Intrinsic alignments of bulges and discs

TL;DR

This study investigates how different galaxy components, especially bulges and discs, exhibit intrinsic alignments in a cosmological simulation, revealing bulges align similarly to elliptical galaxies and emphasizing the importance of galaxy dynamics in understanding these effects.

Contribution

It provides the first detailed analysis of intrinsic alignments of galaxy bulges and discs separately in hydrodynamical simulations, showing bulges have alignment signals comparable to ellipticals.

Findings

Bulges exhibit significantly higher intrinsic alignment signals than discs.

The IA amplitude for bulges is similar to that of elliptical galaxies.

Galaxy dynamics are crucial for understanding intrinsic alignments and their impact on cosmology.

Abstract

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.