The work during my PhD condensed in this first first-author paper, in which I studied the distribution of molecular hydrogen in the Local Group galaxy M33 by developing novel methods to generate high-resolution (18.2″, ∼75 pc) hydrogen column density maps from Herschel far‑infrared data. Using continuum observations between 160 μm and 500 μm, I first derived total hydrogen column densities via pixel‑by‑pixel modified‑blackbody SED fits that incorporate spatially variable emissivity index β and dust absorption coefficient \(\kappa_0\). In parallel, I devised a second, more direct approach that translates only the 250 μm map into NH at the same angular resolution using those same variable dust parameters.
Both methods yielded consistent total column density maps which, after subtracting a VLA H I component, produced H₂ column density maps tracing the molecular gas distribution across M33. I found that the SED‑based method recovers more granular structure in the inter‑arm and outer disk regions, whereas the single‑band approach delivers smoother, more extended emission.
By dividing the IRAM CO(2–1) integrated intensity map by these dust‑derived H₂ column densities, I produced a spatially resolved CO‑to‑H₂ conversion factor (\(X_\mathrm{CO}\)) map, revealing strong local variations around a disk‑average of 1.8×10²⁰ cm⁻² (K km s⁻¹)⁻¹. I extracted column‑density probability distribution functions from the NH, NH₂, and H I maps and found predominantly log‑normal shapes in the diffuse ISM with emerging power‑law tails at high column densities, indicative of self‑gravitating gas. The variable‑β, variable‑κ₀ framework I developed allowed us to build an intrinsic dust‑to‑gas ratio map that captures metallicity‑driven trends without imposing fixed values. I assessed biases in both methods and compared our results to existing literature maps, demonstrating improved depth and resolution. The resulting high‑resolution products afford a refined view of giant molecular cloud complexes and their relation to the spiral‑arm structure in M33. We conclude that these new datasets provide a robust foundation for future studies of cloud matching, star formation, and ISM structure in nearby galaxies, and we publicly release all column density maps for community use.
Read the full A&A article here.