PHANGS-ALMA GMC clustering

When analysing the 2PCFs, we generated several control samples that reflect different null hypotheses on large spatial scales. We stacked integrated intensity CO emission profiles around the position of catalogued GMCs to probe the gas distribution on scales between the observational resolution and the typical GMC-GMC spacing. Our measurements of cloud spacing and the number of neighbours show that GMC clustering follows the large-scale gas distribution.

Once we accounted for this contribution, the peak excess clustering relative to the null hypothesis in the 2PCF dropped from 1+{imega}~2.3 to 1.3, with the power-law slope flattened from -0.25 to 0. Stacks of CO intensity around local maxima show a strong clustering signal on scales smaller than the typical GMC-GMC separation. We show that this is largely the same signal captured by the "GMC size" measured by CPROPS, with an additional ~20% of the flux in an extended component beyond 500 pc.

We find that our stacked profiles can be fit with a double Gaussian function plus a constant offset. The broad Gaussian component accounts for 70% of the over-density power above the constant background and is stronger around massive and gravitationally bound GMCs. Our measurements yield a general statistical description of the structure of CO emission from ~150pc to galactic scales that can serve as a benchmark for simulations of molecular cloud formation and destruction in galaxy disks.

Our results indicate that galactic structure exerts a strong influence on the GMC distribution in galaxy disks and that the formation of massive gravitationally bound GMCs is related to strong local gas clustering.