Volume 5, Issue 35 February 2008
The Hunt for Dark Matter
Leo Blitz shows an Allen Telescope Array dish feed to Paul Allen, the telescope's namesake, at the facility's October dedication. The feed enables the telescope to convert faint cosmic radio signals into measurable voltages. Photo credit: Colby Kraybill
What you see is not what you get everywhere in the universe. Leo Blitz, a Berkeley professor of astronomy, encountered this disturbing discrepancy early in his scientific career. As a postdoctoral fellow at Berkeley in the 1970s, he found that the outer parts of the Milky Way spin far faster than a galaxy of its size should while still holding together.
Similar discrepancies have cropped up in the motion of other galaxies, galactic clusters, and even light traveling past distant objects. Further studies implied the existence of a substance that neither absorbs nor emits light yet is abundant enough to affect the movements of the universe. Known as dark matter, this stealthy substance makes up nearly a quarter of the cosmos. "It completely dominates the gravity on large scales between galaxies because there's so much of it," Blitz says.
An expert in dark matter and the evolution of galaxies, Blitz also serves as director of the university's Radio Astronomy Laboratory. In this role, he manages the Combined Array for Research in Millimeter-wave Astronomy (CARMA), located high in California's Inyo Mountains, and the brand-new Allen Telescope Array (ATA) near Mount Lassen.
For Blitz and other astronomers seeking insights into dark matter, computer simulations offered some useful hints. These models predict that the universe should contain far more dwarf galaxies than the tiny fraction that astronomers can identify. Could the rest exist as galaxies made up solely of dark matter?
The Allen Telescope Array, located near Mount Lassen, conducts both conventional astronomy studies and scans the skies for intelligent alien life. Photo credit: Seth Shostak
If so, Blitz thinks he knows how to find them. "Imagine them plopping through the gas of the outer Milky Way," he says. "They might create some sort of splash or ripple."
These distant reaches are relatively calm, making such disturbances possible to detect. Blitz explains, "It's like throwing darts at a board. As these dark galaxies come at the Milky Way, they're likely going to hit the outer parts because there's more surface area there."
To pinpoint any dark galaxy splashes, Blitz and his research group are mapping the structure of the Milky Way. In the process, they have been able to characterize the warping of our generally flat galaxy is. "It's like hitting cymbals; it's held in the middle and the outer parts are free to vibrate," he says.
Within this structure, Blitz has identified areas of very localized vibrations-an encouraging sign-and is now searching other galaxies for similar characteristics. "That's exactly the kind of signature we look for if the Milky Way were being hit by these dark matter galaxies," he says.
Atomic hydrogen in the nearby galaxy M31 as imaged by the ATA. This is the first radio image of nearly an entire galaxy taken in a single pointing, thanks to the telescope's broad field of view. The hole near the center suggests that the galaxy is running out of gas to fuel future star formation. Image credit: Courtesy Leo Blitz
As promising as the mapping looks, Blitz is hedging his bets with a second approach: seeking gassy cores that could be embedded even in dark galaxies. "We're trying to survey regions of the sky to see if there are concentrations of atomic hydrogen that are not associated with known galaxies," he says. "I'm hoping that by making a large enough survey of the sky, we'll be able to find galaxies that contain only hydrogen and no stars. By looking at the motions of the hydrogen, we'll be able to determine the properties of the dark matter that's within it as well."
The resulting map of interstellar hydrogen could help answer another paradox in astronomy: why today's galaxies haven't yet run out of gas. According to observations, most galaxies have just enough fuel left to make stars for another billion years or so. Yet galaxies have endured for most of the age of the universe, making it unlikely that so many should blink out at once.
Blitz thinks they could be topping up their tanks with interstellar gases. As galaxies interact gravitationally, gases from their edges will get torn loose. These gases may eventually fall onto other galaxies, just as water vapor gets recycled back into rain. "There should be enough material between galaxies to be able to make up for the stars that are currently being formed," he says. "That's measurable with the Allen Telescope."
