Galaxies in the Present-Day Universe
Galaxies in the local universe are excellent laboratories to study the physical relationships between the components that make up galaxies - namely the stars, the dust and gas. Due to their proximity young star formation sites, clusters of older stars, dust heated by these young and old stars as well as the gas in all its different phases (ionized, atomic, molecular) can be well resolved and separated.
On the other hand astronomers have a privileged view onto these galaxies by not sitting in the middle of a disk as it is the case for our own Milky Way. Stars are born out of gas and dust and they return this material in a processed form to the interstellar medium that consists of neutral and ionized gas, large molecules such as poly-aromatic hydro-carbonates (PAHs), and dust grains.
The spatial distribution of the stars and dark matter determine the visible structure of galaxies - often showing amazing spiral arms, rings, or bars. Nearby galaxies cover a large range not only in physical conditions of the interstellar medium and the amount of new stars forming, but also in galactic environments: spiral arms, bars, disks, spheroids, active galactic nuclei (AGN), starburst. All physical models that are developed for the formation and evolution of, e.g., the formation of molecular gas and its clouds, the onset and evolution of star formation as well as the dynamical evolution of galactic structure itself must be able to reproduce and explain the observations. In particular galactic nuclei offer the opportunity to study their formation via observations that resolve their constituents.
Dedicated surveys of galaxy components across the full electro-magnetic spectrum utilizing ground- and space-based observatories are key to reveal the relationships between the gas, star and dust and to help develop physical models that can in turn be apply in the young universe where spatial resolution is no longer sufficient.
One of the latest surveys is PHANGS providing a comprehensive and detailed view of the constituents of the star formation process. It builds on the insights gained by THINGS looking at the atomic gas, HERACLES and PAWS imaging the molecular gas as well as S4G and KINGFISH providing information on the stellar populations as well as the ionized gas and dust. These observations are combined with state-of-the-art modeling to obtain insights into the radiation field (from stars and AGN), densities and other physical parameters of the galaxy components.