Comparison of Cloud Identification Methods: Implications for Observations and Theory

Giant Molecular Clouds (GMCs) are the largest gas structures of galaxies and are composed of mainly cold gas. They are the primary sites of star formation, so their properties have strong implications for the star formation process. Since these clouds are dense and cold, their gas content can only be inferred indirectly from the emission of tracer molecules such as CO. However, these emission maps carry only 3D information (x,y,v_z) of the clouds making it non-trivial to identify individual clouds or infer cloud properties. On the other hand, numerical simulations and theoretical models rely on fundamentally different GMC definitions, thus properties of these clouds are possibly biased by the identification method of choice, which can lead to false results and confusion. In this project, we utilize a simulated Milky Way (MW) like galaxy to compare different definitions of GMCs on the same data set: those used by observers (based on CO emission maps, CO clouds) and theorists (based on gravitational boundedness, Bound clouds).

In our analysis, we visually compare Bound Clouds to CO Clouds and find that there is no one-to-one mapping between them. CO clouds are more massive and are typically unbound, and sometimes contain Bound Clouds. Furthermore, the two types of clouds follow different mass-size and linewidth size relations.