possible to achieve only in the galactic neighborhood. Not surprisingly, these constraints have been woven into the modern theoretical framework for galaxy formation and evolution.
To appreciate the impact of the galactic neighborhood, first consider studies of the universe on the largest scales. The interpretation of observations of the most distant galaxies is built on a foundation of knowledge established in the galactic neighborhood, including knowledge about the evolution of stellar populations, the existence of dark matter, the scaling relations of supermassive black holes, the effects of feedback from supernovae, the importance of accretion, the relationship between star formation and gas density, and the stellar initial mass function, among many others. Likewise, the evidence for dark energy from high-redshift supernovae was predicated on years of characterization of the properties of supernovae in nearby galaxies, along with more mundane constraints on the properties of dust extinction and exhaustive calibrations of the local distance scale.
The impact of the galactic neighborhood has been equally significant on smaller scales. The galaxies of the Local Group offer millions of observationally accessible stars, assembled into systems with a common distance and foreground extinction. The resulting samples of stars, their ancestors, and their descendants (e.g., planetary nebulae, supernova remnants, variable stars, transients, supernovae, molecular clouds, H II regions, X-ray binaries, etc.) can be analyzed with fewer uncertainties than in the Milky Way, where unknown distances and reddenings present challenging obstacles to assembling large samples. Moreover, such samples span a wide range of environment and metallicity, adding these new dimensions to the understanding of the physics of stellar evolution and the interstellar medium. The galactic neighborhood is also the only region where one can study the smallest scales of galaxy formation, revealing the presence of galaxies whose masses are scarcely more than a globular cluster. This fact is particularly important for assessing processes of feedback from star formation to the ISM, CGM, and IGM.
In assessing the scientific potential of the galactic neighborhood over the coming decade, the Panel on the Galactic Neighborhood faced a difficult task, given that the galactic neighborhood is the arena within which the interaction of nearly all astrophysical systems can be witnessed. Thus, narrowing down the scientific potential to only four key questions involved both the exclusion of research areas and unavoidable overlap with the scientific realms covered by other Science Frontiers Panels participating in the National Research Council’s (NRC’s) Astronomy and Astrophysics (Astro2010) Survey. This panel chose to focus its questions on areas in which the constraints from the galactic neighborhood are most powerful and unique. As a result, the four science questions developed by the panel exploit the use of the galactic neighborhood as a venue for studying interconnected astrophysical systems, for constraining complex physical processes, and for probing small scales. The key science questions are as follows: