Asymmetries in core-collapse supernovae from maps of radioactive 44Ti in Cassiopeia A

Asymmetry is required by most numerical simulations of stellar core-collapse explosions, but the form it takes differs significantly among models. The spatial distribution of radioactive Ti-44, synthesized in an exploding star near the boundary between material falling back onto the collapsing core and that ejected into the surrounding medium(1), directly probes the explosion asymmetries. Cassiopeia A is a young(2), nearby(3), core-collapse(4) remnant from which Ti-44 emission has previously been detected(5-8) but not imaged. Asymmetries in the explosion have been indirectly inferred from a high ratio of observed Ti-44 emission to estimated Ni-56 emission(9), from optical light echoes(10), and from jet-like features seen in the X-ray(11) and optical(12) ejecta. Here we report spatial maps and spectral properties of the Ti-44 in Cassiopeia A. This may explain the unexpected lack of correlation between the Ti-44 and iron X-ray emission, the latter being visible only in shock-heated material. The observed spatial distribution rules out symmetric explosions even with a high level of convective mixing, as well as highly asymmetric bipolar explosions resulting from a fast-rotating progenitor. Instead, these observations provide strong evidence for the development of low-mode convective instabilities in core-collapse supernovae.