On the Time Variation of Dust Extinction and Gas Absorption for Type~Ia Supernovae Observed Through Non-uniform Interstellar Medium

TL;DR

This paper investigates whether non-uniform interstellar medium causes observable time variations in dust extinction and gas absorption in Type Ia supernovae, concluding that such effects are minimal and unlikely to affect cosmological measurements.

Contribution

The study demonstrates that steep ISM density spectra lead to negligible time variations in supernova observations, supporting the reliability of SNe Ia in cosmology.

Findings

Time variations in dust extinction are small due to ISM density fluctuations.

Non-uniform ISM density does not significantly impact SNe Ia cosmology.

Predictions align with observations of SN 2012cu and SN 2014J.

Abstract

For Type Ia supernovae (SNe Ia) observed through a non-uniform interstellar medium (ISM) in its host galaxy, we investigate whether the non-uniformity can cause observable time variations in dust extinction and in gas absorption due to the expansion of the SN photosphere with time. We show that, owing to the steep spectral index of the ISM density power spectrum, sizable density fluctuation amplitudes at the length scale of typical ISM structures ($\gtrsim \text{ 10 pc}$) will translate to much smaller fluctuations on the scales of a SN photosphere. Therefore the typical amplitude of time variation due to non-uniform ISM, of absorption equivalent widths and of extinction, would be small. As a result, we conclude that non-uniform ISM density should not impact cosmology measurements based on SNe Ia. We apply our predictions based on the ISM density power law power spectrum to the observations of two highly reddened SNe Ia, SN 2012cu and SN 2014J.