The blue anbd visual absolute magnitude distributions of Type IA supernovae

Abstract

Tully-Fisher (TF), surface brightness fluctuation (SBF), and Hubble law distances to the parent galaxies of Type Ia supernovae (SNs Ia) are used in order to study the SN Ia blue and visual peak absolute magnitude (M_B_ and M_v_) distributions. We propose two objective cuts, each of which produces a subsample with small intrinsic dispersion in M. One cut, which can be applied to either band, distinguishes between a subsample of bright events and a smaller subsample of dim events, some of which were extinguished in the parent galaxy and some of which were intrinsically subluminous. The bright events are found to be distributed with an observed dispersion of 0.3 <~σ_obs_(M) <~0.4 about a mean absolute magnitude (M^bar^_b_ or M^bar^_v_). Each of the dim SNs was spectroscopically peculiar and/or had a red B - V color; this motivates the adoption of an alternative cut that is based on B - V rather than on M. To wit, SNs Ia that are both known to have -0.25 <B - V < +0.25 and not known to be spectroscopically peculiar show observational dispersions of only σ_obs_(M_B_) = σ_obs_(M_V_) = 0.3. Because characteristic observational errors produce σ_err_(M) > 0.2, the intrinsic dispersion among such SNs Ia is σ_int_(M) <= 0.2. The small observational dispersion indicates that SNs Ia, the TF relation, and SBFs all give good relative distances to those galaxies that produce SNs Ia. The conflict between those who use SNs Ia in order to determine the value of the Hubble constant (H_0_) and those who use TF and SBF distances to determine H_0_ results from discrepant calibrations.