Type Ia Supernova Spectral Features in the Context of Their Host Galaxy
  Properties
  We analyse spectroscopic measurements of 122 type Ia supernovae (SNe Ia) with
z<0.09 discovered by the Palomar Transient Factory, focusing on the properties
of the Si II 6355 and Ca II `near-infrared triplet' absorptions. We examine the
velocities of the photospheric Si II 6355, and the velocities and strengths of
the photospheric and high-velocity Ca II, in the context of the stellar mass
(Mstellar) and star-formation rate (SFR) of the SN host galaxies, as well as
the position of the SN within its host. We find that SNe Ia with faster Si II
6355 tend to explode in more massive galaxies, with the highest velocity events
only occuring in galaxies with Mstellar > 3*10^9 solar mass. We also find some
evidence that these highest velocity SNe Ia explode in the inner regions of
their host galaxies, similar to the study of Wang et al. (2013), although the
trend is not as significant in our data. We show that these trends are
consistent with some SN Ia spectral models, if the host galaxy stellar mass is
interpreted as a proxy for host galaxy metallicity. We study the strength of
the high-velocity component of the Ca II near-IR absorption, and show that SNe
Ia with stronger high-velocity components relative to photospheric components
are hosted by galaxies with low stellar mass, blue colour, and a high sSFR.
Such SNe are therefore likely to arise from the youngest progenitor systems.
This argues against a pure orientation effect being responsible for
high-velocity features in SN Ia spectra and, when combined with other studies,
is consistent with a scenario where high-velocity features are related to an
interaction between the SN ejecta and circumstellar medium (CSM) local to the
SN.
