  Galaxies exhibit coherent alignments with local structure in the Universe.
This effect, called Intrinsic Alignments (IA), 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 nonlinear alignment model amplitude of $A_I =
2.98^{+0.36}_{-0.37}$ compared to the amplitude for the galaxies as a whole
(both components), $A_I = 1.13^{+0.37}_{-0.35}$. 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.
