Fraction of Local Group Motion Due to the Shapley Supercluster

Determine the fraction of the Local Group’s approximately 600 km/s motion relative to the cosmic microwave background that is attributable to the gravitational influence of the Shapley Supercluster.

Background

The paper focuses on the Shapley Supercluster, one of the most massive nearby structures, and its role in driving peculiar velocities in the local universe. A central scientific issue is understanding the origin of the Milky Way and Local Group motion relative to the CMB and quantifying the specific contribution from large-scale structures.

Because galaxy redshift surveys often do not extend beyond the Shapley Supercluster and supernova observations in its direction have been sparse, direct empirical constraints on Shapley’s dynamical influence have been limited. The WHALES survey aims to discover and measure Type Ia supernovae in and around Shapley to provide new constraints on bulk-flow models and lay groundwork for improved supercluster mass and motion estimates.

References

Here, we present data from the first two seasons targeting the Shapley Supercluster ($0.02<z<0.06$), which is responsible for a large but unknown fraction of our local group's motion.

— The Weighing Halos Accurately, Locally, and Efficiently with Supernovae (WHALES) Survey Overview and Initial Data Release  (2510.26731 - Acevedo et al., 30 Oct 2025) in Abstract