Photo by Andrew Campbell
Photo courtesy NASA/CXC/MIT/F.K. Baganoff et al.
Photo courtesy of NRAO
Photo courtesy of NASA/CXC/SAO
When Farhad Zadeh was a boy growing up in Iran, his father would send him outside at twilight on Saturdays to search the sky for the first star. The family, however, was not particularly interested in the heavens, explains Zadeh, now a Northwestern professor of physics and astronomy. Rather, the request was a practical one: The first visible star marks the official end of the Jewish Sabbath, meaning his dad could light up a cigarette.
Zadeh tells the story with a deep rolling laugh, adding that it would be a good one to explain why he first became so fascinated with the skies. But the truth is, as a boy Zadeh had no idea he would join an elite team of scholars probing the universe with the world’s most sophisticated telescopes. Even after he finished his formal studies in astronomy — graduating with a degree from Columbia University and going to work on a National Research Council fellowship at NASA’s Goddard Space Flight Center in Greenbelt, Md. — Zadeh had no burning desire to peer into the cosmos. He was more interested in theory and physics, so he followed in the footsteps of his Columbia adviser by knitting others’ observations into theoretical frameworks and mathematical models. At least that’s what he tried to do.
Zadeh’s turning point came in the early 1980s, when he attempted to pin a theoretical framework on others’ observation that radio waves were being emitted from the center of the Milky Way Galaxy.
“We tried to model it, we had a theory to explain the observation, but we never could get it to work,” he says. “We thought maybe our assumptions were wrong.”
Astronomers believed the emissions came from densely packed hydrogen clouds — signposts of stellar nurseries — but attempts to explain the compact sources of energy failed. So when a new observatory, located in New Mexico and called the Very Large Array radio telescope, became available to Zadeh and his colleagues in 1983, they decided to reobserve the phenomenon themselves.
“It turned out the previous observation had seen the trees but not the forest, so to speak,” he says. “It was no wonder why we couldn’t explain it. The ‘forest’ turned out to be something no one had ever seen before. It became intriguing to me.”
What Zadeh and the other astronomers found were filaments of magnetic structures that stretched like narrow jets 50 to 100 light-years across the sky. Since then, astronomers have found a number of similar structures. “There are lots of ideas around about what’s causing them, but there’s no consensus,” he says.
That discovery was enough to lure Zadeh out of the realm of physical possibilities and into the domain of the observable universe. He has never looked back. Instead, his focus became the center of the galaxy, a dynamic place filled with exotic phenomena that manifest in regions of space packed with matter and that are strongly influenced by massive gravitational forces and energy. “Unusual conditions form unusual features,” says Zadeh. “All kinds of things are happening in this very small volume in our own galaxy. This includes the evidence for the most massive black hole located exactly at the center of our galaxy. This black hole weighs more than a million times the mass of our sun.”
In this world of extremes, Zadeh is at home. “The galactic center is interesting to some, frustrating to others,” he says. “It’s very popular now to study other galaxies and cosmology, but I like looking at the Milky Way because there’s a lot of information in every wavelength that you look at. When you have something nearby, you can do spectroscopic imaging. You can learn how things move on the plane of the sky.”
Zadeh belongs to a new school of astronomers who do not specialize in studying a particular region of the electromagnetic spectrum, such as radio waves, optical light, or X-ray and gamma ray radiation. “I like the multi-wavelength approach. You can use all the available instruments.”
Though his roots are in radio astronomy, Zadeh has been spending much of his time studying observations from NASA’s Chandra X-ray Observatory, an orbiting sister telescope to the Hubble observatory that is sensitive to high-energy X-ray radiation.
The different observatories detect different wavelengths, or energy levels, of radiation. The faintest, lowest-temperature emissions manifest as radio, microwave and infrared energy; the hottest and most energetic radiation emits in X-ray and gamma rays.
Using Chandra, Zadeh and colleagues have made several intriguing discoveries about structures and processes in the heart of our galaxy. For example, they found what may be the cause of a ridge of high-energy X-ray emissions that extend along the plane of the Milky Way. The origin of the ridge had been a mystery for 30 years.
