The Search for Cosmic Supernova Explosions
Several members of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC)Masao Sako, Roger Romani, Chen Zheng, Roger Blandford and Steve Kahnhave recently joined the Sloan Digital Sky Survey II (SDSS-II) collaboration to participate in a massive search for supernova explosions from the distant universe. The survey uses the SDSS 2.5-meter telescope at Apache Point Observatory in New Mexico to obtain repeated multi-color images of the same 300 square-degree patch of the sky approximately every other night. A unique large-format CCD camera array with a total of 120 million pixels is capable of recording 200 gigabytes of imaging data in a single night of observing, allowing efficient discoveries of supernova explosions out to redshift z~0.4, or about one-third of the way back to the big bang.
The search consists of three campaigns each lasting from September 1 through November 30 of 2005, 2006 and 2007. During the first season, which just ended, the primary focus was to study type Ia supernovae, a class of supernovae that are associated with thermonuclear explosions of white dwarfs. The campaign was a remarkable success and a total of 130 supernova Ia in the redshift interval of 0.01 < z < 0.42 (called the "redshift desert", since other searches have not found many supernovae in this range) were spectroscopically confirmed during the 3-month period. This unique set of supernovae will help understand and quantify the systematics of type Ia supernovae as distance indicators, and will provide independent constraints on dark matter, dark energy and the expansion history of the universe. Interestingly, the redshift range of the SDSS-II supernovae search covers most of the period where recent work suggests that dark energy is the dominant constituent of the cosmos.
Stanford's partnership in the 9.2-meter Hobby-Eberly Telescope (HET) has allowed KIPAC members to follow-up and obtain optical spectra of ~50 high-redshift supernova Ia, which make up a large chunk of the "distant" sample most useful for cosmological studies. The KIPAC group has also targeted different types of supernovae including the peculiar Ib/c hypernova that are believed to be associated with cosmic gamma-ray bursts and other rare types of supernovae that are poorly sampled in other surveys.