Excitation of the molecular gas in the nuclear region of M82

TL;DR

This study uses high-resolution spectroscopy to analyze molecular gas in M82's nucleus, revealing multiple gas components with varying densities, temperatures, and radiation fields, emphasizing the importance of multi-component models.

Contribution

It provides a detailed multi-component model of molecular gas in M82's nucleus based on high-resolution spectral data, advancing understanding of starburst galaxy environments.

Findings

Most molecular gas is in low-density, low-UV field clouds.

A small fraction of gas is in high-density, high-UV field clouds.

High-J CO lines originate from a very dense, highly irradiated component.

Abstract

We present high resolution HIFI spectroscopy of the nucleus of the archetypical starburst galaxy M82. Six 12CO lines, 2 13CO lines and 4 fine-structure lines are detected. Besides showing the effects of the overall velocity structure of the nuclear region, the line profiles also indicate the presence of multiple components with different optical depths, temperatures and densities in the observing beam. The data have been interpreted using a grid of PDR models. It is found that the majority of the molecular gas is in low density (n=10^3.5 cm^-3) clouds, with column densities of N_H=10^21.5 cm^-2 and a relatively low UV radiation field (GO = 10^2). The remaining gas is predominantly found in clouds with higher densities (n=10^5 cm^-3) and radiation fields (GO = 10^2.75), but somewhat lower column densities (N_H=10^21.2 cm^-2). The highest J CO lines are dominated by a small (1% relative surface filling) component, with an even higher density (n=10^6 cm^-3) and UV field (GO = 10^3.25). These results show the strength of multi-component modeling for the interpretation of the integrated properties of galaxies.