Are GMCs real? Searching for a virialized scale in NGC 253

Giant Molecular Clouds (GMCs) are widely described as the key structures of early star formation. However, the hierarchical and complex nature of the interstellar medium (ISM) means that cloud-like objects can be seen above and below the canonical GMC scales of 50-100 pc, raising the question of whether GMCs are physically motivated, well-defined entities. In this work, I will describe a search for the characteristic size scale of gravitationally bound objects. I will present the ALMA Zoom field, which maps 12CO(2-1) at 0.4'' ~= 7 pc spatial resolution over 1.4 kpc^2 in the disk of NGC 253. Using dendrograms, we have conducted a scale-free, hierarchical decomposition of these data to identify 2463 candidate structures with sizes ranging from R~2 to 500 pc. We estimate the bound state of each structure, i.e. whether we expect it to be gravitationally coherent or not, using the virial parameter. By comparing the virial parameter against physical properties including the size, mass, surface density, and hierarchical position of structures, we have carried out a comprehensive search for a preferred scale at which bound structures emerge. Ultimately, we do not find any emergent scale for cloud-like structures in our data. These findings suggest that gravitational binding cannot be used to define GMCs and emphasize the need for structure-agnostic approaches to characterize the ISM.