Probing the molecular interstellar medium of M82 with Herschel-SPIRE spectroscopy

TL;DR

This study uses Herschel SPIRE spectroscopy to analyze M82's molecular gas, revealing a warm, turbulent component at about 500 K, and demonstrates the instrument's effectiveness for ISM studies in galaxies.

Contribution

First detection of high-J CO lines in M82's central region, providing detailed constraints on warm molecular gas properties and heating mechanisms.

Findings

Warm molecular gas at ~500 K identified

Turbulent dissipation likely heats the gas

SPIRE FTS proves effective for galaxy ISM studies

Abstract

We present the observations of the starburst galaxy M82 taken with the Herschel SPIRE Fourier Transform Spectrometer. The spectrum (194-671 {\mu}m) shows a prominent CO rotational ladder from J = 4-3 to 13-12 emitted by the central region of M82. The fundamental properties of the gas are well constrained by the high J lines observed for the first time. Radiative transfer modeling of these high-S/N 12CO and 13CO lines strongly indicates a very warm molecular gas component at ~500 K and pressure of ~3x10^6 K cm^-3, in good agreement with the H_2 rotational lines measurements from Spitzer and ISO. We suggest that this warm gas is heated by dissipation of turbulence in the interstellar medium (ISM) rather than X-rays or UV flux from the straburst. This paper illustrates the promise of the SPIRE FTS for the study of the ISM of nearby galaxies.