Jeff Cooke

ARC Future Fellow - Centre for Astrophysics & Supercomputing
Swinburne University of Technology, PO Box 218, Mail number H30, Hawthorn, VIC 3122 Australia
office: +61 3 9214 5392 -- fax: +61 3 9214 8797 -- email:

AR 315

The mean ultraviolet spectrum of type Ia supernovae using the Hubble Space Telescope

Type Ia supernovae are believed to be the deaths of white dwarf stars which are the remnant cores of low-mass stars (like our Sun) after they have died. A type Ia supernova may occur either when a white dwarf star accretes matter from a very nearby giant star that is in the process of bloating and expanding its outer envelope of gas or when two white dwarf stars are in a close orbit and eventually merge together (see the figures to the right).

When transforming the light curves of type Ia supernovae using observed relationships, these events become excellent distance indicators and, because of their extreme brilliance, are effective out to great distances. Type Ia supernovae have been used to show that the expansion rate of the universe is accelerating, uncovering the existence of dark energy. This work earned type Ia supernova researchers Brian Schmidt, Saul Permutter, and Adam Riess the 2011 Nobel Prize in physics.

Very deep observations are necessary to explore the early universe and examine the conditions during the first half of cosmic time. To reach the faintest supernovae (and, hence the farthest), astronomers use sensitive optical detectors on large telescopes. Because of cosmological redshift, high redshift optical observations sample restframe ultraviolet (UV). Recent work has found a dispersion in the UV that could have a important impact on future surveys and the utility of type Ia to accurately probe the expansion rate of the early Universe.

Our team undertook a large project to measure the mean type Ia supernova UV spectrum and search for any dispersion. This project is quite difficult because the UV can only be observed from space. For this program, we used the STIS spectrograph on the Hubble Space Telescope. To catch the short-lived supernovae near their peak brightness and send off identified type Ia events to the Hubble Space Telescope to be observed 10-12 days later, we had to scour the skies and search hundreds of thousands of galaxies in hopes of finding a few faint, initial outbursts of supernovae as they were just beginning to brighten with time (7-16 days before peak brightness). In addition, we had to "take over" many of the world's largest telescopes the next night (or soon thereafter) to confirm their identifications. An illustration of the timeline of the program to achieve these results is shown in Figure 1 below.

Figure 1: Large areas of the sky containing hundreds of thousands of galaxies were monitored night after night by the Palomar Transient Factory in an effort to detect early type Ia supernovae. The blue squares show the days before peak brightness and the observed magnitude of the supernovae found by the Palomar Transient Factory. The gold triangles are the ages and magnitudes of the supernovae when we (rapidly) observed them using large telescopes all over the world to confirm their identity. Finally, the red diamonds indicate the ages and magnitudes of the supernovae when they were observed by the Hubble Space Telescope. The dashed curves indicate the distances of the supernovae in redshift (z) and their approximate brightness evolution with time.

This project could not have been done without the hard work and intelligent processing of an enormous amount of data by the Palomar Transient Factory Team. Remarkably, the team was able to detect type Ia supernovae as early as 15 days before they reached their maximum brightness (~3-4 days after explosion). After the supernovae were discovered, we utilzed telescopes such as the Palomar 5m, VLT 8m, William Herschel Telescope, Gemini 8m, and Keck 10m, to spectroscopically confirm and phase the events. The UV spectra taken by Hubble Space Telescope are shown in Figure 2 below.

Our team reported the mean near UV spectrum and dispersion of type Ia SNe in Cooke et al. (2011) which was constructed from the individual spectra shown below.

Figure 2: The spectra of 15 type Ia supernovae taken by the STIS spectrograph on the Hubble Space Telescope. The blue spectra were the supernovae caught quick enough to be combined and compared to spectra of supernovae at higher redshift. Each spectrum is squashed in the vertical direction here in order to fit them all on the plot.

Click here to access the ADS link displaying a list of articles describing this work and other research of mine.



White dwarf star in close orbit with a giant star. The white dwarf steals matter from the evolving giant star as it slowly expands.
Two white dwarf stars in a close orbit. These two stars spiral inward and merge.
(Image credits: NASA/CXC/M. Weiss)

The Hubble Space Telescope

The Milky Way can be seen, as well as two of our closest companion galaxies, the Large and Small Magellenic Clouds, in this long-exposure image of the 4 meter telescope at the CTIO located in the Southern Hemisphere (Chile).
  Astronomy 110
Physics 20A  
  Physics 7D
Curriculum Vitae
  Astro Grad Seminar
Centre for Astrophysics & Supercomputing
  Caltech Astronomy Department
Center for Cosmology
UC Irvine
  Center for Astrophysics and Space Sciences
UC San Diego
W. M. Keck Observatory  
  Palomar Observatory