Galaxy spin alignment in filaments and sheets: observational evidence

TL;DR

This study provides observational evidence that galaxy spins are aligned with cosmic web structures, with ellipticals perpendicular to filaments and spirals aligned along them, shedding light on galaxy formation processes.

Contribution

It offers the first observational confirmation of galaxy spin alignments with filaments and sheets, using SDSS data and the Bisous filament detection model.

Findings

Elliptical galaxy minor axes are perpendicular to filaments.

Spiral galaxy spins align with filaments.

Alignment strength varies with distance from filament axis.

Abstract

Galaxies properties are known to be affected by their environment. One important question is how their angular momentum reflects the surrounding cosmic web. We use the SDSS to investigate the spin axes of spiral and elliptical galaxies relative to their surrounding filament/sheet orientations. To detect filaments a marked point process with interactions (the "Bisous model") is used. Sheets are found by detecting "flattened" filaments. The minor axes of ellipticals are found to be preferentially perpendicular to hosting filaments. A weak correlation is found with sheets. These findings are consistent with the notion that elliptical galaxies formed via mergers which predominantly occurred along the filaments. The spin axis of spiral galaxies is found to align with the host filament, with no correlation between spiral spin and sheet normal. When examined as a function of distance from the filament axis, a much stronger correlation is found in outer parts, suggesting that the alignment is driven by the laminar infall of gas from sheets to filaments. When compared with numerical simulations our results suggest that the connection between DM halo and galaxy spin is not straightforward. Our results provide an important input to the understanding of how galaxies acquire their angular momentum.