Schematic showing our model of giant molecular cloud (GMC) evolution. Dense gas cores within the clouds collapse to form hot, young stars, which ionize their surrounding medium, disrupt the gas around them, and eventually leave exposed Hii regions with no associated GMC. On the right, I illustrate the "matching fraction" method we developed to associate GMCs and their Hii region neighbors (or children).
ALMA 12CO(2−1) (blue) and VLT/MUSE Hα (orange) maps of 8 nearby, star-forming, main sequence galaxies from the PHANGS survey. Molecular gas is strongly associated with Hα emission, but the emission peaks are offset. We used catalogs of giant molecular clouds (GMCs, derived from CO) and star-forming Hii regions (derived from Hα) to associate spatially correlated objects, as shown in the bottom right.
Having matched GMCs with associated Hii regions, we explored their complementary properties. Here, I show the star formation rate (SFR) as a function of GMC mass for each galaxy. We estimated the median depletion time (gas mass / SFR) for each galaxy, finding significant scatter that reflects the range of local environmental conditions.
GMC and Hii region catalogs
- Galaxy sample: 8 nearby, star-forming, main sequence spirals
- Angular resolution: 1''
- Number of GMCs: 3324
- Number of Hii regions: 9031
How is gas associated with star formation?
Clusters of massive stars form in the densest parts of giant molecular clouds (GMCs), ionizing the gas around them to create Hii regions. To understand their co-evolution, we need to relate the progenitor GMCs to their associated Hii regions. I explored several approaches to match GMCs and Hii regions, eventually recommending a "fractional overlap" approach. We used this to describe the impact of Hii regions on GMC properties in Zakardjian et al. (2023).