Investigating Galaxy-Filament Alignments in Hydrodynamic Simulations
  using Density Ridges
  In this paper, we study the filamentary structures and the galaxy alignment
along filaments at redshift $z=0.06$ in the MassiveBlack-II simulation, a
state-of-the-art, high-resolution hydrodynamical cosmological simulation which
includes stellar and AGN feedback in a volume of (100 Mpc$/h$)$^3$. The
filaments are constructed using the subspace constrained mean shift (SCMS;
Ozertem & Erdogmus (2011) and Chen et al. (2015a)). First, we show that
reconstructed filaments using galaxies and reconstructed filaments using dark
matter particles are similar to each other; over $50\%$ of the points on the
galaxy filaments have a corresponding point on the dark matter filaments within
distance $0.13$ Mpc$/h$ (and vice versa) and this distance is even smaller at
high-density regions. Second, we observe the alignment of the major principal
axis of a galaxy with respect to the orientation of its nearest filament and
detect a $2.5$ Mpc$/h$ critical radius for filament's influence on the
alignment when the subhalo mass of this galaxy is between $10^9M_\odot/h$ and
$10^{12}M_\odot/h$. Moreover, we find the alignment signal to increase
significantly with the subhalo mass. Third, when a galaxy is close to filaments
(less than $0.25$ Mpc$/h$), the galaxy alignment toward the nearest galaxy
group depends on the galaxy subhalo mass. Finally, we find that galaxies close
to filaments or groups tend to be rounder than those away from filaments or
groups.
