SMA Observations of the Extended CO(6-5) Emission in the Starburst Galaxy NGC253

TL;DR

This study uses SMA observations to analyze the CO(6-5) emission and dust properties in NGC253, revealing detailed molecular gas conditions, heating mechanisms, and dust characteristics in the starburst galaxy.

Contribution

First detailed SMA observations of CO(6-5) emission in NGC253, providing insights into molecular gas temperatures, heating processes, and dust components in a starburst galaxy.

Findings

CO(6-5) emission follows lower transition distributions with little ratio variation.

Molecular gas modeled as two-temperature components influenced by shocks and PDRs.

Detected dust temperatures range from 10-30K with evidence of hotter, optically thick dust.

Abstract

We present observations of the $^{12}$CO(6-5) line and 686GHz continuum emission in NGC253 with the Submillimeter Array at an angular resolution of ~4arcsec. The $^{12}$CO(6-5) emission is clearly detected along the disk and follows the distribution of the lower $^{12}$CO line transitions with little variations of the line ratios in it. A large-velocity gradient analysis suggests a two-temperature model of the molecular gas in the disk, likely dominated by a combination of low-velocity shocks and the disk wide PDRs. Only marginal $^{12}$CO(6-5) emission is detected in the vicinity of the expanding shells at the eastern and western edges of the disk. While the eastern shell contains gas even warmer (T$_{\rm kin}$>300~K) than the hot gas component (T$_{\rm kin}$=300K) of the disk, the western shell is surrounded by gas much cooler (T$_{\rm kin}$=60K) than the eastern shell but somewhat hotter than the cold gas component of the disk (for similar H$_2$ and CO column densities), indicative of different (or differently efficient) heating mechansisms. The continuum emission at 686GHz in the disk agrees well in shape and size with that at lower (sub-)millimeter frequencies, exhibiting a spectral index consistent with thermal dust emission. We find dust temperatures of ~10-30K and largely optically thin emission. However, our fits suggest a second (more optically thick) dust component at higher temperatures (T$_{\rm d}$>60K), similar to the molecular gas. We estimate a global dust mass of ~10$^6$Msun for the disk translating into a gas-to-dust mass ratio of a few hundred consistent with other nearby active galaxies.