The Weighing Halos Accurately, Locally, and Efficiently with Supernovae (WHALES) Survey Overview and Initial Data Release

TL;DR

WHALES introduces a dedicated SN Ia survey targeting the Shapley Supercluster to probe local bulk flows and constrain supercluster masses. Using SkyMapper for discovery, WiFeS spectroscopy for redshifts, and complementary ATLAS data, the project identified 12 SNe Ia across two seasons (8 with spectroscopy) and built initial light curves and a preliminary Hubble diagram. Simulations with Uchuu reveal that subcluster structure and Fingers-of-God effects complicate mass inference, but a sample of ~100 well-measured SNe Ia could achieve better than 25% precision on the total Shapley mass. This work demonstrates the feasibility of environment-specific SN surveys and underscores the need for multi-year, wide-field programs (and future LSST data) to disentangle substructure and robustly constrain supercluster masses.

Abstract

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 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 8 with spectroscopic confirmation. We present the first light curves of these SNe and combine our observations with data from ATLAS. 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 multi-year timespan. 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.

Figures (6)

• Figure 1: A visualization of our survey design. The black points are galaxies located in or near the Shapley supercluster as identified in Quintana_2020. The boxes represent the CCD layout of SkyMapper, which is in a $2\times8$ grid. The red boxes are Field 1 (Season 1 & 2), while the blue and green boxes are Fields 2 and 3 (Season 2), respectively.
• Figure 2: Examples image and light-curves from SN 2024kux. Images span $200" \times 200"$ and are taken in the $r$-band. Upper Left: Science image from MJD 60471.4 (phase $-$5.5 days relative to optical maximum). Upper Right: Template image built from three images taken over a year before the explosion. Lower Left: Difference image of the upper two panels with the template image convolved to fit the science image. Lower Right: SNANA SALT3 SN Ia fit to the light-curve.
• Figure 3: Evaluating the consistency of measured magnitudes for a given star by analyzing the distribution of observed values relative to the mean. Each magnitude is compared to the average and plotted as a function of magnitude and represented in blue based on the number of data points within each bin. The legend provides the overall median and RSD of the residuals for stars brighter than 15 mag, while binned statistics are shown in pink.
• Figure 4: SALT3 light curves for the combined WHALES and ATLAS data.
• Figure 5: Hubble diagram (top) and Hubble residuals (bottom). The shaded region represents the redshift range of the Shapley Supercluster as defined in Quintana_2020 ($0.02<z<0.06$) with the darker line marking the center redshift of the supercluster.