  The violent merger of two carbon-oxygen white dwarfs has been proposed as a
viable progenitor for some Type Ia supernovae. However, it has been argued that
the strong ejecta asymmetries produced by this model might be inconsistent with
the low degree of polarisation typically observed in Type Ia supernova
explosions. Here, we test this claim by carrying out a spectropolarimetric
analysis for the model proposed by Pakmor et al. (2012) for an explosion
triggered during the merger of a 1.1 M$_{\odot}$ and 0.9 M$_{\odot}$
carbon-oxygen white dwarf binary system. Owing to the asymmetries of the
ejecta, the polarisation signal varies significantly with viewing angle. We
find that polarisation levels for observers in the equatorial plane are modest
($\lesssim$ 1 per cent) and show clear evidence for a dominant axis, as a
consequence of the ejecta symmetry about the orbital plane. In contrast,
orientations out of the plane are associated with higher degrees of
polarisation and departures from a dominant axis. While the particular model
studied here gives a good match to highly-polarised events such as SN 2004dt,
it has difficulties in reproducing the low polarisation levels commonly
observed in normal Type Ia supernovae. Specifically, we find that significant
asymmetries in the element distribution result in a wealth of strong
polarisation features that are not observed in the majority of currently
available spectropolarimetric data of Type Ia supernovae. Future studies will
map out the parameter space of the merger scenario to investigate if
alternative models can provide better agreement with observations.
