In this study (again condensed during my PhD), I studied the physical properties of giant molecular clouds (GMCs) in the flocculent spiral galaxy M33 by applying the Dendrogram algorithm to both a novel 2D dust‑derived H₂ column density map at 18.2″ resolution that I have generated as described in the previous paper and archival IRAM 30 m ¹²CO(2–1) data, using a pixel‑by‑pixel \(X_\mathrm{CO}\) conversion factor instead of a constant value. I identified over 300 dust‑traced and nearly 200 CO‑traced GMC structures, measured their projected areas and deconvolved radii (mean ∼58 pc from dust, ∼68 pc from CO), and computed masses by summing H₂ column densities over each structure.
I found that the most massive associations reach ∼8×10⁶ M⊙ in dust and ∼5×10⁶ M⊙ in CO, but M33 lacks the very high‑mass (\(>10^6\,\mathrm{M_\odot}\)) GMC population seen in the Milky Way.
Mean surface mass densities are \(22\,\mathrm{M_\odot\,pc^{-2}}\) (dust) and \(16\,\mathrm{M_\odot\,pc^{-2}}\) (CO), roughly an order of magnitude lower than typical Milky Way values, while beam‑averaged number densities remain low (∼5 cm⁻³ and ∼3 cm⁻³), reflecting unresolved substructure. Comparing to Milky Way catalogs, we find a similar upper limit of ∼150 pc on GMC/association size, suggesting a common regulating mechanism—perhaps disk scale height or stellar feedback. We observe no strong gradients in GMC properties with galactocentric radius, although central clouds are modestly denser and more massive, containing ∼30% of M33’s molecular mass. The GMC mass spectrum across the disk follows a power law of index α ≈ 2.3 (dust) and α ≈ 1.9 (CO), consistent with self‑similar structure and hierarchical fragmentation. Our results indicate that, while GMCs in M33 are broadly similar to those in the Milky Way in terms of size distribution and mass spectrum, M33’s lower overall metallicity and galaxy mass lead to systematically lower masses and densities and a deficit of the most massive clouds. These findings imply that basic GMC scaling relations hold across environments, but the upper end of the cloud mass function is sensitive to global galaxy properties.
Read the full A&A article here.