Intrinsic alignments of bulges and discs

Galaxies exhibit coherent alignments with local structure in the Universe. This effect, called intrinsic alignments (IAs), is an important contributor to the systematic uncertainties for wide-field weak lensing surveys. On cosmological distance scales, intrinsic shape alignments have been observed in red galaxies, which are usually bulge-dominated; while blue galaxies, which are mostly disc-dominated, exhibit shape alignments consistent with a null detection. However, disc-dominated galaxies typically consist of two prominent structures: disc and bulge. Since the bulge component has similar properties as elliptical galaxies and is thought to have formed in a similar fashion, naturally one could ask whether the bulge components exhibit similar alignments as ellipticals? In this paper, we investigate how different components of galaxies exhibit IA in the TNG100-1 cosmological hydrodynamical simulation, as well as the dependence of IA on the fraction of stars in rotation-dominated structures at z = 0. The measurements were controlled for mass differences between the samples. We find that the bulges exhibit significantly higher IA signals, with a non-linear alignment model amplitude of AI = 2.98(-0.37)(+0.36) compared to the amplitude for the galaxies as a whole (both components), A(I) = 1.13(-0.35)(+0.37). The results for bulges are statistically consistent with those for elliptical galaxies, which have A(I) = 3.47(-0.57)(+0.57). These results highlight the importance of studying galaxy dynamics in order to understand galaxy alignments and their cosmological implications.