How Do the Galaxy Stellar Spins Acquire a Peculiar Tidal Connection?

Abstract 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 dark matter (DM) counterparts, which tend to be perpendicular to them, regardless of their masses. Analyzing the halo and subhalo catalogs from IllustrisTNG 300 hydrodynamic simulations at z ≤ 1, we determine the cosines of the alignment angles, cos α , between the galaxy stellar and DM spins. Creating four cos α -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 ≤ 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 black hole-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 anticorrelations with cos α . 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.