The ATA's exceptionally wide field of view and superior sensitivity can scan local galaxies efficiently for interstellar hydrogen at its current strength of 42 dishes. Once the telescope attains its full complement of 350 dishes, it will be powerful enough to more fully illuminate two very dark areas of astronomy.
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Statistical Challenges in Genomics
Sandrine Dudoit is Associate Professor of Biostatistics and Statistics at UC Berkeley and is a faculty affiliate of the Center for Computational Biology and the California Institute for Quantitative Biosciences (QB3). Image credit: Alain Dudoit
At first glance, it bears an uncanny resemblance to a piece of modern art. A grid of red, yellow, and green spots glows against a glassy black backdrop in an abstract composition no larger than a microscope slide. But in addition to having visual appeal, this stylish object possesses tremendous scientific value. Called a DNA microarray, it is a miniature laboratory on a chip. In a single experiment it can deliver a detailed snapshot of the thousands of genes and proteins interacting in an organism, whether bacterium or human.
For biologists, DNA microarrays have been boon and curse alike. Researchers routinely use these assays to monitor gene expression patterns in cells from cancer patients, with the aim of deriving better diagnosis and treatment strategies for the disease. They can now obtain unprecedented insights into the activities of genes and cells with a minimum of experimental effort. At the same time, they are struggling to make sense of the tidal wave of data that ensues.
"Each microarray experiment yields thousands and thousands of measurements for just one person," says Sandrine Dudoit, a Berkeley professor of Biostatistics and Statistics. "Microarrays and other high-throughput biological assays are raising challenging statistical design and analysis questions and are a driving force for our discipline. The scale and complexity of the data are unprecedented and far greater than traditional methods allow you to handle."
DNA microarrays allow biologists to monitor gene expression levels for entire genomes. Image credit: Wikipedia
Dudoit specializes in developing statistical and computational methods to analyze and comprehend the mind-bogglingly large and intricate datasets generated by high-throughput biotechnologies such as DNA microarrays. She has pioneered approaches for combining and synthesizing information from multiple and diverse data sources that concern different aspects of gene expression. She develops statistical methods to uncover relationships among a patient's entire genome; demographic and environmental variables such as age, sex, ethnicity, and diet; and medical outcomes such as survival prognosis and response to treatment.
Dudoit regularly fields requests from colleagues on campus and around the world seeking assistance with their unwieldy data collections. "Biologists are contacting statisticians with fascinating questions and new tools to measure biological variables we'd never thought we'd be able to investigate just a few years ago. They are wondering how to gain biological insight from their experimental data and the wealth of information publicly available on the Internet," she remarks. "It's a very exciting time to be a statistician."
This stained glass window at Cambridge University commemorates Sir Ronald Aylmer Fisher (1890-1962), one of the founding fathers of statistics and a pioneer in the application of statistics to genetics. It represents a Latin square, a table that can be applied to design DNA microarray experiments.Image credit: Wikipedia
Her collaborators encounter a sympathetic ear. As a postdoctoral researcher Dudoit had line-of-fire experience with the formidable data analysis challenges faced by biologists. On her first day in the lab, she was presented with "raw" image data from a microarray experiment and wondered how one could possibly extract biological knowledge from these thousand-dimensional numeric matrices. She recalls, "It was facing the real world in a biology lab that made me realize what applied statistics was all about. It highlighted the critical importance of effective communication between biologists and statisticians. It also emphasized the crucial role of statistical computing as a link between the development of statistical methodology and its timely impact on biology."
Dudoit and her students rely on computing to make the results of their research available to the scientific community. They implement their novel approaches using the R language for statistical computing and graphics and release their software as part of the open-source Bioconductor Project.
"Being a biostatistician today allows me to combine my interests in both mathematics and biology. There's a real need for statistical principles and methodology and our contributions are being used right away to address relevant biological questions. With a next generation of DNA sequencing machines entering the scene, we are facing new and even greater statistical and computational challenges," Dudoit says. "You feel like your work really matters; it's being applied immediately, with the goal of elucidating fundamental scientific questions and improving public health."
