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

The ACES captured a map of the cold gas in the Central Molecular Zone in motion. The colors in the photo represent identified chemical species and gas velocities using spectroscopy. (Credit: ALMA(ESO/NAOJ/NRAO)/S. Longmore/D. Minniti et al. via CNN Newsource)

Scientists have captured the most complete, high-resolution map of the cold gas at the centre of the Milky Way, which contains the raw material from which stars and planets are made. Information from 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 the most powerful telescopes on Earth, the Atacama Large Millimetre/submillimeter Array, or ALMA, a collection of more than 50 radio antennae spread across a high plateau in the Chilean Andes.

“We’ve never had a picture of what’s happening right in the centre 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’ve had lots of detailed studies on small regions, but this is the first time that we’ve had an entire map of the cold gas in the centre of our galaxy.”

Previous observations of the Milky Way have been like snapshots taken in different spots of the same city, Longmore explained. This Milky Way image, however, is like a top-down view of the entire city. “You don’t get the full story of a city unless you have a total map of it,” he said.

A map of molecular gas

The galactic centre of the Milky Way — known as the Central Molecular Zone, or CMZ — is far denser, hotter and more turbulent than the regions of space closer to Earth, Longmore said. At its very core is Sagittarius A*, a supermassive black hole roughly four million times more massive than our sun.

This part of the galaxy has the strongest gravitational pull, “so everything is trying to fall into that,” Longmore said. He compared it to a draining bathtub — the black hole acts as the drain and vast clouds of molecular gas act as the swirling water.

The new image maps the molecular gas, which is 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 in the galaxy that collapse will happen is the central mystery the ACES survey was designed to investigate.

“We’re looking at star-forming material in this extreme environment. It’s the first really detailed look at how that gas is distributed in 3D space,” said Richard Teague, a professor of planetary science at Massachusetts Institute of Technology, who was not involved with the project.

Not the typical Milky Way photo

The Milky Way images most people are familiar with, depicting the sweeping spiral galaxy from above, are illustrations, not photographs, Longmore said. “They’re just what we think it looks like,” he added.

What ACES captured is a map of the gas in motion. By measuring the precise frequencies of light emitted by specific molecules, scientists can detect tiny shifts caused by the Doppler effect — the same phenomenon that makes an ambulance siren sound higher-pitched as it approaches and lower-pitched as it recedes, Longmore explained. Using a technique called spectroscopy, this principle can be applied to light from gas clouds, revealing whether the gas is moving toward or away from Earth, and how fast.

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

What can we learn from the image?

The rich colours in the ACES images are not what the human eye would see if the Milky Way were to be viewed from the vantage point of the telescope. In fact, the colours were not actually picked up by the telescope as visible light. Instead, the telescope identified chemical species and gas velocities using spectroscopy, and the images were then edited to assign specific colours to the different galactic features.

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

Altogether, 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 are thought to be precursors to amino acids, the building blocks of proteins.

Longmore sees the galactic center as a proxy for the early universe. The conditions there closely resemble those of 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 back then were very much like the gas we see now in the galactic centre.”

A project of immense scale

One of the most incredible parts of this project was its scale, Longmore said. The 160-person team, consisting of collaborators from around the world, “had to stitch together many of these individual images. That took a huge amount of work from people.”

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

“It’s really a huge amount of work from scientists and universities, but also engineers and telescope operators based in Chile, that made it possible,” Teague added. “I think astronomy on this scale is really no longer about small individual people pushing in their labs, but about huge international collaborations. And I think that’s what’s particularly impressive about this piece of work, just the scale of that collaboration that you need to make it happen.”