Intrinsic alignments of bulges and discs
  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.
