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

TL;DR

This paper reports the first extragalactic detection of 13CO(J=6-5) in NGC 253, revealing warm, dense molecular gas likely heated by cosmic rays or turbulence, with implications for starburst feedback mechanisms.

Contribution

First detection of 13CO(J=6-5) in an external galaxy, providing new insights into the heating processes of molecular gas in starburst environments.

Findings

35-60% of molecular ISM is warm and dense

UV and X-ray photons are unlikely heating sources

Cosmic rays or turbulence likely heat the gas

Abstract

We report the detection of 13CO(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 13CO(J=6-5) transition, which traces warm, dense molecular gas. We employ a multi-line LVG analysis and find ~ 35% - 60% of the molecular ISM is both warm (T ~ 110 K) and dense (n(H2) ~ 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.