The nature of Galactic interstellar extinction is tested using reddening line parameters for several fields in conjunction with equivalent widths $W(lambda4430)$ for the diffuse interstellar band at $4430$ AA. The Cardelli et al.$;$relations [29] at
infrared, optical, and ultraviolet wavelengths are inconsistent with the newly-derived quadratic variation of $R_V({rm observed})$ on reddening slope $X$. A minimum of $R_V=2.82pm0.06$ exists for $X=0.83pm0.10$, and is argued to represent true Galactic extinction described by $A(lambda)propto lambda^{-1.375}$. It matches expectations for a new description of extinction in the infrared, optical, and ultraviolet by Zagury [32]. Additional consequences, reddened stars with no 2175 AA$;$feature and a correlation of normalized $lambda4430$ absorption with $X$, are not predicted by the Cardelli et al.$;$relation [29]. Known variations in $X$ from 0.62 to 0.83, and corresponding variations in $R_V({rm observed})$ from 4.0 to 2.8, presumably result from forward-scattered starlight in the ultraviolet contaminating optical light of stars affected by dust extinction. A new understanding of the true nature of interstellar extinction is important for establishing an accurate picture of the extragalactic distance scale, which in turn is related to our understanding of the nature of the Universe.
The Supernova Legacy Survey (SNLS) has produced a high-quality, homogeneous sample of Type Ia supernovae (SNe Ia) out to redshifts greater than z=1. In its first four years of full operation (to June 2007), the SNLS discovered more than 3000 transien
t candidates, 373 of which have been confirmed spectroscopically as SNe Ia. Use of these SNe Ia in precision cosmology critically depends on an analysis of the observational biases incurred in the SNLS survey due to the incomplete sampling of the underlying SN Ia population. This paper describes our real-time supernova detection and analysis procedures, and uses detailed Monte Carlo simulations to examine the effects of Malmquist bias and spectroscopic sampling. Such sampling effects are found to become apparent at z~0.6, with a significant shift in the average magnitude of the spectroscopically confirmed SN Ia sample towards brighter values for z>0.75. We describe our approach to correct for these selection biases in our three-year SNLS cosmological analysis (SNLS3), and present a breakdown of the systematic uncertainties involved.
