBlack II
  universe
  We investigate the redshift evolution of the intrinsic alignments (IA) of
galaxies in the \texttt{MassiveBlackII} (MBII) simulation. We select galaxy
samples above fixed subhalo mass cuts ($M_h>10^{11,12,13}~M_{\odot}/h$) at
$z=0.6$ and trace their progenitors to $z=3$ along their merger trees. Dark
matter components of $z=0.6$ galaxies are more spherical than their progenitors
while stellar matter components tend to be less spherical than their
progenitors. The distribution of the galaxy-subhalo misalignment angle peaks at
$\sim10~\mathrm{deg}$ with a mild increase with time. The evolution of the
ellipticity-direction~(ED) correlation amplitude $\omega(r)$ of galaxies (which
quantifies the tendency of galaxies to preferentially point towards surrounding
matter overdensities) is governed by the evolution in the alignment of
underlying dark matter~(DM) subhaloes to the matter density of field, as well
as the alignment between galaxies and their DM subhaloes. At scales
$\sim1~\mathrm{cMpc}/h$, the alignment between DM subhaloes and matter
overdensity gets suppressed with time, whereas the alignment between galaxies
and DM subhaloes is enhanced. These competing tendencies lead to a complex
redshift evolution of $\omega(r)$ for galaxies at $\sim1~\mathrm{cMpc}/h$. At
scales $>1~\mathrm{cMpc}/h$, alignment between DM subhaloes and matter
overdensity does not evolve significantly; the evolution of the galaxy-subhalo
misalignment therefore leads to an increase in $\omega(r)$ for galaxies by a
factor of $\sim4$ from $z=3$ to $0.6$ at scales $>1~\mathrm{cMpc}/h$. The
balance between competing physical effects is scale dependant, leading to
different conclusions at much smaller scales($\sim0.1~\mathrm{Mpc}/h$).
