Exploring the origin of ultra-diffuse galaxies in clusters from their primordial alignment

TL;DR

This study reveals that ultra-diffuse galaxies in a cluster tend to align with large-scale filaments, suggesting their origins are linked to primordial angular momentum acquisition before cluster infall.

Contribution

It provides evidence for the primordial alignment of UDGs in clusters, supporting a formation scenario outside the cluster environment prior to accretion.

Findings

Bright, outer-region UDGs show significant alignment with the cluster's large-scale filament.

Alignment suggests UDGs acquire angular momentum from large-scale vortices before cluster infall.

Primordial alignment persists despite cluster interactions, indicating an external origin.

Abstract

We find that the minor axes of the ultra-diffuse galaxies (UDGs) in Abell 2634 tend to be aligned with the major axis of the central dominant galaxy, at a $\gtrsim 95\%$ confidence level. This alignment is produced by the bright UDGs with the absolute magnitudes $M_r<-15.3$ mag, and outer-region UDGs with $R>0.5R_{200}$. The alignment signal implies that these bright, outer-region UDGs are very likely to acquire their angular momenta from the vortices around the large-scale filament before they were accreted into A2634, and form their extended stellar bodies outside of the cluster; in this scenario, the orientations of their primordial angular momenta, which are roughly shown by their minor axes on the images, should tend to be parallel to the elongation of the large-scale filament. When these UDGs fell into the unrelaxed cluster A2634 along the filament, they could still preserve their primordial alignment signal before violent relaxation and encounters. These bright, outer-region UDGs in A2634 are very unlikely to be the descendants of the high-surface-brightness dwarf progenitors under tidal interactions with the central dominant galaxy in the cluster environment. Our results indicate that the primordial alignment could be a useful probe of the origin of UDGs in large-scale structures.