Galaxy-halo internal alignments across cosmic time

Abstract

Investigations of intrinsic alignments suggest a link between the alignment of a central galaxy's major axis with the galaxy distribution and its internal galaxy-halo shape alignment. In contrast, blue central galaxies typically exhibit almost no alignment signal, owing to a stronger internal misalignment with their halo. 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 central galaxies 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. We used data from the IllustrisTNG300-1 run and selected a sample of bright central galaxies at z=0. 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. Also, we used the merger trees to determine the number of major mergers between z=20 and z=0, and to track their shapes along their main branch. We examined secondary dependencies of the galaxy-halo alignment on properties such as color and merger history. We analyzed how shape alignments relate to the dynamical coupling between the angular momentum directions of the stellar and dark matter components. The results show that massive centrals tend to align with the shape of their inner halo, and they are typically red and have undergone numerous mergers. Lower-mass red centrals, and those that have experienced many mergers exhibit the strongest evolution toward alignment. Blue centrals, 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.