March 22, 2010
Hubble Treasury Project To Survey Cosmic Time, Dark Energy
The Hubble Space Telescope will devote an unprecedented amount of time over the next few years to documenting galaxy evolution in the early universe and to studying whether distant supernovae can be extremely reliable measures of distance across vast regions of the cosmos.
In this ambitious new project, Hubble will image more than 250,000 distant galaxies in five directions, providing the first comprehensive view of the structure and assembly of galaxies over the first third of cosmic time. It will also yield crucial data on the earliest stages in the formation of supermassive black holes and find distant exploding stars called Type Ia supernovae, which are the cosmic yardstick for studying dark energy and the accelerating expansion of the universe.
"A better understanding of Type Ia supernovae as distance indicators is critical for the definition of the next generation of dark-energy missions," said Alex Filippenko, a professor of astronomy at the University of California, Berkeley, and one of several dozen scientists on the team. "Future, highly precise measurements of dark energy utilizing thousands of Type Ia supernovae as distance indicators require the resolution of uncertainties which lie at the foundation of supernova cosmology: their progenitor systems and their related possible evolution."
Project leader Sandra Faber of UC Santa Cruz said the effort relies on Hubble's powerful new infrared camera, the Wide Field Camera 3 (WFC3), as well as the telescope's Advanced Camera for Surveys (ACS). The proposal, which brings together a large international team of collaborators, was awarded a record 902 orbits of observing time as one of three large-scale projects chosen for the Hubble Multi-Cycle Treasury Program. The observing time, totaling about three and a half months, will be spread out over the next three years.
"This is an effort to make the best use of Hubble while it is at the apex of its capabilities, providing major legacy data sets for the ages," said Faber, a University Professor and chair of astronomy and astrophysics at UC Santa Cruz.
Twelve years ago, astronomers and physicists using Type Ia supernovae to measure the nearby universe found that the expansion of the universe was accelerating, propelled by some unknown force, dubbed "dark energy."
"When we made this discovery, our jaws dropped in astonishment," said Filippenko. "It has revolutionized our view of the universe."
Subsequently, independent studies not using supernovae confirmed the result.
But to make further progress and constrain the properties of dark energy, more precise measurements of Type Ia supernovae need to be made, he said. The precision is currently limited by uncertainties about the origin and explosion mechanism of Type Ia supernovae, which are thought to be exploding white dwarfs "“ burnt-out, sun-like stars at the end of their lives. While astronomers are able to determine the peak power of a supernova reasonably well, these uncertainties affect the conclusions.
The new Hubble project will look for Type Ia supernovae out past 9 billion light years, when the universe was less than one-third its current age and dark energy had little effect. Such a survey will determine whether Type Ia supernovae change over time, while a count of Type Ia supernovae will help refine theories of how they explode and how bright they become.
"The extremely distant supernovae will allow us to refine their use as custom yardsticks, providing greater precision that will allow us to better determine the properties of dark energy," Filippenko said.
With the newly found Type Ia supernovae, the team also will be able to further test various theories of dark energy, including Einstein's own early idea, that a "cosmological constant" would cause an accelerated expansion.
"If we find that the nature of dark energy is not changing with time," noted Filippenko, "then the evidence for Einstein's cosmological constant will be even greater."
The study focuses on several patches of sky where deep observations with other instruments are providing data in multiple wavelengths of light, including x-ray data from NASA's Chandra X-ray Observatory. X-ray emissions reveal the presence of a supermassive black hole at the core of a galaxy powering an "active galactic nucleus." Understanding the role of black holes in the evolution of galaxies is an important aspect of this project, Faber said.
"We don't know if the black holes form later or are a central feature of these galaxies from the very beginning. We hope to observe the earliest stages of black hole growth," she said.
The Cosmology Survey Multi-Cycle Treasury Program team expects the first data from its observations to be available by the end of the year. Data from this project will be made available to the entire astronomy community with no proprietary period for Faber's team to conduct its own analysis. The likely result will be a race among teams of scientists to publish the first results from this new treasure trove of data. But Faber said the project will yield such rich data that it will keep astronomers busy for years to come.
"We're very excited, not only about the 900 orbits, but also about what this new camera can do. It's just amazing what it sees," Faber said. "This project is the biggest event in my career, the culmination of three decades of work using big telescopes to study galaxy evolution."
"The new instruments on the refurbished Hubble Space Telescope have been producing stunning results," said Filippenko. "The images I've seen so far are truly breathtaking."
The committee that reviewed proposals for the Hubble Multi-Cycle Treasury Program asked Faber to combine her initial proposal with a similar one led by Henry Ferguson, an astronomer at the Space Telescope Science Institute (STScI), which operates the Hubble telescope. Faber and Ferguson will work together to manage the project, which involves more than 100 investigators from dozens of institutions around the world.
By Robert Sanders, UC Berkeley Media Relations, and Tim Stephens, UC Santa Cruz
Image 1: Supernova 1997ff, imaged here by the Hubble Space Telescope's Wide Field and Planetary Camera 2, bolstered the case for the existence of dark energy. The Hubble's new replacement camera will embark on a major project to find new and more distant supernovae to more precisely test theories of dark energy. This panel of images shows the supernova's cosmic neighborhood; its home galaxy; and the dying star itself. (NASA, Adam Ries, Space Telescope Science Institute, Baltimore, MD)
Image 2: Three of the most distant supernovae known were discovered using the Hubble Space Telescope as a supernova search engine. The stars exploded back when the universe was approximately half its current age. Hubble's new survey will explore the universe when it was only one-third its current age. (NASA and A. Riess, STScI)
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