Low-Resolution Sodium D Absorption is a Bad Proxy for Extinction

Understand

Dust extinction is generally the least tractable systematic uncertainty in astronomy, and particularly in supernova science.

• Often in the past, studies have used the equivalent width of Na I D absorption measured from low-resolution spectra as proxies for extinction, based on tentative correlations that were drawn from limited data sets.
• We show here, based on 443 low-resolution spectra of 172 Type Ia supernovae for which we have measured the dust extinction as well as the equivalent width of Na I D, that the two barely correlate.
• We briefly examine the causes for this large scatter that effectively prevents one from inferring extinction using this method.