Galaxy–halo internal alignments across cosmic time

Context. Previous observational studies have shown that the principal shape axes of red central galaxies are strongly aligned both with other galaxies in their host group and with the surrounding large-scale cosmic structure. Simulation-based investigations of these intrinsic alignments suggest a link between the alignment of a central galaxy’s major axis with the large-scale galaxy distribution and its internal galaxy–halo shape alignment. In contrast, blue central galaxies typically exhibit little to no alignment signal, owing to a stronger internal misalignment with their halo. Aims. We investigated how the internal alignment between the principal axes of the stellar and dark matter components evolves over time as a function of the total mass of the central galaxy at z = 0. In particular, we aim to understand why disk-dominated, blue central galaxies often show weak or absent alignment signals with the galaxy distribution in their group and in the larger-scale cosmic structure. Methods. In this work, we used data from the IllustrisTNG300-1 run and selected a sample of bright central galaxies at z = 0. From the 3D data, we computed the principal axes of the stellar and dark matter components, along with their angular momenta, to obtain the various alignment angles analyzed in this study. In addition, we used the SUBLINK merger trees to determine the number of major mergers each central galaxy experienced between z = 20 and z = 0, and to track their shapes at higher redshifts along their main branch. We examined secondary dependencies of the galaxy-halo alignment on properties such as color and merger history after first dividing the sample into mass bins. Also, we analyzed how shape alignments relate to the dynamical coupling between the angular momentum directions of the stellar and dark matter components. Results. The results show that central galaxies with MTot > 1013 M⊙ tend to align with the shape of their inner halo, largely independent of color or major merger history, although the most massive systems are typically red and have undergone numerous mergers. For lower-mass central galaxies, those that are red and have experienced many mergers exhibit the strongest evolution toward alignment. Blue central galaxies, in contrast, are more strongly influenced by the link between the stellar and dark matter angular momenta, such that they evolve toward either alignment or misalignment with both the shape and angular momentum of the inner halo.