DETECTION OF THE 13CO J=6 → 5 TRANSITION IN THE STARBURST GALAXY NGC 253

We report the detection of (CO)-C-13 J = 6 -> 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 (CO)-C-13 J = 6 -> 5 transition, which traces warm, dense molecular gas. We employ a multiline LVG analysis and find approximate to 35%-60% of the molecular interstellar medium is both warm (T similar to 110 K) and dense (n(H2) similar to 10(4) cm(-3)). We analyze the potential heat sources and conclude that ultraviolet 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.