Molecular cloud matching in CO and dust in M33 II. Physical properties of giant molecular clouds

TL;DR

This study compares the physical properties of giant molecular clouds in M33 and the Milky Way, revealing similarities in size but differences in mass and density, and highlighting the influence of galactic environment on GMC characteristics.

Contribution

It introduces a novel analysis of GMCs in M33 using dust and CO data with a dendrogram algorithm, providing new insights into their physical properties and environmental dependence.

Findings

M33 GMCs are similar in size to Milky Way GMCs.

M33 contains less massive, lower-density GMCs than the Milky Way.

GMC properties show weak dependence on galactic environment.

Abstract

Understanding mass, size, and surface mass density of giant molecular clouds (GMCs) in galaxies is key to insights into star formation processes. We analyze these in M33 using Herschel dust and archival IRAM 30m telescope data, compared to Milky Way CO data. A Dendrogram algorithm on a 2D dust map and a Xco factor map are used for M33 instead of a constant value. Dust and CO-derived values are similar, with mean radii of $\sim\,$$58\,$pc for the dust and $\sim\,$$68\,$pc for CO. Largest GMAs are about $150\,$pc in radius, similar to the Milky Way, suggesting a size-limiting process. M33 contains less massive, lower-density GMCs compared to the Milky Way. The highest mass GMCs observed in the Milky Way are mostly absent in M33. M33's mean surface mass density is much lower, due to the Milky Way's higher column densities despite similar GMC areas. No systematic gradients in M33's physical properties were found with galactocentric radius, but higher densities and masses near the center suggest increased star formation. In both galaxies, 30% of molecular mass is central. The GMC mass power-law spectrum index is $\alpha=2.3\pm0.1$ and $\alpha=1.9\pm0.1$ for dust and CO in M33, respectively. We conclude that M33 and Milky Way GMCs are mostly similar, though M33 lacks high-mass GMCs, with no clear explanation. GMC properties weakly depend on galactic environment, with stellar feedback as a factor needing further study.