Gas and dust cooling along the major axis of M33 (HerM33es): ISO/LWS CII observations

TL;DR

This study investigates gas cooling via [CII] emission in M33, revealing how photoelectric heating efficiency varies across the galaxy and the contribution of different interstellar medium phases.

Contribution

It provides the first detailed analysis of [CII] cooling and heating efficiency variation along M33's major axis using multi-wavelength data and PDR models.

Findings

Heating efficiency remains constant at 0.8% within 4.5 kpc of the galaxy center.

Efficiency increases to about 3% at 6 kpc, linked to lower densities and FUV fields.

The CNM contributes approximately 15% to [CII] emission in the inner 2 kpc, rising to 40%+-20% further out.

Abstract

We aim to better understand the heating of the gas by observing the prominent gas cooling line [CII] at 158um in the low-metallicity environment of the Local Group spiral galaxy M33 at scales of 280pc. In particular, we aim at describing the variation of the photoelectric heating efficiency with galactic environment. In this unbiased study, we used ISO/LWS [CII] observations along the major axis of M33, in combination with Herschel PACS and SPIRE continuum maps, IRAM 30m CO 2-1 and VLA HI data to study the variation of velocity integrated intensities. The ratio of [CII] emission over the far-infrared continuum is used as a proxy for the heating efficiency, and models of photon-dominated regions are used to study the local physical densities, FUV radiation fields, and average column densities of the molecular clouds. The heating efficiency stays constant at 0.8% in the inner 4.5kpc radius of the galaxy where it starts to increase to reach values of ~3% in the outskirts at about 6kpc radial distance. The rise of efficiency is explained in the framework of PDR models by lowered volume densities and FUV fields, for optical extinctions of only a few magnitudes at constant metallicity. In view of the significant fraction of HI emission stemming from PDRs, and for typical pressures found in the Galactic cold neutral medium (CNM) traced by HI emission, the CNM contributes ~15% to the observed [CII] emission in the inner 2kpc radius of M33. The CNM contribution remains largely undetermined in the south, while positions between 2 and 7.3kpc radial distance in the north of M33 show a contribution of ~40%+-20%.