Article: One in a Million main content.

Article: One in a Million

The Sloan Digital Sky Survey, a catalog of all the space objects detectable in one-quarter of the night sky, was designed for size. The intent of the project was to provide fodder for large-scale maps and statistical studies about the arrangement and evolution of space objects across the Universe. But its breadth has also enabled researchers to find “small” surprisespeculiar space objects that are, literally, one in a million. 

A map of the distribution of stars in the northern hemisphere of the Milky Way galaxy. The Virgo overdensity (the remnant of another galaxy) is visible as the red area near the top.
Zeljko Ivezić

You’d think that a dense area of stars 5,000 times that of the full moon, situated in our own galaxy, would be hard to miss. Especially if you’re an astrophysicist. But Mario Jurić and Zeljko Ivezić, a research pair from Princeton University and the University of Washington, respectively, and other members of the Sloan team had to map 48 million stars in the Milky Way in 3D to be able pick the clump out. The “Virgo overdensity,” its technical name at the moment, is likely the remnant of a smaller galaxy swallowed by the Milky Way billions of years ago.

The Milky Way galaxy is not sprinkled evenly with stars. Some areas are cluttered, others are sparse. Ivezić likens the overall shape of the Milky Way to a pizza pie (the galactic disk) floating in a haze of smoke (the halo). The halo is comprised of thinly distributed stars often too faint to appear on conventional telescopes. The exact layout of stars across the "pizza" and "smoke" is only now being understood via wide-range surveys like Sloan. “Former surveys did find some dim stars,” says Ivezić, “but in just a tiny fraction of the galaxy.” Sloan, however, can spot the dimmest stars in a large swath of sky and estimate their distances away from Earththe data used for the 3D map.

One reason the Virgo overdensity is significant is that it strengthens one of two opposing theories about the development of our galaxy. It suggests that the Milky Way built up gradually by attracting smaller, less-dense galaxies with its intense gravitational clout, as opposed to forming all at once. 

This red point is the most distant quasar known. Its light takes 13 billion light-years to reach Earth.
SDSS Collaboration

In 2003, University of Arizona astronomer Xiaohui Fan used the Sloan data to locate the farthest of the far: the most distant quasar known in the Universe. 

Quasars occur at the cores of galaxies where an extremely active, extremely massive black hole resides. Many galaxies have central black holes, but one whose overwhelming gravity is actively attracting matter will radiate energy in the process. That radiationthe quasaris intensely luminous. That’s why quasars can be seen at great distances, beyond 12 billion light-years from Earth. In fact, quasars are the most distant objects in the Universe.

To find the record-breaker, Fan and his colleagues used a computer program to narrow down hundreds of millions of objects to a thousand prime quasar candidates. The program flagged objects that emit very red light. What we call “white” light is actually a combination of light all the colors of the rainbow, running from red to violet. This is called a spectrum. Different classes of space objects--quasars, galaxies, stars--emit light in characteristic colors on the spectrum based on their chemical composition. Each specific color has a specific wavelength.

Fan and his team examined each of the thousand candidates further, some by eye, discarding any that appeared to be photographic defects. Later they took a detailed spectrum of a select hundred objects using spectrographs on other telescopes such as the 10 m Keck optical telescope in Hawaii.  By analyzing these spectra, Fan’s team was able to measure the precise wavelength of the light emission from hot hydrogen gas in the quasar. The more distant the quasar is, the redder the light.

In 2003, Fan and his colleagues at the Sloan Digital Sky Survey discovered the most distant quasar found so far in the Universe. “However, we haven’t broken the record in three years, and I doubt we will,” says Fan. “We are at the limit.” Sloan’s instruments aren’t designed to detect wavelengths any more red than that of this farthest quasar, and Sloan is nearly finished with its survey area. Finding further quasars will be a job left for those projects beyond Sloan with instrumentation sensitive to wavelengths of light beyond red.

“Quasars are fascinating objects,” says Fan, “Because their light has traveled so far to reach us, they can tell you about the evolution of the Universe.”

The red speck between the lines is a brown dwarf in the Virgo constellation, as seen by the Sloan telescope.
SDSS Collaboration

While looking for the most distant quasars, Fan accidentally stumbled upon another notable object in Sloan’s data: the first free-floating brown dwarf. Brown dwarfs are considered failed stars. They are objects without sufficient mass to sustain the fusion reaction required to shine brightly. Brown dwarfs are not quite stars, and not quite planetsand too dim to show up on most telescopes.

The first of brown dwarf was discovered in 1995 orbiting another dwarf. Naturally, scientists wondered if brown dwarves could exist on their own in interstellar space, with no gravitational ties to a companion. Fan answered that question in 1999. “We were experimenting with how to find distant quasars in the Sloan data,” he says. “And a very red object showed up.” Fan thought it must be a quasar, but a spectrograph revealed its chemical composition was quite differentit featured strong methane gas and water vapor rather than hydrogen gas. “At first, we had absolutely no idea what it was,” he says. But then Fan and his colleagues recalled a recent lecture by a Caltech astronomer, Davy Kirkpatrick, who had suggested that Sloan’s data may be useful in revealing elusive brown dwarfs. Brown dwarfs, it turns out, also emit light at the red end of the spectrum.

The mystery object turned out to be the first free-floating brown dwarf located in space. And, surprisingly, Sloan’s data happened to be full of them. Now brown dwarves are considered among the most numerous objects in the Universe. “I’m lucky if I find a distant quasar among the pesky brown dwarves,” says Fan.

Researchers are now trying map brown dwarfs across the galaxy and compare their frequency to that of stars like the Sun. “Because brown dwarfs are between planets and stars, finding more of them presumably can teach us something about planet formation,” says Fan.

Related Links

SDSS: Three Distant Quasars Found at Edge of the Universe
Original press release on the Universe's most distant quasar known.

SDSS: Sky Survey Scientists Discover New Celestial Dwarfs
Original press release on the new class of brown dwarfs found via the Sloan telescope.

Science Bulletins: Eight New Einstein Rings Discovered
Another remarkable discovery from the Sloan Digital Sky Survey and the Hubble Space Telescope.