Alignment of galaxies relative to their local environment in SDSS-DR8

Abstract

Aims. We study the alignment of galaxies relative to their local environment in SDSS-DR8 and, using these data, we discuss evolution scenarios for different types of galaxies. Methods. We defined a vector field of the direction of anisotropy of the local environment of galaxies. We summed the unit direction vectors of all close neighbours of a given galaxy in a particular way to estimate this field. We found the alignment angles between the spin axes of disc galaxies, or the minor axes of elliptical galaxies, and the direction of anisotropy. The distributions of cosines of these angles are compared to the random distributions to analyse the alignment of galaxies. Results. Sab galaxies show perpendicular alignment relative to the direction of anisotropy in a sparse environment, for single galaxies and galaxies of low luminosity. Most of the parallel alignment of Scd galaxies comes from dense regions, from 2 . . . 3 member groups and from galaxies with low luminosity. The perpendicular alignment of S0 galaxies does not depend strongly on environmental density nor luminosity; it is detected for single and 2 . . . 3 member group galaxies, and for main galaxies of 4 . . . 10 member groups. The perpendicular alignment of elliptical galaxies is clearly detected for single galaxies and for members of ≤10 member groups; the alignment increases with environmental density and luminosity. Conclusions. We confirm the existence of fossil tidally induced alignment of Sab galaxies at low z. The alignment of Scd galaxies can be explained via the infall of matter to filaments. S0 galaxies may have encountered relatively massive mergers along the direction of anisotropy. Major mergers along this direction can explain the alignment of elliptical galaxies. Less massive, but repeated mergers are possibly responsible for the formation of elliptical galaxies in sparser areas and for less luminous elliptical galaxies.

Galaxy orientation with the cosmic web across cosmic time

“…Our novel predictions for the alignment between virtual galaxies and the cosmic web based on Horizon-AGN are finally also in good agreement with observational results that tend to find disks with a spin lying in the plane of walls and parallel to filaments, as well as ellipticals elongated along filaments and walls implying their spins are instead perpendicular to them (see e.g Lee & Erdogdu 2007;Li et al 2013;Zhang et al 2013;Huang et al 2016;Hirv et al 2017;Lee, Kim & Rey 2018), despite other conflicting 9 The alignment of the shapes (i.e minor axes) with the filaments and walls could also be predicted using a tidal stretching model together with the anisotropy of the cosmic web but this is left for future works. observational results (e.g Varela et al 2012).…”

The Cosmic Ballet: spin and shape alignments of haloes in the cosmic web

“…This result has since been reproduced in multiple cosmological simulations with and without baryons (Hahn et al 2010;Codis et al 2012;Libeskind et al 2013;Trowland et al 2013;Dubois et al 2014;Forero-Romero et al 2014;Wang & Kang 2017). The alignment has been confirmed by observational studies, most outstandingly so in the finding by that massive elliptical galaxies tend to have their spin perpendicular to their host filaments while the spin of less massive bright spirals has a tendency to lie parallel to their host filaments (see also Jones et al 2010;Tempel & Libeskind 2013;Zhang et al 2013Zhang et al , 2015Hirv et al 2017). The transition mass from halo spins preferentially perpendicular to preferentially parallel to their host haloes is known as the spin flip mass.…”

Mysterious Coherence in Several-megaparsec Scales between Galaxy Rotation and Neighbor Motion

“…Finally, we try to reconcile this scenario with the previous findings that the spin axes of galaxies are aligned with large-scale filaments. According to recent studies in simulations (Navarro et al 2004;Aragón-Calvo et al 2007;Brunino et al 2007;Cen 2014;Dubois et al 2014;Liu 2017;Lee et al 2018) and in observations (Tempel & Libeskind 2013;Zhang et al 2013Zhang et al , 2015Hirv et al 2017;Kim et al 2018;Jeong et al 2019), late-type galaxies in a filament tend to have spin axes parallel with the filament direction, while spin axes of early-type galaxies tend to be perpendicular to it. Since the galaxies with strong coherence signals in our results may be mainly late-type galaxies, if they are located in filaments, their spin axes may be aligned to be parallel with filaments according to those studies.…”