ALMA telescope poised to peer into hidden corners of the cosmos
The ALMA radio telescope array set to come on line this week will give astronomers an unprecedented look into areas of the universe obscured from other telescopes – from star nurseries in other galaxies to the black hole at the center of the Milky Way.
Sitting atop a plateau some 16,500 feet high, a growing array of upturned dishes has started to plumb the secrets of planet formation and star formation in early galaxies, and promises to take astronomers to the very brink of a supermassive black hole at the center of the Milky Way.
That may be bad news for fish, but for astronomers, the desert's extreme aridity is welcome. The array observes the universe in a region of the electromagnetic spectrum whose radiation is readily absorbed by water vapor. The radiation ranges from the extreme high end of radio frequencies to the extreme low end of infrared light. In this range, cosmic objects obscured by dust or hidden deep within the cold interstellar clouds where stars eventually form – barely visible to most telescopes – burst into brilliance.
ALMA has been a long time coming. "I attended my first ALMA planning meeting in 1983," says Al Wootten, a researcher at the National Radio Astronomy Observatory headquarters in Charlottesville, Va., and the program's project scientist.
Now, he says, the observatory has 22 dish antennas installed, each just over 39 feet across. Sixteen of the 22 are now operational, en route to 66 antennas by 2013.
Signals coming in from each are combined to build images of the objects astronomers are studying, and the antennas are mobile, allowing the facility to vary the level of detail the array can capture. In their most compact array, the antennas have the ability to capture detail comparable to a single dish 525 feet across. At their maximum spread, the 66 antennas will collectively match the resolving power of a single dish 10 miles wide.
That capability opens the way for a range of new observations, Dr. Wootten says.
Seeing beyond the dust
Hints of the potential for studying the distant cosmos at ALMA's wavelengths began to appear in 1998. Researchers were trying to determine when the universe underwent its most intense burst of star formation. Based on visible and ultraviolet images gathered by the Hubble Space Telescope, it appeared that star formation peaked between 4 billion and 6 billion years ago. Not much appeared to be happening earlier than that.
But a team using the James Clerk Maxwell submillimeter telescope on Hawaii's Mauna Kea observed galaxies producing new stars at enormous rates back to about 8 billion years ago – activity Hubble couldn't see because it was obscured by dust.
This ability of submillimeter telescopes like ALMA to see into the murky regions of the universe could lead to further discoveries. For instance, astrophysicists want to know if these locally bright, early starburst galaxies had dimmer, "normal" kin more like the Milky Way. ALMA is the first instrument sensitive enough to detect dimmer galaxies that might be present during those early times, Wootten says.
"Were there galaxies such as the Milky Way, which we would call normal galaxies, or were galaxies fundamentally different then?" he asks.
Witness to planet formation?
Another objective is to capture images of planet formation around other stars at its earliest stages, when objects are still shrouded with dust.
One early target is a star known as AU Microscopii, which is only about 1 percent of the sun's age.
If planets are orbiting the star, some 33 light-years away, they could leave "observable signatures on the dust," says David Wilner, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
Those signatures would appear as "long-lived concentrations, such as clumps or eccentric rings," he says.
An eight-dish telescope on Mauna Kea known as the Submillimeter Array has identified several stars where these features may be present, Dr. Wilner says, but this smaller facility can't produce images with the level of detail needed to spot the features.
"This is where ALMA will really shine," Wootten says.
Window on a black hole
For some researchers, one of the most exciting prospects for ALMA involves using it as a window on the innermost regions around the supermassive black hole that sits at the center of the Milky Way.
Black holes in the million to billion solar-mass range are thought to lie at the heart of virtually every galaxy. The Milky Way's monster tips the scales at some 4 million times the mass of the sun.
Although ALMA will be able to make some valuable images on its own, the major advances are likely to come when ALMA is linked to the array on Mauna Kea, as well as to subillimeter telescopes elsewhere to act as one virtual dish thousands of miles across – a project known as the Event Horizon Telescope.
Such a telescope would allow astronomers to view the space around the black hole in sufficient detail to image processes taking place at the black hole's event horizon, the slippery slope down which matter around the object falls, never to escape.
Although a black hole's gravity is so intense that not even light can escape, these objects signal their presence by the effect they have on matter falling into them. As matter approaches, it gets compressed under the intensifying gravity, heats, and emits radiation.
Using a smaller array in 2008 and a slightly larger one in 2009, scientists spotted radiation coming from a very tiny source where the Milky Way's black hole is said to lurk. And they saw changes from one year to the next. "We now know there's something very tiny, and it's changing," says Sheperd Doeleman, an astronomer with the Massachusetts Institute of Technology's Haystack Observatory, in Westford, Mass., who worked on the array in 2008 and 2009 and is leading the Event Horizon Telescope effort.
This opens the possibility of observing matter as it makes its final spiral into the event horizon, he says.
One feature the team will seek is a black hole's shadow. The black hole's gravity is so strong that it can bend radiation all the way around it. Theory predicts that this would generate a ring of radiation around the black hole that, when seen edge on, would be brightest on the side where matter is orbiting toward an observer and dimmer where orbiting away. Seen from above, it would look like bit like a doughnut – a bright arc with a dim center.
"This only happens where you get an object that is so incredibly dense that light is being bent by gravity in a very severe way," Dr. Doeleman says.
To see this, "the key, really, is ALMA," with its unprecedented resolution and sensitivity, he says.