The Dark Energy Bedrock All-Sky Supernova Program: Motivation, Design, Implementation, and Preliminary Data Release

TL;DR

The DEBASS program aims to provide a large, well-calibrated low-redshift supernova dataset to improve cosmological measurements, demonstrating promising initial results with low scatter in the Hubble diagram residuals.

Contribution

This paper introduces the DEBASS program, a new survey that delivers a large, uniformly calibrated low-$z$ SN Ia dataset using the same instruments as DES, enhancing supernova cosmology.

Findings

Early data release includes 77 SNe Ia within the DES footprint.

Median absolute residuals of the Hubble diagram are approximately 0.10 mag.

Initial host-galaxy mass step measured at 0.06±0.04 mag.

Abstract

Precise measurements of Type Ia supernovae (SNe Ia) at low redshifts ($z$) serve as one of the most viable keys to unlocking our understanding of cosmic expansion, isotropy, and growth of structure. The Dark Energy Bedrock All-Sky Supernovae (DEBASS) program will deliver the largest uniformly calibrated low-$z$ SN Ia data set in the southern hemisphere to date. DEBASS utilizes the Dark Energy Camera to image supernovae in conjunction with the Wide-Field Spectrograph (WiFeS) to gather comprehensive host galaxy information. By using the same photometric instrument as both the Dark Energy Survey (DES) and the DECam Local Volume Exploration Survey, DEBASS not only benefits from a robust photometric pipeline and well-calibrated images across the southern sky, but can replace the historic and external low-$z$ samples that were used in the final DES supernova analysis. DEBASS has accumulated more than 400 spectroscopically confirmed SNe Ia in the redshift range of $0.01<z<0.08$ from 2021 to mid-2025, and, in this paper along with a companion paper Acevedo et al. submitted, we present an early data release of 77 SNe within the DES footprint to demonstrate the merit and constraining power of the data set. Here, we introduce the DEBASS program, discuss its scientific goals and the advantages it offers for supernova cosmology, and present our initial results. With this early data release, we find a robust median absolute standard deviation of Hubble diagram residuals of $\sim$0.10 mag and an initial measurement of the host-galaxy mass step of $0.06\pm0.04$ mag, both before performing bias corrections. This low scatter shows the promise of a low-$z$ SN Ia program with a well-calibrated telescope and high signal-to-noise ratio across multiple bands.