  We present polarization spectra near maximum light for the strongly
subluminous Type Ia supernova 1999by that show that the supernova is
intrinsically polarized. SN 1999by has an observed, overall level of
polarization of ~0.3 to 0.8%, a rise of the polarization P redward of 6500 A,
and a change in polarization across the Si II 6150 A feature of about 0.4%. The
distribution of points with wavelength using an empirical Q-U plane method
reveals that SN 1999by has a well-defined axis of symmetry and suggests an
interstellar polarization (ISP) vector with P(ISP)=0.3% and position angle
Theta = 150 deg with an error circle in the Q-U plane of radius about 0.1%.
Synthetic NLTE-spectra for axisymmetric configurations based on delayed
detonation models have been computed assuming ellipsoidal geometry. Both flux
and polarization spectra can be reasonably well reproduced by models with an
asphericity of ~20 % observed equator-on. The general properties of the
polarization can be understood as a consequence of the structure of subluminous
models. The low upper limits for polarization determined for many normal events
in contrast to the high polarization in SN 1999by may suggest a relation
between the asymmetry we observed and the mechanism that produces a subluminous
Type Ia. Among various mechanisms, rapid rotation of the progenitor white
dwarf, or an explosion during a binary white dwarf merger process are likely
candidates to explain the asphericity in SN 1999by.
