The volume densities of GMCs in M81

TL;DR

This study estimates the volume densities of GMCs in M81 using photodissociation models, revealing densities similar to the Milky Way and explaining the low CO emission observed.

Contribution

It introduces a method to determine GMC densities in M81 from HI features and UV fluxes, providing new insights into GMC properties in this galaxy.

Findings

GMC volume densities range from 1 to 200 cm^-3 with a mean of 17 cm^-3.

GMC densities in M81 are comparable to those in the Milky Way.

Low molecular densities explain the weak CO emission in M81.

Abstract

HI features near young star clusters in M81 are identified as the photodissociated surfaces of Giant Molecular Clouds (GMCs) from which the young stars have recently formed. The HI column densities of these features show a weak trend, from undetectable values inside R = 3.7 kpc and increasing rapidly to values around 3 x 10^21 cm^-2 near R ~ 7.5 kpc. This trend is similar to that of the radially-averaged HI distribution in this galaxy, and implies a constant area covering factor of ~ 0.21 for GMCs throughout M81. The incident UV fluxes G0 of our sample of candidate PDRs decrease radially. A simple equilibrium model of the photodissociation-reformation process connects the observed values of the incident UV flux, the HI column density, and the relative dust content, permitting an independent estimate to be made of the total gas density in the GMC. Within the GMC this gas will be predominantly molecular hydrogen. Volume densities of 1 < n < 200 cm^-3 are derived, with a geometric mean of 17 cm^-3. These values are similar to the densities of GMCs in the Galaxy, but somewhat lower than those found earlier for M101 with similar methods. Low values of molecular density in the GMCs of M81 will result in low levels of collisional excitation of the CO(1-0) transition, and are consistent with the very low surface brightness of CO(1-0) emission observed in the disk of M81.