[Paper Review] A Sample of Dust Attenuation Laws for DES Supernova Host Galaxies

J. Duarte, S. González–Gaitán+59 more|arXiv (Cornell University)|Nov 25, 2022

Gamma-ray bursts and supernovae3 references4 citations

TL;DR

This study analyzes dust attenuation laws in 162 Dark Energy Survey supernova host galaxies using multi-band photometry and stellar population modeling, revealing significant variations in attenuation slopes linked to host galaxy properties like stellar mass and age. The findings challenge the assumption of a universal color-luminosity relation in Type Ia supernova standardization, showing that host galaxy dust cannot fully account for the observed 'mass step' in Hubble residuals, especially for redder SNe Ia.

ABSTRACT

Type Ia supernovae (SNe Ia) are useful distance indicators in cosmology, provided their luminosity is standardized by applying empirical corrections based on light-curve properties. One factor behind these corrections is dust extinction, accounted for in the color-luminosity relation of the standardization. This relation is usually assumed to be universal, which could potentially introduce systematics into the standardization. The ``mass-step'' observed for SNe Ia Hubble residuals has been suggested as one such systematic. We seek to obtain a completer view of dust attenuation properties for a sample of 162 SN Ia host galaxies and to probe their link to the ``mass-step''. We infer attenuation laws towards hosts from both global and local (4 kpc) Dark Energy Survey photometry and Composite Stellar Population model fits. We recover a optical depth/attenuation slope relation, best explained by differing star/dust geometry for different galaxy orientations, which is significantly different from the optical depth/extinction slope relation observed directly for SNe. We obtain a large variation of attenuation slopes and confirm these change with host properties, like stellar mass and age, meaning a universal SN Ia correction should ideally not be assumed. Analyzing the cosmological standardization, we find evidence for a ``mass-step'' and a two dimensional ``dust-step'', both more pronounced for red SNe. Although comparable, the two steps are found no to be completely analogous. We conclude that host galaxy dust data cannot fully account for the ``mass-step'', using either an alternative SN standardization with extinction proxied by host attenuation or a ``dust-step'' approach.

Motivation & Objective

• To investigate the diversity of dust attenuation laws in Type Ia supernova host galaxies to assess their impact on cosmological standardization.
• To examine whether host galaxy dust properties can explain the observed 'mass step' in Hubble residuals of SNe Ia.
• To test the validity of assuming a universal color-luminosity relation in SN Ia standardization by probing variations in attenuation slopes.
• To determine whether alternative standardization methods using host attenuation or a 'dust step' correction can fully eliminate the mass step.
• To explore the connection between dust reddening and residual systematics in SN Ia cosmology, particularly for red SNe Ia.

Proposed method

• Utilized multi-epoch photometry from the Dark Energy Survey (DES) for 162 SN Ia host galaxies, covering optical bands from 4 kpc and global scales.
• Fitted spectral energy distributions (SEDs) using composite stellar population models to infer host galaxy properties and dust attenuation.
• Derived attenuation laws by fitting the observed photometry to model SEDs, estimating the attenuation slope (β) and optical depth (τ) for each host.
• Compared the resulting τ–β relation to the standard Milky Way-like extinction law (R_V = 3.1) and to relations derived directly from SN light curves.
• Applied statistical modeling to assess the significance of the mass step and dust step in Hubble residual distributions.
• Used corner plots and SED fit visualizations (available at GitHub) to validate model fits and parameter constraints.

Experimental results

Research questions

• RQ1 How do dust attenuation laws vary across a sample of 162 DES SN Ia host galaxies?
• RQ2 To what extent do host galaxy properties such as stellar mass and age correlate with attenuation slope and optical depth?
• RQ3 Can host galaxy dust properties fully account for the observed 'mass step' in SN Ia Hubble residuals?
• RQ4 How do the mass step and dust step compare in magnitude and significance, particularly for red SNe Ia (c > 0)?
• RQ5 Is the assumption of a universal color-luminosity relation in SN Ia standardization invalidated by host galaxy dust diversity?

Key findings

• A significant variation in attenuation slopes was found across the host galaxy sample, with strong correlations to stellar mass and age, indicating that a universal attenuation law is not valid.
• The τ–β relation derived from host galaxies differs significantly from the standard Milky Way extinction law (R_V = 3.1), suggesting different star-to-dust geometry depending on galaxy orientation.
• Evidence for both a mass step and a two-dimensional 'dust step' was found in the Hubble residual analysis, with both effects more pronounced for red SNe Ia (c > 0).
• The mass step and dust step are comparable in magnitude but not identical, implying that host galaxy dust data alone cannot fully explain or eliminate the mass step.
• The study concludes that neither host-attenuation-based standardization nor a dust-step correction can fully account for the residual systematics, indicating that additional physical effects may be at play.
• The results suggest that the color-luminosity relation in SN Ia standardization may require more nuanced, host-dependent corrections rather than a universal prescription.