DEBASS: Dark Energy Bedrock All-Sky Supernova

• DEBASS is a uniform, low-redshift Type Ia supernova program that employs DECam’s precise photometric calibration to anchor cosmological distance scales.
• It integrates rigorous cross-calibration, simulation-driven bias corrections, and detailed host galaxy analysis to minimize systematic uncertainties.
• Forecasts show DEBASS can reduce dark energy parameter uncertainties by up to 30-60%, reinforcing its role in next-generation cosmological research.

The Dark Energy Bedrock All-Sky Supernova (DEBASS) Program is an observational initiative focused on assembling a homogeneous, precisely calibrated sample of low-redshift Type Ia supernovae (SNe Ia) using the Dark Energy Camera (DECam), with a view to provide the definitive low-redshift anchor for cosmological studies of dark energy. DEBASS leverages uniform photometric instrumentation, rigorous cross-calibration, and comprehensive host-galaxy characterization to minimize systematics that have limited previous SN Ia datasets. The program’s early releases and forecasts position it as a foundational dataset for next-generation dark energy analyses, particularly in synergy with high-redshift supernova and large-scale structure surveys.

1. Scientific Motivation and Role in Cosmology

Low-redshift SNe Ia are essential for anchoring the cosmological distance scale employed in supernova Hubble diagrams. Historically, the available low-z SN Ia samples are heterogeneous, resulting from a patchwork of surveys with disparate photometric systems, calibration strategies, and reduction pipelines. Such heterogeneity introduces cross-calibration uncertainties and complicates the propagation of systematic errors into constraints on cosmological parameters, particularly the dark energy equation of state $w$ and its evolution $w_a$.

DEBASS is designed to supersede these historical samples by producing the largest uniformly calibrated low-z SN Ia dataset in the southern hemisphere, reducing calibration systematics below the level of statistical uncertainties. This approach enables improved precision and accuracy in the extraction of cosmological information, particularly when combined with high-redshift SN Ia data from modern surveys such as DES and future programs with the Vera Rubin Observatory (LSST).

2. Program Design and Observational Strategy

DEBASS is centered on the use of the Dark Energy Camera (DECam), an imager with a wide field of view mounted on the Blanco 4-m telescope at CTIO. By using DECam for both DEBASS and the Dark Energy Survey (DES), the program achieves direct cross-calibration and eliminates filter transformation uncertainties.

Key operational strategies include:

• Uniform all-sky coverage: Observations target a broad area in the southern sky, providing an all-sky character to the low-z SN sample. The survey has so far accumulated more than 400 spectroscopically confirmed SNe Ia in $0.01Sherman et al., 14 Aug 2025).
• Photometric calibration pipeline: Nightly observations are calibrated against DELVE and DES tertiary standards, with further cross-comparisons to Pan-STARRS1. Zeropoint accuracy is tracked and verified at the $\sim 10$ millimagnitude level (Acevedo et al., 14 Aug 2025).
• Host galaxy characterization: In parallel with SN photometry, DEBASS employs the WiFeS (Wide-Field Spectrograph) system for integral-field spectroscopy of the host galaxies. This provides environmental data—host mass, star formation rate, and global/local metallicity—enabling precise studies of host-dependent corrections ('mass step') in SN Ia standardization (Sherman et al., 14 Aug 2025).

3. Photometric Calibration, Data Reduction, and Systematics

Rigorous control of calibration systematics underpins DEBASS’s design. The pipeline proceeds in several stages:

• Single-epoch image processing: Stellar catalogs are extracted from each image, with zeropoints established by cross-matching with DELVE/PS1 and utilizing DECam’s stable instrument signature.
• Global calibration checks: Band-to-band offsets among DEBASS, DES, and Pan-STARRS1 are tracked to within $\sim$0.01–0.02 mag, and tests are performed for inter-night and inter-field stability (Acevedo et al., 14 Aug 2025). Calibration 'flavors'—distinct zeropoint solutions—are routinely compared to quantify systematic shifts in SN distance moduli at the millimagnitude scale.
• Light-curve fitting and bias correction: SN light curves are modeled with the SALT3 framework, which returns stretch ($x_1$), color ($c$), and amplitude parameters. The distance modulus estimator is

$$\mu = m_x + \alpha x_1 - \beta c - M - \Delta\mu_\text{bias} + \Delta\mu_\text{host},$$

where the bias term $\Delta\mu_\text{bias}$ is calibrated from simulations, and $\Delta\mu_\text{host}$ corrects for host mass effects.

