Astronomers capture the most detailed image yet of our galaxy’s center

Scientists have captured the most comprehensive, high-resolution map of the cold gas at the center of the Milky Way, which contains the raw material from which stars and planets are made. The information in the image could help astronomers understand the origin of our solar system.

The image is the product of a four-year international effort using one of Earth’s most powerful telescopes, the Atacama Large Millimeter/submillimeter Array, or ALMA, an array of more than 50 radio antennas spread across a high plateau in the Chilean Andes.

“We’ve never had a picture of what’s happening right in the center of our galaxy before,” said Steven Longmore, a professor of astrophysics at Liverpool John Moores University who led the project called the Atacama Large Millimeter Array Central Molecular Zone Exploration Survey, or ACES. “We have done many detailed studies of small regions, but this is the first time we have a complete map of the cold gases at the center of our galaxy.”

Previous observations of the Milky Way were like snapshots taken at different locations in the same city, Longmore explained. However, this image of the Milky Way looks like a top-down view of the entire city. “You can’t know the complete history of a city without having a complete map,” he said.

A map of molecular gases

The Milky Way’s galactic center — known as the central molecular zone, or CMZ — is much denser, hotter and more turbulent than regions of space closer to Earth, Longmore said. At its heart is Sagittarius A*, a supermassive black hole about 4 million times more massive than our sun.

This part of the galaxy has the strongest gravitational pull, “so everything is trying to fall in there,” Longmore said. He compared it to a draining bathtub: the black hole acts like a drain, and vast clouds of molecular gases act like swirling water.

The new image maps molecular gas, made up of molecules including hydrogen, carbon monoxide and dozens of more complex compounds that will eventually collapse under their own gravity to form new stars and planetary systems, he added. Understanding when and where this collapse will occur in the galaxy is the central mystery on which the ACES investigation was designed.

“We’re studying star-forming materials in this extreme environment. This is the first really detailed look at how this gas is distributed in 3D space,” said Richard Teague, professor of planetary sciences at the Massachusetts Institute of Technology., who was not involved in the project.

This is not your typical Milky Way photo

The images of the Milky Way that most people are familiar with, depicting the vast spiral galaxy from above, are illustrations, not photographs, Longmore said. “They are exactly what we think they look like,” he added.

What ACES captured is a map of gas in motion. By measuring the precise frequencies of light emitted by specific molecules, scientists can detect tiny changes caused by the Doppler effect — the same phenomenon that makes an ambulance’s siren sound louder when it approaches and louder when it moves away, Longmore explained. Through a technique called spectroscopy, this principle can be applied to light from gas clouds, revealing whether the gas is moving closer or further away from Earth and at what speed.

Such a level of detail, maintained consistently across the entire mapped area, has never been achieved before, Longmore said. Teague added that previous surveys covered large areas at low resolution or zoomed in on small areas at high resolution, but ACES does both in a balanced way.

What can we learn from the image?

The rich colors of the ACES images do not match what the human eye would see if the Milky Way were observed from the telescope’s vantage point. In fact, the colors were not actually captured by the telescope as visible light. Instead, the telescope identified chemical species and gas velocities through spectroscopy, and the images were then edited to assign specific colors to different galactic features.

“Each of the molecules tells us something about the conditions there,” Longmore explained. Red areas can indicate the presence of molecules such as silicon monoxide, which only appear when huge clouds of gas collide. Blue, on the other hand, signals calmer, more stable regions, he said.

In total, the survey observes more than 70 different molecular spectral lines – signatures of simple two-atom molecules, complex organic compounds, such as methanol and ethanol, and everything in between. Longmore noted that some of the complex molecules would be precursors to amino acids, the building blocks of proteins.

Longmore considers the galactic center to be a proxy for the early universe. Conditions there are very similar to those in galaxies billions of years ago, when our own solar system was forming.

“The universe has given us a laboratory to understand our own origins,” he said. “Our own solar system, the sun and our own planets formed a long time ago, about 4.5 billion years ago, and the galaxies were very different. The galaxies of the time looked a lot like the gas we see today in the galactic center.”

A project of immense scale

One of the most incredible aspects of this project was its scale, Longmore said. The 160-person team, made up of collaborators from all over the world, “had to put together a lot of these individual images. It took a huge amount of work on people’s part.”

In the field of submillimeter astronomy, the scale of collaboration is among the largest, Teague noted.

“It’s really a huge amount of work by scientists and universities, but also engineers and telescope operators based in Chile, that made this possible,” Teague added. “I think astronomy on this scale is no longer really about small individuals working in their laboratories, but about huge international collaborations. And I think that’s what’s particularly impressive about this work, just the scale of that collaboration that you need to achieve this.”

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