A numerical detection of the mass-dependent spin transition of the galaxies is presented. Analyzing a sample of the galaxies with stellar masses in the range of 109 < (M ⋆/M ⊙) ≤ 1011 from the IllustrisTNG300-1 simulations, we explore the alignment tendency between the galaxy baryon spins and the three eigenvectors of the linearly reconstructed tidal field as a function of M ⋆ and its evolution in the redshift range of 0 ≤ z ≤ 1.5. Detecting a significant signal of the occurrence of the mass-dependent transition of the galaxy spins, we show that the centrals differ from the satellites in their spin transition type. As M ⋆ increases beyond a certain threshold mass, the preferred directions of the central galaxy spins transit from the minor to the intermediate tidal eigenvectors (type two) at z = 0.5 and 1, while those of the satellites transit from the minor to the major tidal eigenvectors (type one) at z = 1 and 1.5. It is also shown that the mass range and type of the spin transition depend on the galaxy morphology, the degree of the alignments between the baryon and total spin vectors, and the environmental density. Meanwhile, the stellar spins of the galaxies are found to yield a weak signal of the T1 transitions at z = 0, whose strength and trend depend on the degree of the alignments between the stellar and baryon spins. The possible mechanisms responsible for the T1 and T2 spin transitions are discussed.

We present the statistical properties of a volume-limited sample of 7,429 nearby (z = 0.033 -0.044) galaxies from the Sloan Digital Sky Survey Data Release 7. Our database includes morphology distribution as well as the structural and spectroscopic properties of each morphology type based on the recent re-measurements of spectral line strengths by Oh and collaborators (2011). Our database does not include galaxies that are apparently smaller and flatter, because morphology classification of them turned out to be difficult. Our statistics confirmed the up-to-date knowledge of galaxy populations, e.g., correlations between morphology and line strengths as well as the derived ages, etc. We hope that this database will be useful as a reference.

Galaxies in pairs show enhanced star formation (SF) compared to their counterparts in isolation, which is often explained by the tidal effect of neighboring galaxies. Recent observations, however, reported that galaxies paired with early-type neighbors do not undergo the SF enhancement. Here we revisit the influence of neighbors using a large sample of paired galaxies from the Sloan Digital Sky Survey and a carefully constructed control sample of isolated counterparts. We find that star-forming neighbors enhance SF, and even more so for more star-forming (and closer) neighbors, which can be attributed to collisions of interstellar medium (ISM) leading to SF. We further find that, contrary to the anticipated tidal effect, quiescent neighbors quench SF, and even more so for more quiescent (and closer) neighbors. This seems to be due to removal of gas reservoirs via ram pressure stripping and gas accretion cutoff by hot gas halos of quiescent neighbors, on top of their paucity of ISM to collide to form stars. Our findings, especially the intimate connection of SF to the status and strength of neighbors' SF, imply that the hydrodynamic mechanisms, along with the tidal effect, play a crucial role during the early phase of galactic interactions.

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.

A numerical detection of the radius-dependent spin transition of dark matter halos is reported. Analyzing the data from the IllustrisTNG simulations, we measure the halo spin vectors at several inner radii within the virial boundaries and investigate their orientations in the principal frames of the tidal and velocity shear fields, called the Tweb and Vweb, respectively. The halo spin vectors in the high-mass section exhibit a transition from the Tweb intermediate to major principal axes as they are measured at more inner radii, which holds for both the dark matter and baryonic components. The radius threshold at which the transition occurs depends on the smoothing scale, R f , becoming larger as R f decreases. For the case of the Vweb, the occurrence of the radius-dependent spin transition is witnessed only when R f ≥ 1 h −1 Mpc. Repeating the same analysis but with the vorticity vectors, we reveal a critical difference from the spins. The vorticity vectors are always perpendicular to the Tweb (Vweb) major principal axes, regardless of R f , which indicates that the halo inner spins are not strongly affected by the generation of vorticity. It is also shown that the halo spins, as well as the Tweb (Vweb) principal axes, have more directional coherence over a wide range of radial distances in the regions where the vorticity vectors have higher magnitudes. The physical interpretations and implications of our results are discussed.