How the Galaxy Stellar Spins Acquire a Peculiar Tidal Connection?
  We explore how the galaxy stellar spins acquire a peculiar tendency of being
aligned with the major principal axes of the local tidal fields, in contrast to
their DM counterparts which tend to be perpendicular to them, regardless of
their masses. Analyzing the halo and subhalo catalogs from the IllustrisTNG 300
hydrodynamic simulations at $z\le 1$, we determine the cosines of the alignment
angles, $\cos\alpha$, between the galaxy stellar and DM spins. Creating four
$\cos\alpha$-selected samples of the galaxies and then controlling them to
share the same density and mass distributions, we determine the average
strengths of the alignments between the galaxy stellar spins and the tidal
tensor major axes over each sample. It is clearly shown that at $z\le 0.5$ the
more severely the galaxy stellar spin directions deviate from the DM
counterparts, the stronger the peculiar tidal alignments become. Taking the
ensemble averages of such galaxy properties as central blackhole to stellar
mass ratio, specific star formation rate, formation epoch, stellar-to-total
mass ratio, velocity dispersions, average metallicity, and degree of the cosmic
web anisotropy over each sample, we also find that all of these properties
exhibit either strong correlations or anti-correlations with $\cos\alpha$. Our
results imply that the peculiar tidal alignments of the galaxy stellar spins
may be caused by anisotropic occurrence of some baryonic process responsible
for discharging stellar materials from the galaxies along the tidal major
directions at $z<1$.
