We report the detection of 13 CO 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 13 CO J = 6→ 5 transition, which traces warm, dense molecular gas. We employ a multiline LVG analysis and find ≈35%-60% of the molecular interstellar medium is both warm ( T ∼ 110 K) and dense ( n H 2 ∼ 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.
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Hailey-Dunsheath et al. (2008) studied this question.
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