Kinematics and Host-Galaxy Properties Suggest a Nuclear Origin for
  Calcium-Rich Supernova Progenitors
  Calcium-rich supernovae (Ca-rich SNe) are peculiar low-luminosity SNe Ib with
relatively strong Ca spectral lines at ~2 months after peak brightness. This
class also has an extended projected offset distribution, with several members
of the class offset from their host galaxies by 30 - 150 kpc. There is no
indication of any stellar population at the SN positions. Using a sample of 13
Ca-rich SNe, we present kinematic evidence that the progenitors of Ca-rich SNe
originate near the centers of their host galaxies and are kicked to the
locations of the SN explosions. Specifically, SNe with small projected offsets
have large line-of-sight velocity shifts as determined by nebular lines, while
those with large projected offsets have no significant velocity shifts.
Therefore, the velocity shifts must not be primarily the result of the SN
explosion. There is an excess of SNe with blueshifted velocity shifts within
two isophotal radii (5/6 SNe), indicating that the SNe are moving away from
their host galaxies and redshifted SNe on the far sides of their galaxies are
selectively missed in SN surveys. Additionally, nearly every Ca-rich SN is
hosted by a galaxy with indications of a recent merger and/or is in a dense
environment. We propose a progenitor model which fits all current data: The
progenitor system for a Ca-rich SN is a double white dwarf (WD) system where at
least one WD has a significant He abundance. This system, through an
interaction with a super-massive black hole (SMBH) is ejected from its host
galaxy and the binary is hardened, significantly reducing the merger time.
After 10 - 100 Myr (on average), the system explodes with a large physical
offset. The rate for such events is significantly enhanced for galaxies which
have undergone recent mergers, potentially making Ca-rich SNe new probes of
both the galaxy merger rate and (binary) SMBH population. (abridged)
