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Risa Wechsler: Shedding Light on the Dark Side

RISA H. WECHSLER MAY HAVE ESTABLISHED A RECORD AMONG KAVLI-AFFILIATED SCIENTISTS. An assistant professor of physics at Stanford University and the SLAC National Accelerator Laboratory, Wechsler is a member of the Kavli Institute for Particle Physics and Cosmology (KIPAC). It's a position she accepted after serving as a member of the Kavli Institute for Cosmological Physics, University of Chicago. In the course of her career, she has also attended several workshops on galaxy formation, cosmology, and gravitational lensing at the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara, and twice participated in the Kavli Frontiers of Science program.

At the Kavli Institute for Particle Astrophysics and Cosmology, Risa Wechsler looks for visible clues to unlock the riddles of the unseen Universe.

Risa Wechsler, assistant professor of physics at Stanford University and the SLAC National Accelerator LaboratoryRisa Wechsler, assistant professor of physics at Stanford University and the SLAC National Accelerator Laboratory.

In short, her career path has led to three Kavli institutes and one program, giving her particularly extensive roots in the Kavli community.

But if unique, it shouldn't be surprising. As a cosmologist, Wechsler is interested in finding connections between the visible and the unseen. "I am mostly interested in how structure forms and how galaxies form, and how we can use that knowledge to understand the ‘dark side’ of the Universe … how what we see in telescopes is connected to the dark matter of the Universe.”

In other words, Wechsler is pursuing the kind of work that distinquish the Kavli institutes -- research that advances basic science and our fundemental understanding of existence. At KIPAC, this means cracking open a window for observing the unobservable.

The Big Questions

Wechsler's own interest in science dates back to high school, when she made a personal discovery -- she “wanted to ask the big questions, like what is the Universe made of, and how did it evolve.” About two decades later, her work in cosmology has gone far to illuminate the relationship of invisible dark matter to the formation of galaxies and is helping scientists close in on the even more mysterious dark energy.

As Wechsler explains, cosmologists now believe that most of the matter in the Universe gives off no light, heat or other electromagnetic radiation. It’s thus “dark” as well as cold. More recently – since the turn of the millennium – scientists have embraced the theory that most of the universe is not matter at all. Roughly three-quarters of it appears to consist of “dark energy,” which counteracts gravity and powers the Universe’s expansion. This means only about 4% of the Universe can be observed directly with human eyes and instruments (though the presence of dark matter can be deduced indirectly from its gravitational effects, such as the bending of light).

A slice from a catalog of model galaxies representing what may be observed by the upcoming Dark Energy Survey. A slice from a catalog of model galaxies representing what may be observed by the upcoming Dark Energy Survey. The figure shows the galaxy distribution over 1/8 of the sky. Because light takes a finite time to travel, looking at distant galaxies gives us a glimpse into the past. This image shows galaxies formed as early as 6 billion years ago. Blue spheres indicate the location of massive clumps of dark matter which are traced by the galaxy population. Credit: M. Busha & R. Wechsler (Stanford University)

To Wechsler, however, that small visible slice can be a window into the much larger invisible Universe when the links between the two realms are better understood. She explains: “I am mostly interested in how structure forms and how galaxies form, and how we can use that knowledge to understand the ‘dark side’ of the Universe … how what we see in telescopes is connected to the dark matter of the Universe.”

She focuses in particular on the relationship of galaxies to the dark-matter “halos” that surround them. The halos are believed to have evolved from fluctuations in the density of the early Universe, and Wechsler theorizes that they constitute a framework for the billions of galaxies we see today. By the same token, she says those visible galaxies can tell us a lot about the size and structure of the halos. She has found that the luminosity of a galaxy – the number and brightness of its stars -- is closely connected to the mass of its dark-matter halo, though the factors that determine the relationship are not yet understood. 

Galaxies with very large halos don’t have as great a fraction of their matter producing light, because they are too hot to allow much star formation. Dwarf galaxies, about 20 of which are known to exist in the Milky Way’s dark-matter halo, also seem to have a high proportion of dark matter around them, she says. The Milky Way itself, somewhere in between these extremes, has turned about 20% of its available gas into stars.

