The critical constituents of galaxies—dark matter, stars, gas, dust, and super-massive black holes—are strongly coupled to one another. The program recommended here will allow major progress in our understanding of this cosmic order. Large multiobject spectroscopic surveys with new instruments would measure the stellar populations and the internal motions of thousands of distant galaxies in a single observation. High-angular-resolution optical and near-infrared integral-field-unit spectrographs on intermediate-class and large-aperture ground-based telescopes would trace in detail the internal velocity fields of galaxies. Meanwhile, while JWST will provide observations on the assembly of galaxies over cosmic time, IXO would obtain X-ray observations of the warm and hot gas in the dark matter halos that surround galaxies.
High-mass stars embedded in dense gas within galaxies can be inventoried with CCAT and studied in detail with ALMA. These stars are thought to be the main agents for injecting mass and energy into the interstellar medium and for driving galactic outflows. They do this through powerful stellar winds and supernova explosions, both of which are also responsible for accelerating cosmic rays and amplifying magnetic fields. The proposed ACTA facility will advance understanding of the mechanisms involved. The cycling of gas from galaxies to the surrounding intergalactic medium and back again could also be studied with a GSMT telescope, using high-resolution optical spectra to study gas absorption lines highlighted by background quasars along many sight-lines, but a future UV space mission will be needed for a complete inventory. This program of observations will move the subject of galaxy evolution from one dominated largely by surveys to one of integrated measurements of the buildup of dark matter, gas, stars, metals, and structure over cosmic time. These observations will lay the foundation for the ultimate aim of a complete ab initio theory of galaxy formation and evolution.
Understanding of the structure and evolution of stars is the foundation on which the knowledge of galaxies and the rest of the universe is built. ATST will provide tools for the study of solar (and hence stellar) rotation and magnetic fields. The time-domain information obtained from LSST would provide an unprecedented view of magnetic activity in other stars. LSST would also yield a large sample of Type Ia supernovae that could be followed up immediately by a GSMT in order to identify the progenitor stars and better understand the physical processes involved in their explosions. Likewise LSST would detect many Type II supernovae and find new types of rare or faint outcomes of massive-star evolution that have never been seen before. Key properties of compact stellar remnants such as neutron stars will be measured in new radio pulsar surveys that are less biased against detecting the fastest-rotating pulsars.