Merger Effects on the Spin and Shape Alignments of Galaxy Stellar, Cold
  and Hot Gas, and Dark Matter Components
  We present a numerical evidence supporting the scenario that the peculiar
alignments of the galaxy stellar spins with the major principal axes of the
local tidal tensors are produced during the quiescent evolution period when the
galaxies experience no recent merger events. Analyzing the merger tree from the
TNG300-1 simulation of the IllustrisTNG project, we find the latest merger
epochs, $a(z_{m})$, of the galaxies, and create four $a(z_{m})$-selected
samples that are controlled to share the identical mass and density
distributions. For each sample, we determine the spin and shape vectors of the
galaxy stellar, cold and hot gas, and dark matter components separately, and
compute the average strengths of their alignments with the principal directions
of the local tidal fields as well as their mutual alignment tendencies. It is
found that the stellar (cold gas) spin axes of the galaxies whose latest merger
events occur at earlier epochs are more strongly aligned (weakly anti-aligned)
with the major principal axes of the tidal fields. It is also shown that
although the mass-dependent transition of the galaxy DM spins have little
connection with the merger events, the morphologies, spin-shape and shape-shear
alignment strengths of the galaxy four components sensitively depend on
$a(z_{m})$. Noting that the stellar components of the galaxies which undergo
long quiescent evolution have distinctively oblate shapes and very strong
spin-shape alignments, we suggest that the local tidal field might be traced by
using the stellar shapes of galaxies without signatures of mergers as a proxy
of their stellar spins.
