Detection of the 13CO(J=6-5) Transition in the Starburst Galaxy NGC 253

摘要

We report the detection of 13CO(J=6-5) emission from the nucleus of thestarburst galaxy NGC 253 with the redshift (z) and Early Universe Spectrometer(ZEUS), a new submillimeter grating spectrometer. This is the firstextragalactic detection of the 13CO(J=6-5) transition, which traces warm, densemolecular gas. We employ a multi-line LVG analysis and find ~ 35% - 60% of themolecular ISM is both warm (T ~ 110 K) and dense (n(H2) ~ 10^4 cm^-3). Weanalyze the potential heat sources, and conclude that UV and X-ray photons areunlikely to be energetically important. Instead, the molecular gas is mostlikely heated by an elevated density of cosmic rays or by the decay ofsupersonic turbulence through shocks. If the cosmic rays and turbulence arecreated by stellar feedback within the starburst, then our analysis suggeststhe starburst may be self-limiting.