Is the Big Dipper scooping dark matter?

Scientists peered through a galactic window in the ladle of the Big Dipper, using the Herschel telescope to look 10 billion years backwards in time and investigate the origins of galaxies, which turn out to require 20 times less dark matter than previously calculated.

By , Staff writer

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    The Big Dipper appears over Charlestown, Rhode Island's Frosty Drew Observatory. Scientists peered through the Big Dipper's Lockman Hole, one of the clearest windows to the ancient universe and found that the amount of dark matter needed to build a young galaxy is far less than previously thought.
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IBM's Watson is the new Jeopardy champ? Answer this one, big guy: How much dark matter do you need to build a young galaxy whose rate of star birth is set to overdrive?

Enough dark matter to tip the scales at 5 trillion times the mass of the sun? Last week, right, but this week: Wrong! Try 300 billion solar masses worth of the enigmatic dark stuff.

That's the estimate a team of scientists produced in what lead researcher Asantha Cooray bills as the first attempt to estimate this important quantity in galaxy formation based on observation, and not just through computer simulations.

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The number is substantially smaller – as in, almost 20 times smaller – than the estimates computer produced by computer simulations. It implies a gravitational sweet spot that billions of years ago allowed galaxies nestled inside vast halos of dark matter to undergo intense star formation and grow, leading to the zoo of galaxies astronomers observe today.

With too little dark matter, "a developing galaxy would peter out," says Dr. Cooray, an astrophysicist at the University of California at Irvine. The dark matter's gravity would be too weak to counteract the effect of young, hot stars, whose radiation tends to sweep their neighborhoods clean of the gas needed to make additional stars.

Too much dark matter, on the other hand, would trigger the formation of many small galaxies instead of one large one, Cooray adds.

The team reported its results in Thursday's issue of the journal Nature.

Dark matter is an unseen form of matter whose presence astronomers infer from its gravitational influence on other objects, such as galaxies.

Without dark matter's gravity, galaxies would fly apart; the combined gravity of all the stars, dust, and gas they contain isn't enough to hold a galaxy together.

In trying to find the mass of a dark-matter halo that supports the most efficient star formation in young galaxies, the team used the European Space Agency's Herschel orbiting telescope, which operates at so-called submillimeter wavelengths. This permits the telescope to peer through dust and gas to reveal otherwise obscured objects.

In this case, the team aimed the telescope at a spot in the Milky Way known as the Lockman Hole, which lies near the pointer stars in the Big Dipper. The hole is one of the clearest galactic windows on the distant, younger universe. It contains far less gas to obscure astronomers' views than other parts of the galaxy they have tried to peer through.

The team used the telescope to map the distribution of the infrared glow from star-forming galaxies some 10 billion to 11 billion years old, when the universe underwent its most intensive period of star formation.

These young galaxies contain dense clouds of dust and gas – the nurseries for the stars that form. The dust obscures the view of these galaxies at visible wavelengths, but at submillimeter wavelengths, they shine.

Cooray cautions that the telescope can't spot individual galaxies at such distances. Instead, the team mapped the distribution and strength of the infrared glow these galaxies generated.

The team also relied on the current understanding of how galaxies form and evolve, and on data documenting the amount and distribution of dark matter in the early universe. This distribution is encoded in subtle differences in the strength of pervasive microwave signals that scientists have identified as the afterglow of the Big Bang, which spawned the universe.

Armed with these data, the team used a glass-slipper approach, hunting for the best fit between the infrared patterns they observed and those produced in the simulation, as it sought the halo mass that most readily allowed a galaxy to reach peak star formation.

The ultimate test comes with completion of the Atacama Large Millimeter Array, an observatory under construction in Chile's Atacama Desert, Cooray says. The facility will host at least 66 dish-style telescopes that not only pierce the dust, but can pick out and measure the distant galaxies, one by one.

This will allow the team to measure the movement of matter in those galaxies, which will yield far more precise estimates of the mass of each galaxy and the dark-matter halo surrounding it.

"That will be a direct confirmation" of the amount of dark matter needed to nurture these early star-forming galaxies, he says.

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