Illuminating the Local Universe: Large-Scale Structure from ZTF Type Ia Supernovae

TL;DR

This study analyzes the distribution of Type Ia supernovae in the local universe using ZTF data, revealing significant inhomogeneities and environmental effects that impact cosmological measurements.

Contribution

It provides the first detailed comparison of observed SN Ia distributions with local matter density reconstructions, highlighting environmental dependence of SN rates.

Findings

Significant SN Ia rate excess at z ≈ 0.02-0.04 not explained by survey effects.

Local SN rate enhancements reach factors of 2-5 in dense structures.

SN Ia rate correlates with environment but exceeds matter overdensity expectations.

Abstract

Within the volume-limited subsample at $z<0.06$ of the Zwicky Transient Facility (ZTF) DR2 sample, we confirm a statistically significant excess of Type Ia supernovae (SNe Ia) at $z \simeq 0.02$-$0.04$, previously reported but not explained by survey selection effects. Forward simulations assuming a uniform volumetric SN Ia rate and realistic ZTF detection efficiencies fail to reproduce the feature at the $5$-$7\sigma$ level. We also detect an excess in the rates compared to our survey simulations at $z \simeq 0.08$ and $0.14$, albeit at smaller significance. To investigate the origin of these inhomogeneities, we compare the observed SN distribution to constrained reconstructions of the local matter density field from the Manticore project, based on Bayesian forward modelling of the 2M++ galaxy catalogue. While SN overdensities are spatially associated with prominent nearby structures such as the Perseus, Coma, and Hercules superclusters, the amplitude of the SN excesses significantly exceeds that expected from matter overdensities alone. By reconstructing a redshift-dependent volumetric SN Ia rate, we find that local enhancements can reach factors of two to five within specific clusters, while the sample-averaged rate remains consistent with previous low-redshift measurements. These results indicate that the SN Ia rate is not a linear tracer of the underlying matter density and suggest a strong environmental dependence in dense structures. We discuss possible physical origins and highlight the implications for low-redshift SN cosmology, including correlated peculiar velocities and additional covariance beyond standard linear corrections.