Comparing data from Chandra with millimeter wavelength radio information obtained by the Nobeyama Radio Observatory in Japan and centimeter radio observations by the Very Large Array, the astronomers found that hot and cold gases are coexisting in the ridge. The discovery bolsters a theory that low-energy cosmic rays are striking and heating the cold gas clouds, producing X-rays.
Chandra also aided Zadeh in another discovery, namely that an extremely hot pocket of gas, which stretches 30 light-years, is surrounding a cluster of massive young stars called the Arches Cluster. The telescope tracked the gas, which moves at about 600 miles per second around the cluster. Zadeh and his collaborators believe the gas is the result of stellar winds and charged particles being blown out from the young stars. The energy is so high because about 150 stars are crammed into the diameter of one light-year, or about six trillion miles. It’s the most compact cluster known in the Milky Way.
“We don’t need a supernova explosion to explain what we see in the nucleus of our own galaxy,” says Zadeh.
The force of the stellar winds within the cluster would be like running into a wall at 1,000 kilometers (621 miles) per second, he adds. “All of the energy, all of a sudden, is transformed into X-ray radiation. It’s a nightmare in there.”
In addition the density of stars makes the region in and around the Arches Cluster a microcosm of what is likely occurring in starburst galaxies — the most prodigious stellar nurseries known. “This cluster is one of the best local analogues of a starburst galaxy, yet it is in our backyard, not millions of light-years away,” says Casey Law, a Northwestern graduate student who works with Zadeh.
Not content to rest on his laurels, the intellectually restless Zadeh has lately turned his attention to an odd class of objects called masers, which are naturally occurring microwave lasers. “They have interesting properties and characteristics,” he says.
As is the case with most of his research initiatives, Zadeh is attracted to a particular group of masers that are signposts to an extreme condition in the cosmos. “When you have equilibrium, generally all the molecules are in the lowest state [of energy] possible,” he says.
“Nature is a little lazy,” Zadeh adds. “Molecules tend to stay in their lowest energy levels, and then as they get hotter, the energy levels move up higher and higher. If you want to make a molecule or an atom go to a higher energy, you have to pump energy into it, but once the energy goes out, it comes back down again as the molecules cool down.”
With masers, the situation is reversed. “All these molecules and atoms want to stay at an excited energy level, and they’re not coming down. They just sit there,” he says. “When a light passes through or something happens to trigger them, suddenly all of them come down simultaneously. The light gets amplified and becomes extremely bright. But it’s very different than the light we see from the sun or a light bulb.”
Zadeh is particularly keen on hunting masers that appear where the hot, expanding fragments of a dying star, called a supernova remnant, are colliding with a cool cloud of gas and dust that actually provided the raw materials for the star’s birth in the first place. The energy released from the stellar fragments crashing into the cool cloud, which in fact is collapsing under the gravitational pull of its own mass, in turn triggers a new generation of star formation.
“You have two interesting situations,” he says. “One is the end state of the evolution of a star when it blows up, and the other is when it gives birth to a new generation of stars. … You have totally different physical situations, so it’s a fascinating way to find where the interactions are.
“Basically, we’re studying shock fronts and shock waves,” Zadeh says. “Masers are a good way to find the interaction sites. Once you do, you can look at them with many different telescopes — infrared, radio or X-ray. Some people even think gamma ray observatories may work because it’s a very powerful shock that’s going through this cloud, and it may even produce some gamma rays.”
There’s plenty at the center of the galaxy to hold Zadeh’s interest for a lifetime. He has proposals pending or in the works that utilize several observatories, both space- and ground-based, and he’s a partner on a project to use NASA’s new Space Infrared Telescope Facility, currently scheduled for launch in April.
“You have to have a very good reason if you want to explore,” says Zadeh. “You have to make a strong case that the region you want to look at addresses important questions. Telescope time is so valuable.
“But there is room for searching for something new, and if you find something, that’s great. If not, you just move on.”
Irene Brown (J82) is a freelance science writer based in Melbourne Beach, Fla.
RETURN TO TOP