The Physical Nature of the Giant Molecular Cloud Population in NGC 300

Abstract

Giant Molecular Clouds (GMCs) are the cradles of stellar birth across the Universe, and thus understanding their properties and evolution is central to addressing questions in astrophysics at a wide range of scales from planet formation to galaxy evolution. Long-held theories of universality in GMC structure based on the relatively uniform properties and scaling relations observed in nearby Milky Way clouds have recently been challenged by extragalactic observations showing hints of environmentally-driven variations in these properties and relations. However, these extragalactic studies have limited sensitivity and resolution, and thus it is unclear if the structures they identify are truly analogous to Milky Way GMCs. To address the question of universality vs. environmental dependence, I have undertaken a series of high-resolution, high-sensitivity studies of molecular gas and star formation in NGC~300, a low mass, star forming spiral galaxy that at a distance of about 2 Mpc provides an ideal laboratory for resolved GMC studies and comparison to our own Galaxy. In this dissertation, I will describe the results of a campaign of multiwavelength observations of NGC 300 culminating in an ALMA study that achieves 10 pc and 1 km/s resolution, fully resolving GMC scales and detecting 250 clouds. These studies indicate that despite large differences between global properties of the Milky Way and NGC 300, the GMC populations in the disks of these galaxies appear to be remarkably similar. The data resolve the GMCs sufficiently to measure robust velocity gradients and see signatures of rotation and measure their angular momenta. If the velocity gradients are due to rotation, my results suggest that GMCs form from a combination of top-down accumulation and turbulent fragmentation. Furthermore, the relationship between star formation and molecular gas identified in the Milky Way also holds over 250 pc spatial scales in NGC 300, and the molecular gas depletion time is similar between these two galaxies at these scales. Local physical properties such as midplane pressure of the interstellar medium, which is similar between the Milky Way and NGC 300, may explain observed differences in GMC properties in other galaxies such as M51, as well as in more extreme physical environments such as the Galactic Center. If this scenario holds, it suggests a modification to the idea of universal cloud structure in which parameters such as GMC surface density and structure function coefficient are constant only in the disks of main sequence spiral galaxies having similar pressures. I have also conducted an experiment into the effects of varying parameters in GMC identification algorithms through simulated interferometric observations of Milky Way clouds at extragalactic distances, which suggests that specific, physically-motivated parameter choices may be necessary to accurately identify GMCs and estimate their cloud properties in extragalactic CO observations.