Molecular clouds in the center of M81

TL;DR

This study examines the molecular gas properties in M81's center, revealing low CO emission, a low line ratio indicative of diffuse gas, and massive GMAs with atypical X factors, highlighting unique molecular conditions.

Contribution

It provides detailed observations of CO transitions in M81's center, identifying unusual molecular gas excitation and massive GMAs with high X factors, advancing understanding of molecular cloud physics in this galaxy.

Findings

No CO emission within 300 pc of the nucleus.

Average I_{21}/I_{10} ratio is 0.68, lower than typical galactic nuclei.

Massive GMAs with ~10^5 M_sun and ~250 pc diameter.

Abstract

We investigate the molecular gas content and the excitation and fragmentation properties in the central region of the spiral galaxy Messier 81 in both the ^{12}CO(1-0) and ^{12}CO(2-1) transitions. We have recently observed the two transitions of CO in the M~81 center with A, B, and HERA receivers of the IRAM 30-m telescope. We find no CO emission in the inner $\sim$ 300 pc and a weak molecular gas clump structure at a distance of around 460 pc from the nucleus. Observations of the first two CO transitions allowed us to compute the line ratio, and the average I_{21}/I_{10} ratio is 0.68 for the M~81 center. This low value, atypical both of the galactic nuclei of spiral galaxies and of interacting systems, is probably associated to diffuse gas with molecular hydrogen density that is not high enough to excite the CO molecules. After analyzing the clumping properties of the molecular gas in detail, we identify very massive giant molecular associations (GMAs) in CO(2-1) emission with masses of $\sim$ 10$^{5}$ M$_\odot$ and diameters of $\sim$ 250 pc. The deduced N(H_{2})/I_{CO} ratio for the individually resolved GMAs, assumed to be virialized, is a factor of $\sim$ 15 higher than the \textit{standard} Galactic value, showing - as suspected - that the X ratio departs significantly from the mean for galaxies with an unusual physics of the molecular gas.