Spectroscopy of High-Redshift Supernovae from the ESSENCE Project: The First 2 Years

We present the results of spectroscopic observations of targets discovered during the first 2 years of the ESSENCE project. The goal of ESSENCE is to use a sample of similar to 200 Type Ia supernovae (SNe Ia) at moderate redshifts (0.2 less than or similar to z less than or similar to 0.8) to place constraints on the equation of state of the universe. Spectroscopy not only provides the redshifts of the objects but also confirms that some of the discoveries are indeed SNe Ia. This confirmation is critical to the project, as techniques developed to determine luminosity distances to SNe Ia depend on the knowledge that the objects at high redshift have the same properties as the ones at low redshift. We describe the methods of target selection and prioritization, the telescopes and detectors, and the software used to identify objects. The redshifts deduced from spectral matching of high-redshift SNe Ia with low-redshift SNe Ia are consistent with those determined from host-galaxy spectra. We show that the high-redshift SNe Ia match well with low-redshift templates. We include all spectra obtained by the ESSENCE project, including 52 SNe Ia, five core-collapse SNe, 12 active galactic nuclei, 19 galaxies, four possibly variable stars, and 16 objects with uncertain identifications.