What is a GMC? Are observers and simulators discussing the same star-forming clouds?

Observations and simulations have now reached the point where the giant molecular cloud (GMCs) populations can be studied over a whole galaxy. This is immensely helpful for understanding star formation. Yet, are these two groups really comparing the same objects? While simulators work in 6D (x, y, z, v(x), v(y), v(z)) position- position- position (PPP) space, ob servers see 2 + 1D (RA, Dec, v(los)) projected properties along the line of sight, identifying clouds in position- position- velocity (PPV) space. In this research we generated PPP and PPV data for a high- resolution simulated galaxy and compared the identified clouds in both data sets. The results show that 70% of the clouds have a single counterpart in each data structure. Cloud boundaries of these clouds are indeed the same. Scatter of the derived cloud properties (radius and velocity dispersion) between PPP and PPV are typically within a factor of two. However, this small scatter can make it difficult to determine if a cloud is truly gravitationally bound.