We report the detection of ^ { 13 } CO J = 6 \rightarrow 5 emission from the nucleus of the starburst galaxy NGC 253 with the redshift ( z ) and Early Universe Spectrometer ( ZEUS ) , a new submillimeter grating spectrometer .
This is the first extragalactic detection of the ^ { 13 } CO J = 6 \rightarrow 5 transition , which traces warm , dense molecular gas .
We employ a multi-line LVG analysis and find \approx 35 \% - 60 \% of the molecular ISM is both warm ( T \sim 110 K ) and dense ( n _ { \mathrm { H _ { \mathrm { 2 } } } } \sim 10 ^ { 4 } cm ^ { -3 } ) .
We analyze the potential heat sources , and conclude that UV and X-ray photons are unlikely to be energetically important .
Instead , the molecular gas is most likely heated by an elevated density of cosmic rays or by the decay of supersonic turbulence through shocks .
If the cosmic rays and turbulence are created by stellar feedback within the starburst , then our analysis suggests the starburst may be self-limiting .