It's Dust: Solving the Mysteries of the Intrinsic Scatter and Host-Galaxy Dependence of Standardized Type Ia Supernova Brightnesses (2004.10206v2)

Abstract: The use of Type Ia Supernovae (SNe Ia) as cosmological tools has motivated significant effort to: understand what drives the intrinsic scatter of SN Ia distance modulus residuals after standardization, characterize the distribution of SN Ia colors, and explain why properties of the host galaxies of the SNe correlate with SN Ia distance modulus residuals. We use a compiled sample of $\sim1450$ spectroscopically confirmed, photometric light-curves of SN Ia and propose a solution to these three problems simultaneously that also explains an empirical 11$\sigma$ detection of the dependence of Hubble residual scatter on SN Ia color. We introduce a physical model of color where intrinsic SN Ia colors with a relatively weak correlation with luminosity are combined with extrinsic dust-like colors ($E(B-V)$) with a wide range of extinction parameter values ($R_V$). This model captures the observed trends of Hubble residual scatter and indicates that the dominant component of SN Ia intrinsic scatter is from variation in $R_V$. We also find that the recovered $E(B-V)$ and $R_V$ distributions differ based on global host-galaxy stellar mass and this explains the observed correlation ($\gamma$) between mass and Hubble residuals seen in past analyses as well as an observed 4.5$\sigma$ dependence of $\gamma$ on SN Ia color. This finding removes any need to prescribe different intrinsic luminosities to different progenitor systems. Finally we measure biases in the equation-of-state of dark energy ($w$) up to $|\Delta w|=0.04$ by replacing previous models of SN color with our dust-based model; this bias is larger than any systematic uncertainty in previous SN Ia cosmological analyses.

• The paper introduces a bifurcated color model that separates intrinsic color from dust-induced effects impacting Type Ia supernova brightness.
• It analyzes a sample of 1,450 supernovae to demonstrate that variable R_V values significantly account for intrinsic scatter and host-galaxy mass trends.
• The study warns of potential biases in cosmological parameters and urges adopting refined dust models in future supernova surveys.

An Analysis of the Influence of Dust on Type Ia Supernovae and Its Cosmological Implications

The paper undertaken by Brout and Scolnic provides a rigorous reevaluation of the factors affecting the luminosity of Type Ia Supernovae (SNe Ia), specifically focusing on the intrinsic scatter and host-galaxy dependence. The authors propose a comprehensive model that emphasizes the role of dust, particularly its reddening and total-to-selective extinction parameter, $R_V$, in shaping the observable properties of SNe Ia.

Key Points of the Study

1. Intrinsic and Extrinsic Color Components: The paper introduces a bifurcated color model for SNe Ia, distinguishing between intrinsic colors correlated with luminosity by an intrinsic coefficient, $\beta_{\rm SN}$, and extrinsic colors attributed to dust, characterized by variable $R_V$. This model aims to synthesize the relationships between color and brightness into a coherent framework capable of explaining variations in existing data.
2. Empirical Findings and Data Evaluation: By analyzing a robust sample of roughly 1,450 spectroscopically confirmed supernovae, the authors demonstrate that the primary contributor to the intrinsic scatter is the variation in $R_V$, rather than merely deviations in intrinsic supernova properties. The model also aligns with previously unexplained observations, such as the correlation between host-galaxy mass and Hubble residuals.
3. Influence of Host-Galaxy Properties: A significant finding of this paper is the 4.5 standard deviation dependence of the correlation of Hubble residual scatter on SN Ia color, with distinct $R_V$ distributions noted for high- and low-mass host galaxies. This discovery supports a dust-based explanation for the long-observed host mass step, aligning extinction properties with broader galaxy characteristics.
4. Cosmological Implications: The paper critically evaluates the implications of SNe Ia scatter models on cosmological parameters, such as the equation-of-state parameter $w$. The authors highlight that incorrect handling of color models can introduce bias exceeding 0.04 in $w$, underscoring the necessity of incorporating comprehensive dust models into supernova cosmology.

Implications for Future Research and Cosmological Studies

The results offer new insights into supernova standardization processes and advocate a paradigm shift in cosmological analyses to integrate dust variability. Future surveys such as LSST and WFIRST can benefit from these findings by focusing on acquiring extensive data on blue, "dust-free" supernovae with intrinsic lower scatter and leveraging these refined models to achieve higher precision cosmological measurements.

Furthermore, this research lays groundwork for additional studies exploring the interplay between host-galaxy characteristics and supernova properties, potentially employing machine learning techniques to refine the modeling of astrophysical phenomena.

Conclusion

The insights provided by Brout and Scolnic's paper are pivotal to enhancing our understanding of Type Ia supernovae as cosmological tools. Their comprehensive dust-based model not only reconciles discrepancies in intrinsic scatter and mass step correlations but also illuminates paths toward minimizing potential systematic biases, thereby refining the measurement of cosmological parameters such as $w$.