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Galactic baby boom took place earlier than thought

March 13, 2013|By Geoffrey Mohan
  • Scientists have begun observing distant galaxies using an array of antennas, known as ALMA, built in Chile's Atacama Desert.
Scientists have begun observing distant galaxies using an array of antennas,… (Jorge Saenz / Associated…)

Cosmologists peering into distant, dust-enshrouded galaxies have found that they are far older and more numerous than previously thought.

Their findings, published Wednesday in the journal Nature, push back the birth of these massive star-creation engines and add more precision to the model of how our expanding universe evolved.

“It doesn’t say when the universe began,” said Joaquin Vieira, an observational cosmologist at Caltech and lead author of the paper. “What it does change is when the most massive galaxies in the universe were born. It pushed it back by a billion years.”

Dusty starburst galaxies are extremely difficult to observe, but astronomers can focus on faint radiation signatures with wavelengths of less than a millimeter. Much of that ability is owed to an antenna array in Chile, known as ALMA, the Atacama Large Millimeter Array.

But those observations also were made easier by a cosmological quirk: Galaxies closer to ours bend the radiation from their distant forebears as it passes, and magnify it. The phenomenon is known as gravitational lensing, and the extent of that effect was news in itself.

Images produced by the team look like black bullet holes in white highway signs, surrounded by what looks like curved hills on a red topographic map. They clearly show a strong “lensing” effect and a shift in the wavelength of the radiation due to expansion of the universe.

The wavelength of light and other forms of radiation stretches as it travels because the universe is expanding. (It’s similar to the Doppler effect that changes the perceived sound of a siren). This creates a shift toward the red side of the radiation spectrum.

Measuring this red shift enables astronomers to calculate the age of these sources of radiation that otherwise are obscured by cosmic dust.

Vieira and his colleagues calculated substantially higher red-shift numbers, and effectively dialed back the age of these early galaxies.

First, though, Vieira had to go to the South Pole. There, five years ago, he was part of a team that built a 10-meter telescope that now can take detailed surveys of the sky.

“We practiced building it in Texas. We tore it down and shipped it down,” Vieira said. “Basically we had a month and a half to build it. It was not easy. Almost everyone got frostbite.”

The team surveyed about 720 square degrees of the sky -- about the amount of sky that would be obscured by an average laptop screen on a desk.

One such survey gave astronomers a good idea of where these galaxies were located, and enabled them to turn the power of the ALMA array toward them to peer into their workings.

“We had some idea what they were, in some cases we had anecdotal evidence and in some cases we had observations,” Vieira said.

Vieira and his team chose to look at the radiation from carbon monoxide, and used the red shift to estimate the age of the galaxies that produced it.

“It turns out they had higher red shifts than previously thought,” he said.

The median of red shift was thought to be about 2.3. Vieira and his team calculated a median of about 3.6, meaning a difference of roughly a billion years earlier in the history of the universe. They also effectively doubled the catalogue of galaxies of this type that have a red shift higher than 4.

Vieira, who is accustomed to wrestling with unfathomably large measures of time, credited a more terrestrial time frame for making his work possible: It could have taken years of observation to get their results; ALMA took mere minutes.

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