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Tracking Space Transients
Professor of astronomy Joshua Bloom studies gamma-ray bursts and other transient objects such as supernovae. Image credit: courtesy Josh Bloom
To watch the most powerful explosions in the universe, it helps to be quick on the draw. In seconds to minutes, the titanic blasts known as gamma-ray bursts can release more energy than our sun in 10 billion years. The initial flash of gamma rays is followed by a stream of electromagnetic radiation flowing from the site. The energy bathes nearby stars, nebulae, and galaxies in a brilliant afterglow, providing a floodlit view of the blast's environment.
Joshua Bloom, a Berkeley professor of astronomy, compares the analysis of a gamma-ray burst to a crime scene investigation. "If you blow something up over the course of a second, what you have left is this forensics scene. You have to look for the fingerprints of something that isn't there anymore," he says.
Bloom uses this technique to help explain the origins of these phenomenal explosions. At first, scientists divided gamma-ray bursts into just two types. Long-duration bursts, which persisted for more than two seconds, were followed by a second violent explosion called a supernova, produced by a collapsing star forming a black hole. The origins of short-duration bursts, however, remained unknown because astronomers had not yet closely pinpointed such events on the sky.
Some gamma-ray bursts are the explosive results of a massive star collapsing to form a black hole.
Gamma-rays are released in two jets along the star's axis of rotation. The burst may last from a few milliseconds
to a few minutes.
Image credit: Nicolle Rager Fuller/NSF
In 2005, Bloom observed the afterglow of the first well-localized short burst. The eagle eye's view allowed his team to discern that it had occurred near an elliptical galaxy, a conglomeration of mostly red and old stars. He deduced that the brief blast was likely due to the collision of two neutron stars-the extremely dense, massive remnants of collapsed stars-or a neutron star and a black hole.
From that point on, the tidy two-class burst hypothesis began to break down. Bloom's team has recently observed a short burst from a nearby galaxy which likely came from the surface of a neutron star. They have also observed two long bursts lacking supernovae, implying that there is greater diversity in the mechanisms producing gamma-ray bursts than was once believed.
Bloom's research group manages the robotic telescope PAIRITEL. Bloom has developed a system to allow fully robotic telescopes like PAIRITEL to observe fleeting sky phenomena soon after they appear. Image credit: Joshua Bloom
According to Bloom, the key to advancing the study of gamma-ray bursts and other transient phenomena is using many telescopes to observe these events at the same time. This approach is better able to characterize transients that appear unusual or are near enough to yield a good view. "You're potentially finding a diamond in the rough that you then need to devote more resources to very quickly," he says. "The big progress in gamma-ray bursts came when we got other telescopes around the world looking at them in other wavelengths."
The trick has been enlisting observing help in a timely fashion. To announce news of a blast, scientists once had to make time-consuming phone calls and emails to other astronomers. Bloom has helped develop a way to accelerate this process. He has created a language for today's robotic telescopes to automatically describe and broadcast sightings of gamma-ray bursts and other transient objects to other telescopes. Based on these Real-time Virtual Observatory Event (VOEvent) bulletins, robotic telescope facilities can more quickly respond to important astronomical happenings. The language has already yielded results such as the discovery of new planets around distant stars.
Bloom also seeks to standardize the reams of data being stored by robotic telescopes to allow easy analysis of their digital images. Within these databases, Bloom sees the potential of scientific pay dirt-insights into bursts or other transient events that may have been recorded during other studies of the sky. "It used to be when you wanted to understand this type of object you went and got some telescope time. Now in many cases you don't have to take the data-it already exists," Bloom says. Learning how to mine existing data records, he says, "is a skill that most astronomers don't yet have as part of our formal training, but we desperately need to build this toolbox for future generations."
Infrared images of two supernovae captured by PAIRITEL.
Image credit: Joshua Bloom
Data-mining techniques combined with real-time observations of the dynamic sky is where Bloom sees great opportunity for new discoveries. Referencing the career advice given to Dustin Hoffman's character in the movie "The Graduate," Bloom says that "transients, for astronomers, is the new plastics-it's where it's at."