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 dataset. We also present spectra of SN 2011by 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. 2011; Blondin et al. 2012),
representing the largest set of late-time SN Ia spectra ever analysed. The full
width at half-maximum intensity (FWHM) and velocities of the [Fe III]
{\lambda}4701, [Fe II] {\lambda}7155, and [Ni II] {\lambda}7378 emission
features are measured in most observations of spectroscopically normal objects
where the data have signal-to-noise ratios >20 px^-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/d. The nebular
velocity (v_neb, calculated by taking the average of the [Fe II] {\lambda}7155
and [Ni II] {\lambda}7378 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 {\Delta}m_15(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.
