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Saturday, April 13, 2024

At Last, the Milky Way Gets a Better Close Up

After two years of data-taking and number-crunching, a team of astronomers has dropped a snapshot of, quite literally, cosmic proportions. It’s chock-full of stellar goodness: The image shows the reddish-brown dust clouds clumped along the centerline of our Milky Way teeming with over 3 billion pinpricks of light—nearly all stars, a faint neighboring galaxy here or there. 

The project, based at the Harvard-Smithsonian Center for Astrophysics, is called the Dark Energy Camera Plane Survey, and aims to index celestial objects located in our galactic plane. In January, the researchers published their second data release in The Astrophysical Journal Supplement Series, making it the largest catalog, or index, of stars ever collected by a single instrument, and one of the few instances in which we’ve turned a camera toward the middle of our own galaxy. It’s a space selfie, if you will. 

But while the stars are the showstopper, the other point of this survey is capturing the elusive substance that drifts among them: dust. Because dust masks light, it distorts our view of the cosmos. Knowing how much is out there can help astronomers filter its effects from their data, and more accurately gauge the chemistry and position of stars. Over the next decade, scientists will use this catalog to flesh out galactic dust maps, track down ancient star systems, and study the formation and structure of our Milky Way.   

For the survey, the research team repurposed the Dark Energy Camera, or DECam, an optical instrument at the Cerro Tololo Inter-American Observatory in Chile that was originally built to study faint objects far away from the galactic plane. “We took this instrument that was made for cosmology,” says Eddie Schlafly, an astronomer at the Space Telescope Science Institute, “and we pointed it right at the center of the galactic plane, where there’s tons and tons of stars and dust and gas and nebulosity.” The goal, he says, was to resolve as many individual sources of light as possible. 

That’s quite the tall order: Most astronomers stray from observing the galactic plane because it’s notoriously difficult to image. “The Milky Way is a spiral galaxy. So most of its stars are in a flat pancake,” says Andrew Saydjari, a physics graduate student at Harvard University who spearheaded the survey. Unfortunately for observers on Earth, we sit smack in the middle of that pancake. It’s easy to see above or below our plane in that disc, where the stellar haze is thin. But peering into the center of the galaxy, or backward to the outer edge, is tough because the view is crowded. “A lot of the stars can appear like they’re on top of each other,” Saydjari says. 

Other stuff hanging around the galactic center doesn’t help. Some gas, for example, is hot enough to emit its own photons in a color similar to starlight’s. And dust can make celestial objects appear fainter and redder than they actually are. Both of these can skew astronomers’ measurements of stellar brightnesses and positions.

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The first DECam plane survey was published in 2017, an archive of around 2 billion celestial objects located up to 5 degrees above and below the galactic plane. The second release is a reprocessing of all that information, Saydjari says, plus new observations that more than double the total dataset. The basic setup of their experiment was the same: Each part of the sky was imaged the same number of times, at the same time of night, and in the same colors. But the researchers broadened their view to include measurements of everything up to a galactic latitude of 10 degrees above or below the plane. 

Saydjari also developed state-of-the-art software tools to better interpret this data. He wrote code to disentangle stellar photons from those emitted by hot gas, improving the accuracy of brightness measurements. He also updated the method used in the first data release to resolve individual light sources: Rather than identify each star one at a time, Saydjari enhanced the algorithm to model all objects in a single image simultaneously. This created a wealth of information about the locations and brightnesses of stars in five different photometric bands. (Each band, Saydjari says, is like measuring a star’s brightness through a piece of glass that filters out everything but a specific color.)

Schlafly says the team’s long-term goal is to create detailed, three-dimensional maps of dust sprinkled across the Milky Way. This will help astronomers color-correct their view of the stars. “Nearly all measurements in astronomy are of how bright an object is,” he says. “So we care about anything that impacts light.” 

Dust is the reason why, for example, the sun appears so red at dusk—if you want to know its true color, you have to adjust based on the time of day that you’re measuring. In the same way, galactic dust maps will help astronomers do those corrections for cosmic measurements. Stellar color and brightness are inherently linked to a star’s distance, chemical makeup, and temperature. That’s important for characterizing individual objects, but also helpful in understanding the distribution of different types of stars in the Milky Way.  

Dust is more than just a cosmological nuisance, though. “It’s extraordinarily important in the galaxy,” Saydjari says, even though it makes up less than 1 percent of the Milky Way’s total mass. Stars generate dust when they die, and they are, in part, born from it. It’s an essential ingredient of planetary formation: In some sense, Schlafly says, Earth is just a big pile of dust that coalesced a few billion years ago. What’s more, all of the chemistry in our galaxy—including the processes that eventually led to life—began with molecular hydrogen, which requires dust grains to help it fuse together. Knowing the size and density of galactic dust clouds is important for measuring how much chemical activity is churning in a particular region of space. 

Gautham Narayan, a cosmologist at the University of Illinois Urbana-Champaign who was not involved in the work, believes these dust maps will be pivotal for up-and-coming southern sky scanners like the Vera C. Rubin Observatory, which aims to shoot a 10-year motion picture of the Milky Way to reveal how dark matter shapes galactic evolution. “Knowing how much dust there is on the line of sight as a function of distance in any direction will be tremendously valuable,” Narayan says. The DECam plane survey will also help cross-check early Rubin measurements, serving as a baseline to ensure the telescope is working as expected. 

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Other scientists are excited about what this survey will uncover about our own galactic timeline. “I study the immigration history of the Milky Way,” says Massachusetts Institute of Technology astronomer Rohan Naidu, who says that galaxies like ours are built from smaller star systems that merged together at some point. With datasets like this one, galactic archaeologists can begin to distinguish what came from where. “We can be able to say, ‘Here’s this family of stars that arrived together,’” he says.

Naidu thinks the survey can also help him characterize distant galaxies by unearthing the ancient systems our own galaxy absorbed. “Some of the first galaxies are buried right here within our own Milky Way,” he says, “within these very clouded regions that are very difficult to image, that this dataset has now produced one of the deepest, clearest views of.”

Instruments like the James Webb Space Telescope have detected galaxies that might be 13.6 billion years old, but it will be a long time, if ever, before the tech is advanced enough to probe these distant systems on a star-by-star basis or inventory their chemical makeup. Identifying the oldest galaxies in our vicinity—and “studying them in gory detail,” Naidu says—is a first step toward building templates to understand what’s happening in the far-off universe. 

Schlafly says the next step is to patch together other projects with the DECam plane survey to create a holistic view of the entire southern sky. Combine that with data from Gaia—a European satellite measuring the motions and distances of stars—Narayan says, and astronomers are well on their way toward fleshing out a full, three-dimensional map of the Milky Way.

In the meantime, Schlafly encourages space enthusiasts to check out their team’s interactive data viewer, which lets users pan around our cosmic neighborhood like a Google Maps for the galaxy. “The images are captivating,” Schlafly says. “You can browse around here and find all kinds of cool, weird stuff going on.”

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