Central galaxy alignments

Abstract

Context. Observations indicate that central galaxies’ main shape axes are significantly aligned with other galaxies in their group, as well as with the large-scale structure of the Universe. Simulations have corroborated this finding, providing further insights into how the shape of the stellar component aligns with the surrounding dark matter halo. Recent studies have also investigated the evolution of this alignment in bright central galaxies, revealing that the shapes of the dark matter halo and the stellar component can differ. These results suggest that assembly and merger processes have played a crucial role in the evolution of this alignment.Aims. In this work, we aim to gain a deeper understanding of galaxy alignments by quantifying how this property is related to the mass of the halos hosting central galaxies and to the large-scale environment measured at different scales.Methods. By studying different angles, we describe how the alignments of central galaxies depend on the masses of the halos they inhabit. We explore how the main axes of central galaxies align across different scales, both in three-dimensional and two-dimensional projections. We examine how halo mass influences these alignments and how they vary in the surrounding large-scale environment. Additionally, we analyse the characteristics of these alignments across different environments within the large-scale structure of the Universe. To conduct this study, we employed TNG300 hydrodynamical simulations and compared our results with spectroscopic data from the Sloan Digital Sky Survey Data Release 18 (SDSS DR18).Results. Three types of alignment were analysed: between stellar and dark matter components, between satellite galaxies and the central galaxy, and between the central galaxy and its host halo. The results show that the alignment increases with halo mass and varies with the environment (cluster, filament, cluster outskirt, and others). However, after controlling for local density, we found that most of the observed trends disappear, except for a marginal influence of cosmic filaments on some of the considered alignment angles. The SDSS observations confirm a mass dependence similar to the simulations, although observational biases limit the detection of differences between the different environments.