• Systematics propagation: Extensive simulations using SNANA and PIPPIN are performed, including realistic observing cadences, atmospheric transmission, image noise, and intrinsic scatter models. Selection effects (survey detection efficiency, light-curve sampling, host contamination) are quantified and found to have negligible impact compared to photometric zeropoint shifts (Acevedo et al., 14 Aug 2025).

4. Early Data Release and Comparison with Legacy Samples

The DEBASS early data release (77 SNe; 62 after cosmology-quality cuts) demonstrates robust performance:

• Distance modulus precision and Hubble scatter: The sample exhibits a median absolute standard deviation of Hubble diagram residuals of $\sim$0.10 mag, with a bias-corrected Hubble residual scatter of 0.08 mag. This is attained before full bias correction and signals a high signal-to-noise, well-calibrated dataset (Sherman et al., 14 Aug 2025, Acevedo et al., 14 Aug 2025).
• Host mass step: The initial measurement of the host-galaxy mass step is $0.06\pm0.04$ mag, indicating the sample’s utility in isolating environmental systematics.
• Comparison with Foundation: Direct comparison of DEBASS and Foundation SN distances yields a median residual offset of $0.016 \pm 0.019$ mag (Acevedo zeropoint) and $-0.015 \pm 0.019$ mag (Dovekie zeropoint). This close agreement confirms the uniformity and calibration integrity of DEBASS and underscores the avoidance of legacy cross-survey systematics (Acevedo et al., 14 Aug 2025, 1711.02474).

5. Impact on Dark Energy Parameter Estimation

Substitution of the traditional low-z anchor samples with DEBASS results in measurable cosmological gains:

• Improvement in dark energy constraints: Conservative Fisher-matrix forecasts indicate that replacing legacy low-z samples with the full DEBASS set ($>400$ SNe Ia) yields a 30% reduction in the statistical uncertainty on $w_0$ and a 24% reduction for $w_a$ in $w_0w_a$CDM models. The dark energy Figure of Merit (FoM), derived from the inverse area of the $w_0$–$w_a$ confidence ellipse, increases by up to 60% (Acevedo et al., 14 Aug 2025).
• Systematics control: Systematic shifts in distances due to calibration are constrained below the statistical error floor. The program’s ability to isolate calibration-induced variation (few millimagnitudes) ensures the robustness of cosmological inferences (Acevedo et al., 14 Aug 2025).
• Small-scale structure and tension with $\Lambda$CDM: DEBASS’s high-precision SN distances, in synergy with peculiar velocity and growth of structure measurements, will support improved determination of $f\sigma_8$ at the 25% level, directly informing ongoing efforts to resolve small-scale tensions in the $\Lambda$CDM cosmological model.

6. Synergy with Current and Future Surveys

DEBASS’s approach of using a single-instrument, all-sky survey maximizes cross-comparability and calibration integrity with major efforts such as DES, LSST, and other photometric/spectroscopic programs. It replaces the heterogeneity inherent to earlier patchwork low-z samples, overcoming the dominant calibration systematics. The DEBASS host galaxy program, leveraging WiFeS integral field spectroscopy, enables robust corrections for host-dependent effects and additional constraints on the environmental dependencies of SN Ia luminosity.

With forecasted increases in sample size, continual refinement of calibration (filter transmission tuning, color term corrections), and bias models, DEBASS’s dataset and methodology are expected to remain central to future dark energy investigations, forming a critical component of multi-probe analyses alongside high-z supernova samples and large-scale structure measurements (Weinberg et al., 2013, Blazek et al., 2022, Bilicki, 2015).

7. Future Prospects, Limitations, and Conclusions

As the DEBASS sample grows, statistical uncertainties on cosmological parameters will further decrease. Ongoing cross-calibration enhancements and systematic modeling will incrementally improve measurement precision. The architecture of DEBASS—single-instrument uniformity, robust simulations, and comprehensive host analysis—sets a standard for future low-z SN programs.

Limitations primarily remain in further minimizing low-level calibration systematics and refining bias corrections for subtle population differences. Nevertheless, early results demonstrate the sample’s consistency, low scatter, and agreement with leading surveys.

In summary, the DEBASS program is positioned to deliver the bedrock low-z SN Ia dataset for cosmological studies, providing a foundation for the next generation of dark energy research and for resolving fundamental questions concerning cosmic expansion and the nature of dark energy (Acevedo et al., 14 Aug 2025, Sherman et al., 14 Aug 2025).