Reassessing the ZTF Volume-limited Type Ia Supernova Sample and Its Implications for Continuous, Dust-dependent Models of Intrinsic Scatter

Abstract The volume-limited low-redshift SNe Ia sample made available by ZTF DR2 is excellent for studying a central question for analyses of SNe Ia—whether the remaining diversity of standardized luminosities arises in part from an intrinsic or extrinsic effect—characterized by the color-independent bimodality in progenitor population or color- and host-dependent diversity in dust extinction, for instance. In the initial analyses from the ZTF Collaboration, the authors reported evidence for the former hypothesis, in contrast to much of the previous evidence for the dust hypothesis found across other SNe Ia samples. We reanalyze this ZTF sample (458 SNe after cuts) in the same manner as previous samples, and report consistency with trends seen in the literature, in support of the dust hypothesis. We find the following: (1) a color dependency in the canonical “mass-step” size for SNe, with red SNe having a larger host-dependent residual step than blue SNe by 0.18 ± 0.09 mag; (2) a color-dependent difference in the Hubble residual scatter, with red SNe having a ∼33% larger scatter than blue SNe at >3 σ significance; (3) data’s preference of a model that accounts for color-dependency over a simple “step” model at ∼3 σ ; (4) the strongest evidence to date (3.5 σ ) that the relationship between supernova color, host-galaxy properties, and luminosity is continuous rather than a discrete step. Accounting for (3) and (4) with our new model, Host2D, yields a 4.0 σ improvement over the mass step model. We trace the difference in reported findings to the analysis methods, rather than a difference in the sample itself.