Galaxy spin-filament alignments in the cluster outskirts

Leader: Stefania Barsanti

Collaborators: Matthew Colless and anyone who is interested in collaborating.

Topic: The future fate of ram-pressure affected cluster galaxies

Status: under review by Science Committee

Abstract: Understanding how the galaxy angular momentum is affected by the local and global environments is a key science goal of the Hector Galaxy Survey. Previous works investigating the alignment of the galaxy spin axis with respect to the closest cosmic filament have shown that the spin of galaxies in high density regions are expected to be oriented more perpendicularly (e.g., Codis et al. 2012,Bett et al. 2012, Welker et al. 2014). This is due to the numerous mergers that occur in these environments and that are expected to cause the flipping of the spin–filament alignment. In our previous study, Barsanti et al. (submitted) we found a strong correlation between bulge formation and the galaxy spin-filament alignments for the SAMI Galaxy Survey. In this project, we aim to understand the role of the local environment in changing the these alignments. We will study this signal for isolated galaxies, combining the Hector data with the SAMI Galaxy Survey, and for galaxies in the cluster outskirts, taking advantage of the probed regions beyond the virial radius for the Hector clusters. We will also include low-mass galaxies to understand whether the correlation with bulge mass shows a difference between classical (merger-produced) bulges and pseudo (bar/instability-produced) bulges. This larger sample size will increase the significance of the signal across all environments. Pre-processing occurring in the cluster outskirts is expected to affect the spin–filament alignment and the study of these regions will help us to understand the effects of environmental processes. We will make use of the underlying spectroscopic surveys and of the extractors DisPerSe (Sousbie 2011a, 2011b) and SCONCE (Zhang et al. 2022) to reconstruct the cosmic filaments. We will compare the fractions of perpendicular and parallel tendencies for the galaxies in the different environments. We will analyse the spin-filament alignment of galaxies as a function of their morphology, kinematics, spectral characteristics and their position in the projected phase space diagram, i.e. the galaxy velocity as a function of projected cluster-centric radius. This will allow us to disentangle true infallers from backsplash galaxies, i.e. galaxies that have already traversed the cluster core and are observed close to the maximum distance before their second infall. Backsplash galaxies have likely been affected by ram-pressure stripping occurring in the cluster core, and this might have an impact on their spin orientations. Overall, this study will show new insights about the impact of environmental mechanisms on the spin-filament alignment of galaxies.

Needed data products: kinematic position angles, axial ratios/ellipticities

Publication Date: 30/06/2023