Wechsler is intrigued by these nearby “little baby galaxies,” the smallest of which is only three hundred times more luminous than the sun, because they “can teach us about the extreme ends of galaxy formation and about dark matter.” Each of them is believed to have its own dark-matter halo rotating within the much more massive halo of the Milky Way. The fact that they formed in this way is a clue to the nature of dark matter and of its role in the structure of the Universe. “In a Universe dominated by dark matter in the way we think ours is, there’s hierarchical formation,” Wechsler says. “Small things form first and then merge and grow … so you only get big objects at late times.” The way this process happens can give us clues to the type of dominant matter. If ordinary matter were dominant, “structure would form much more slowly. And if the Universe had a very different type of dark matter, you wouldn’t have all that hierarchical formation. This would result in many fewer small objects orbiting our own galaxy.” 

"You can do a lot with 300 million galaxies."

In keeping with her effort to connect theory and observation, Wechsler is deeply involved in astronomy initiatives. Roger Blandford, director of KIPAC and a professor at Stanford, says Wechsler “is what we call a phenomenologist, working to connect data to simulations, and doing a very good job at that.” Tom Abel, a fellow cosmologist at Stanford, KIPAC and Stanford’s SLAC National Accelerator Laboratory, says Wechsler “has a deep understanding of both the observations and the simulations and is in an ideal situation to document where they agree and where they don't.”

SETI INSTITUTE SEMINAR: "Connecting Galexies, Halos and Star Formation Rates Across Cosmic Time" (October 2009)

Wechsler is playing key roles in developing the theory and analytic tools for two major sky-survey projects, the Large Synoptic Survey Telescope (LSST) and the Dark Energy Survey. The LSST, due to start operating in 2016, is a wide-field telescope in Chile that will photograph the entire observable sky every three nights. The Dark Energy Survey, due to start in 2011 at another Chilean site, will use a new 500-megapixel camera to observe some 300 million galaxies, as much as 8 billion light-years away, over a region covering one eighth of the sky.

“Obviously, you can do a lot with 300 million galaxies,” Wechsler says of the Dark Energy Survey, on which she serves as co-chair of the galaxy evolution science working group. By mapping this huge area and reaching as far back as 8 billion years, she and her colleagues at KIPAC and Stanford hope to get a detailed view of how dark energy works to expand the cosmos. As Blandford notes, Wechsler’s work has helped lay the groundwork for this project by “showing how to use clusters of galaxies to measure the rate at which structure grows in the Universe.”

It has also been announced that a team of more than 100 investigators, including Wechsler, has been awarded 902 orbits of observing time on the Hubble Space Telescope. This is the most time ever awarded to a single project.

The Wide Field Camera 3, installed onto the Hubble Space Telescope last May, will play an important role in the upcoming Cosmology Survey Multi-Cycle Treasury Program. (Photo courtesy of NASA/STScI.)The Wide Field Camera 3, installed onto the Hubble Space Telescope last May, will play an important role in the upcoming Cosmology Survey Multi-Cycle Treasury Program. Courtesy: NASA/STScI

As reported by SLAC Today: “With this observing time, which will begin later this year, astronomers on the Cosmology Survey Multi-Cycle Treasury Program will peer deep into the universe to document the early history of star formation and galaxy evolution. By imaging more than 250,000 distant galaxies, the project will provide 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 supernovae important for understanding dark energy and the accelerating expansion of the universe."

"My main role on the team will be to provide theoretical context based on our understanding of the evolution of structure over this epoch of the Universe," Wechsler told the publication. "Currently our models for star formation in early galaxies are relatively unconstrained, and these data will be very important for informing our models. This is a new and exciting field that's been made possible by Hubble's new infrared camera, the Wide Field Camera 3. It's really amazing that we can see galaxies so far back in time!"

It is just one more way Wechsler continues seeking answers to truly big questions.

Spring, 2010

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