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Asymmetry is required by most numerical simulations of stellar core-collapseexplosions, but the form it takes differs significantly among models. Thespatial distribution of radioactive 44Ti, synthesized in an exploding star nearthe boundary between material falling back onto the collapsing core and thatejected into the surrounding medium, directly probes the explosion asymmetries.CassiopeiaA is a young, nearby, core-collapse remnant from which 44Ti emissionhas previously been detected but not imaged. Asymmetries in the explosion havebeen indirectly inferred from a high ratio of observed 44Ti emission toestimated 56Ni emission, from optical light echoes, and from jet-like featuresseen in the X-ray and optical ejecta. Here we report spatial maps and spectralproperties of the 44Ti in Cassiopeia A. This may explain the unexpected lack ofcorrelation between the 44Ti and iron X-ray emission, the latter being visibleonly in shock-heated material. The observed spatial distribution rules outsymmetric explosions even with a high level of convective mixing, as well ashighly asymmetric bipolar explosions resulting from a fast-rotating progenitor.Instead, these observations provide strong evidence for the development oflow-mode convective instabilities in core-collapse supernovae.

Asymmetries in core-collapse supernovae from maps of radioactive $^{44}$Ti in CassiopeiaA