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Finding Our Way Through the Milky Way

Milky Way infographic
    BENJAMIN WINKEL & THE HI4PI COLLABORATION
WRITTEN BY DIANA MAZZELLA

In southern Australia, west of Sydney, an astronomer was sitting at the controls of a radio telescope, pointing it back and forth over one section of sky.
It was Easter weekend several years ago and no one else was using the Parkes Radio Telescope, so D.J. Pisano, now an associate professor at West Virginia University, went looking for a gas cloud. He knew it existed from old data, but so far he’d only found a very faint trace.

It was Easter weekend several years ago and no one else was using the Parkes Radio Telescope, so D.J. Pisano, now an associate professor at West Virginia University, went looking for a gas cloud. He knew it existed from old data, but so far he’d only found a very faint trace.

So he changed speeds.

While the telescope’s receivers focused on objects moving at a speed of 130 kilometers per second, the cloud looked like a blob, so he set it to look for slower objects, eventually landing at 8 kilometers per second. And there it was (see the red shape in image 1 above). It looks like a backward question mark. It’s massive, made of hydrogen gas, narrow and cold. It probably holds its unusual shape through magnetic fields. But what is it?

Pisano pauses.

“I still don’t know,” he says.

But the nameless cloud has played a part in a wider journey where he has found answers. His curiosity about the cloud helped lead him to create a map of hydrogen in the Milky Way from the southern sky. It’s part of his nearly 20-year search for neutral atomic hydrogen. It took Pisano and a colleague 2,000 hours and about four years until 2008 when they had the clearest map of the hydrogen in the Milky Way from the southern sky ever made. Scientists call the gas HI, and it’s the primary ingredient in stars, the engines of the universe.

Then last year, German scientists who had mapped the Milky Way from the northern sky at the Effelsberg Radio Telescope asked if they could put their data together. That combined data is now the most detailed map (called HI4PI) of hydrogen in the Milky Way. It has twice the sensitivity and four times better angular resolution than the 2005 Leiden-Argentine-Bonn Survey, a previous map that has been cited by astronomers about 2,000 times.

This new map’s purpose is the same as that of Pisano’s southern sky map. To help scientists discover.

“It’s sort of doing what Google Maps does,” Pisano said. “We’re making a map of the Milky Way, and it will be a resource for everybody. We will get something out of it ourselves, but its biggest impact will be all the different ways people want to use it.”

There are lots of questions scientists can ask. Such as: How do hot and cool gas interact in the galaxy? And what does that mean for star formation?

His own questions were about the galactic halo. Pisano goes to his bookshelf in his office and reaches for a F risbee and a golf ball.

“Sorry. I can’t talk about the galaxy without having a prop,” he says.

He holds up the Frisbee. That represents the Milky Way’s galactic plane. And then he holds the golf ball at the Frisbee’s center. That’s the galaxy’s bulge. They’re both densely packed with stars and hydrogen gas. The wispy globe around them would be the galactic halo.

“We started the survey in part because we wanted to know how gas in the halo of our galaxy interacts with the disk, how it goes out of the disk, how it falls onto the disk,” Pisano says.

These questions are part of solving a bigger question that scientists who study star formation don’t have an answer for: Where is all the hydrogen? We’ve accounted for about a tenth of the gas that would be needed to create stars at the rate we’re observing in the universe. In the Milky Way, the hydrogen catalogued so far in the plane of the galaxy would be enough to produce stars for a couple billion years, Pisano says, but that doesn’t match the constant rate of star formation observed in our own and other galaxies.

The assumption is that there is enough hydrogen to constantly form new stars, Pisano said, it’s just out in the halo, perhaps flowing in from intergalactic space. Pisano’s been using his southern sky map to look at the halo for years now, and has made other related discoveries, such as finding an intergalactic river of hydrogen using the Green Bank Observatory in Green Bank, W.Va.

Even if you’re not a scientist and you’re not looking for hydrogen, there’s an appeal to the colorful swirls in the map that you can only find with a gigantic white dish.

And Pisano gets that. When asked what he sees when he looks at the map, he laughs. Probably what most people think when they see it, he says.

“Wow, that looks cool.”

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