Intrinsic alignments of bulges and discs

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.

The SAMI Galaxy Survey: flipping of the spin–filament alignment correlates most strongly with growth of the bulge

“…Overall, studying separately the spin-filament alignments of bulges and discs provides further clues in terms of the pathways that gave rise to the corresponding galaxies. This outcome is in accord with the conclusion of Jagvaral et al (2022), who investigated the intrinsic alignments, i.e. the tendency of galaxies to coherently align with the density field and produce correlations among galaxy shapes, separately for bulges and discs.…”

“…Intrinsic alignments might bias weak lensing measurements and the estimate of cosmological parameters. Jagvaral et al (2022) concluded that the stellar dynamics of the two galaxy components play a significant role in determining the intrinsic alignments.…”

Galaxies and Halos on Graph Neural Networks: Deep Generative Modeling Scalar and Vector Quantities for Intrinsic Alignment

“…In all of the below, we always trained and generated on the full sample, and tests on the subsamples were used to demonstrate the ability of the models to distinguish the variation in IA across the subpopulations. Since past work (e.g., Jagvaral et al 2022) has shown that alignments depend primarily on mass, satellite/central status and morphology, we explicitly check that the GAN can capture these dependencies.…”

“…Here the summation index 𝑛 runs over all particles of a given type in a given galaxy, where 𝑚 𝑛 is the mass of the 𝑛 th particle and 𝑟 is the distance between the galaxy or subhalo centre of mass and the 𝑛 th particle, with 𝑖 and 𝑗 indexing the three spatial directions. We chose to use the simple mass quadrupole moment, since the reduced and the reduced iterative moments generally give very low alignment signals (Jagvaral et al 2022).…”

“…Note that the bulge-dominated sample was controlled for mass effects (i.e. this sample was weighted to match the mass distribution of the disc-dominated sample), as explained in Jagvaral et al (2022). Therefore the implicit mass differences between bulges and discs have already been accounted for in this comparison.…”