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.
accepted for publication in ApJ, 11 figures, revised version