In The Field

AAS 2009: The death rattle of the first stars?

The 10-foot-tall tub, full of 500 gallons of liquid helium, dangled underneath a high-altitude balloon over the skies of Texas for just a few hours. But the bucket came back with a big mystery.

In the 2006 flight, Alan Kogut of Goddard Space Flight Center in Greenbelt, Maryland and his colleagues were aiming to use the seven super-cooled antennae in the tub to hear the faint, relic radio signal of ‘first light’, the epoch a few hundred million years after the Big Bang when big lumbering stars ruled the roost.

But instead, they heard a really loud radio hiss. “To our surprise, we found an unexplained radio static… that fills the early universe and is currently unexplained,” says Kogut, speaking at a press briefing on Wednesday.

Astronomers, using balloons and satellites, have mapped the cosmic microwave background — the faint, 2.7 degree Kelvin fabric created by the Big Bang itself — at slightly shorter wavelengths, and ground based astronomers have conducted radio survey at longer wavelengths. But Kogut says his group was the first to notice something in this strange, in-between regime.

In four papers submitted to the Astrophysical Journal, they are careful to rule out the possibility that the radio background signal came from the Milky Way. It’s much bigger than the total signal from all known radio galaxies, and it’s way too big to have anything to do with the first light stars (they would have looked for the radio signal associated with hot gas near these first stars).

But they were reluctant to speculate what it could be. “We really don’t know,” Kogut says. The balloon experiment didn’t have much resolving power to pinpoint individual sources of the static, and it only looked at a few frequencies. Pressed for ideas, Kogut said that it could be radio radiation from the collapse of these huge, first stars into black holes. There would have hundreds, if not thousands, of these first stars in each proto-galaxy, he says, and their collective death rattle, so many of them across the sky, just might appear as a background signal to his relatively crude balloon instrument. “We may have accidentally backed into the epoch we were interested in.” It would be the first signal from one of the deepest, and heretofore darkest, reaches of the universe.

I was just pleased to see yet again that simple balloon experiments can have a potentially big impact. At $4 million, the experiment, called ARCADE, is another example of how cost effective balloons can be.


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