Berkeley Supernova Ia Program - V. Late-time spectra of Type Ia Supernovae

In this work, we analyse late-time (t > 100 d) optical spectra of low-redshift (z < 0.1) Type Ia supernovae (SNe Ia) which come mostly from the Berkeley Supernova Ia Program (BSNIP) data set. We also present spectra of SN 2011 by for the first time. The BSNIP sample studied consists of 34 SNe Ia with 60 nebular spectra, to which we add nebular spectral feature measurements of 20 SNe Ia from previously published work (Maeda et al. and Blondin et al.), representing the largest set of late-time SN Ia spectra ever analysed. The full width at half-maximum intensity and velocities of the [Fe III] λ4701, [Fe II] λ7155 and [Ni II] λ7378 emission features are measured in most observations of spectroscopically normal objects where the data have signal-to-noise ratios ≳ 20 pixel^-1 and are older than 160 d past maximum brightness. The velocities of all three features are seen to be relatively constant with time, increasing only a few to ∼20 km s^-1 d^-1. The nebular velocity (v_neb, calculated by taking the average of the [Fe II] λ7155 and [Ni II] λ7378 velocities) is correlated with the near-maximum-brightness velocity gradient and early-time ejecta velocity. Nearly all high velocity gradient objects have redshifted nebular lines while most low velocity gradient objects have blueshifted nebular lines. No correlation is found between v_neb and Δm₁₅(B), and for a given light-curve shape there is a large range of observed nebular velocities. The data also indicate a correlation between observed (B - V)_max and v_neb.