2026ApJ...998L..26A

We present an overview of the Weighing Halos Accurately, Locally, and Efficiently with Supernovae (WHALES) survey, the first to discover and measure Type Ia supernovae (SNe Ia) in and around galaxy superclusters. By building a sample of SNe Ia around these massive environments, we aim to provide new constraints on bulk-flow models while laying the groundwork for improved estimates of supercluster masses. 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. Until now, no supernovae (SNe) had been analyzed in the direction of Shapley. Through the WHALES survey, we have identified 12 likely SNe Ia in this region using SkyMapper, including eight with spectroscopic confirmation. We present the first light curves of these SNe and combine our observations with data from the Asteroid Terrestrial-impact Last Alert System. We demonstrate that the low number of discovered SNe Ia per season is consistent with various rate calculations, highlighting the need for future surveys to monitor superclusters over a multiyear time span. Finally, we present simulations of SN Ia observations in the environments of massive galaxy clusters, demonstrating how the inferred peculiar velocities can constrain cluster masses, and highlighting the added complexity within superclusters. We find that a sample of 100 SNe Ia would enable a 25% precision measurement of the total mass of the Shapley Supercluster.