Star formation activity in the universe peaked 2 to 6 billion years after the Big Bang, corresponding to redshifts between 1 and 3. Most of the radiant energy from these stars was absorbed by dust in their nascent molecular clouds, re-radiated in the far infrared part of the spectrum, and can be observed today redshifted to the submillimeter. To measure the intrinsic luminosities and star formation rates of these galaxies, one must therefore observe in the submillimeter bands. Moreover, the star forming interstellar medium (ISM) is cooled by atomic spectral line radiation observable at these wavelengths; hence submillimeter spectroscopy is critical for understanding the star formation process. This project makes use of the scientific capabilities of the ZEUS and ZEUS-2 spectrographs, developed with prior funding from the Advanced Technologies & Instrumentation (ATI) and Major Research Instrumentation (MRI) programs, to observe specific atomic fine-structure lines of carbon, nitrogen, and oxygen in high-redshift galaxies. ZEUS-2 will be used on the Caltech Submillimeter Observatory (CSO) and the Atacama Pathfinder Experiment (APEX), together with other space- and ground-based follow-up observations in the millimeter, submillimeter, and far infrared in order to: (1) extend an ongoing survey of [CII] emission lines to deeper redshift and lower luminosity, tracing the change is star formation mode over time; (2) observe [OI] and [NII] lines in the same redshift range, to trace the changes in stellar populations; (3) observe gravitationally lensed galaxies comparable to the Milky Way in luminosity at redshifts around 2; and (4) study the connection between starbursts and active galactic nuclei. These observations will represent a key stepping stone linking discovery with the Herschel Space Telescope to future spatially resolved investigations with the Atacama Large Millimeter/Submillimeter Array (ALMA). ZEUS-2 also lays the ground-work for instrument development and science opportunities that will be enabled by a future spectrometer on the proposed 25-meter Cerro Chajnantor Atacama Telescope (CCAT). The project will support the active involvement of multiple graduate and undergraduate students, many from under-represented groups, in instrument development and research.
