Aperture fever strikes in the hunt for dark matter

There’s a truism in astronomy that aperture rules. The wider your telescope mirror or lens the more photons you can capture and the better views you’ll get of celestial objects. It turns out that aperture fever strikes professional astronomers as well as amateurs. The latest to fall victim to this malady is Julianne Dalcanton, professor of astronomy at the University of Washington. Last week Dalcanton gave a talk at the UW titled “Building the Universe Piece by Piece.” It was part of the lecture series The Big Bang and Beyond being presented by the UW Alumni Association in celebration of the 50th anniversary of the university’s Department of Astronomy.

Prof. Julianne Dalcanton spoke about galaxy formation and
evolution at the UW Nov. 18. 2015. Photo: Greg Scheiderer.

Dalcanton’s bailiwick is the study of the formation and evolution of galaxies, and she picked up that story where Miguel Morales left off two weeks before in the second lecture of the series. Morales took us up to the “end of the beginning,” the release of the cosmic microwave background, 380,000 years after the Big Bang. Once things cooled down after that, the universe developed more complexity.

“You have intergalactic gas that originally permeated the universe mixed with the dark matter and the light of the cosmic microwave background,” Dalcanton said. “This gas has funneled, along with the dark matter, into these increasingly rich structures and then funneled into galaxies.”

As the galaxies formed, so did stars out of even more densely concentrated areas of gas. Dalcanton noted that the Hubble Space Telescope has given us marvelous photos of stars being born in places like the Orion Nebula or the Eagle Nebula, subject of the now-famous photo “Pillars of Creation.”

Beautiful and deadly

“The Pillars of Creation” is arguably Hubble’s most famous photo.
Image: NASA, Jeff Hester, and Paul Scowen (Arizona State University)

“These scenes of great beauty are scenes of great destruction,” Dalcanton said. “The stars that are born here are the ultimate in ungrateful children. They are just going about their business absolutely destroying the cloud from which they were born.”

Dalcanton pointed out that we can recognize young stars easily because they’re massive, bright, blue, large, and hot. They tend to flame out quickly. On the other hand, smaller, cooler, dimmer red stars like our Sun last a lot longer.

“They all seem so different,” Dalcanton said. “There’s a clear regularity in their properties that must be directly linked to the physics that’s going on inside the stars.”

By looking at other galaxies and noting the distribution of young and old stars, astronomers get clues about how the galaxies evolved and how elusive dark matter works. Then they make computer models and compare the results to what they see around the universe. The theoretical models match the observations pretty well so far.

“Just because you can make it in the computer doesn’t mean that it’s true,” Dalcanton cautioned. “The study of the individual stars and the actual histories of individual galaxies, where we can pick them apart into their individual pieces, gives us a really strong constraint on all of these models. That then gives us the additional leverage to try to break apart various possible theories of dark matter.”

“The key ingredient to all of this is actually detecting individual stars,” she added.

We need a bigger telescope

This is where the aperture fever comes in.

Dalcanton heads up PHAT, the Panchromatic Hubble Andromeda Treasury, a project in which Hubble made nearly 13,000 images of the Andromeda Galaxy and did a billion measurements of 110 million stars. Volunteers in the Andromeda Project helped sift through nearly a terabyte of data, and we learned a lot.

“As awesome as this is, Hubble is not enough,” Dalcanton said. “Hubble’s my babe, but it’s got its limitations.”

She said Andromeda was chosen for this survey because it is the closest, most massive spiral galaxy we can get a good look at.

“Even with the Hubble Space Telescope we can’t really pick apart all of the stars that we actually want to,” Dalcanton said.

HDST is the answer

The HDST would dwarf Hubble or the James Webb Space
Telescope, planned for launch in 2018. Image: C. Godfrey, STscI.

That’s why she’s a big advocate for a new project on the drawing boards called the High Definition Space Telescope (HDST). Hubble’s mirror is 2.4 meters. HDST’s would be nearly 12 meters, and would have 25 times the surface area of Hubble. Dalcanton said that would give it vastly superior sensitivity and clarity.

“We would see fainter stars and we would see them in regions of the universe where they were much more closely packed together,” she said. It would be like going from an old tube TV to your new 60-inch high-definition television. HDST would be strong enough to spot planets orbiting relatively nearby stars, and could see more and more stellar nurseries like the Eagle Nebula.

“We would be able to see those in individual galaxies anywhere in the universe,” with the HDST, Dalcanton said.

“That’s what I’m rooting for.”