Two extra-Galactic surveys are considered. The first takes observations of nearly 200 GMCs across a small sample of local galaxies in the CANON CO survey. In spite of the local nature of the sample, results confirm essential facts about molecular gas distribution in galaxies other than our own, including a confirmed linear relationship between GMC virial mass and CO luminosity, which implies a constant CO-H2 conversion factor and supports a virialization assumption. The second survey, PHANGS-ALMA (Physics at High Angular Resolution in Nearby Galaxies with the Atacama Large Millimetre-submillimetre Array), maps CO emission from galaxies up to 17 Mpc away, with resolutions of 1″–1.5″ encompassing active star-forming galaxies down to total stellar masses ~5 × 109 M☉. Within 11 of those target galaxies considered here, the results offer tens of thousands of measurements at GMCs scales between 20 and 130 pc, comparable to Galactic-scale observations, and one outcome is confirmation of a positive correlation between GMC surface densities and velocity distributions.

The tendency for conservation of angular momentum of a gravitationally collapsing cloud to form a disk gives rise to the disk in our own Galaxy, the Milky Way. We explore the main components, including the disk, bulge, and halo. Studies of Galaxy rotation curves lead us to the existence of “dark matter,” the nature of which is unknown but is detectable through its gravitational interactions with normal, baryonic matter. We finish by exploring the supermassive black hole at the Milky Way’s center.