Science Spotlight: Constraining the Cosmic Evolution of Type IA Supernovae

Stranger than Fiction?

In one of the most surprising developments of the past decade, astronomers discovered that the expansion of the Universe appears to be accelerating rather than decelerating. Perhaps stranger yet, explaining this result led to the proposed existence of a new form of energy—a mysterious “dark energy”—that pervades all of space. At the heart of these developments are observations of type Ia supernovae. They provide most direct evidence for dark energy and the accelerating expansion of the under the assumption that type Ia’s are unevolving standard candles (apart from corrections based on light curve shape).

Results from the NOAO Survey Project ESSENCE

A recent paper by Foley et al. (2007; astro-ph/0710.2338) examines this assumption, by studying the extent to which type Ia’s evolve over cosmological redshifts. In the first large-scale effort of its kind, composite spectra of high-redshift type Ia supernovae were compared with the spectra of low-redshift counterparts. The study made use of the ~100 spectra of high-redshift type Ia supernovae obtained as part of the NOAO survey project ESSENCE. These were compared with a compilation of ~400 spectra of low-redshift supernovae obtained at the Lick and Keck Observatories and with HST and IUE.

Because the spectra of individual high-redshift supernovae have low signal-to-noise ratios, Foley et al. combined the spectra to create higher signal-to-noise composites. The composite spectra of high-redshift supernovae are very similar to the spectra of low-redshift type Ia’s. Once the ESSENCE spectra have been corrected for the likely contribution from the light of the host galaxy, the low- and high-redshift composites of spectra obtained at maximum light differ by less than ~10% at rest-frame optical wavelengths. Foley et al. conclude that type Ia supernovae show little spectral evolution, evolving spectrally by less than 10% at redshifts < 0.8.

Relation to the NOAO System

The ESSENCE project spectra that were used in the study were obtained with telescopes spanning a wide range in aperture: the Keck I and II 10-m telescopes; the VLT, Gemini North and South 8-m telescopes; the Magellan and MMT 6.5-m telescopes; and the Tillinghast 1.5m telescope. Providing access to a similarly broad range of telescope apertures is a goal of the NOAO System of facilities and resources. To learn more about the governing principles and implementation of the NOAO System, see the NOAO Cooperative Agreement proposal, now posted on the NOAO home page.