Astronomers Produce the Largest Image Ever Taken of the Heart of the Milky Way

The central region of our Milky Way, sometimes referred to as the “Bulge,” remains something of an enigma to astronomers. Because it is densely packed with stars and clouds of dust and gas, capturing images of its interior has historically been very difficult. But with advances in radio astronomy over many decades, which can capture light that is otherwise blocked at visible wavelengths, astronomers have made some immensely fascinating finds there. In addition to the well-known supermassive black hole (SMBH), Sagittarius A*, there is chemistry at work that could shed light on the origins of life in our galaxy.

Using the Atacama Large Millimeter/submillimeter Array (ALMA), an international team of astronomers has captured the central region of the Milky Way in unprecedented detail. The image reveals a region measuring 650 light-years in diameter filled with a complex network of filaments composed of dense clouds of cosmic gas, known as the Central Molecular Zone (CMZ). As the largest ALMA image taken to date, the rich dataset will allow astronomers to examine the rich chemistry and how stars evolve in the most extreme region of our galaxy.

The research that led to this dataset was conducted by members of the ALMA CMZ Exploration Survey (ACES), a scientific collaboration of more than 160 scientists from more than 70 institutions across Europe, North and South America, Asia, and Australia. The ACES is dedicated to studying the cold gas and identifying chemical signatures in the CMZ, ranging from simple compounds (such as silicon monoxide) to complex organic molecules (such as hydrocarbons). Their work is described in a series of papers that were published in the *Monthly Notices of the Royal Astronomical Society*.

ACES is the largest survey of its kind conducted with the ALMA array toward the Galactic Center, which produced a mosaic of radio images spanning a section of night sky as big as three full Moons, positioned side-by-side. The project was instigated and led by Principal Investigator Steven Longmore, who was joined by co-PIs from each participating institution. One such person is Ashley Barnes, an astronomer at the European Southern Observatory (ESO), which oversees the ALMA array. As she described their observations of the CMZ in an ESO press release:

It’s a place of extremes, invisible to our eyes, but now revealed in extraordinary detail. The observations provide a unique view of the cold gas — the raw material from which stars form — within the so-called Central Molecular Zone (CMZ) of our galaxy. It is the first time the cold gas across this whole region has been explored in such detail. It is the only galactic nucleus close enough to Earth for us to study in such fine detail. The dataset reveals the CMZ like never before, from gas structures dozens of light-years across all the way down to small gas clouds around individual stars.

The image shows cold molecular gas flowing along filaments that feed into clumps, from which new stars are born. While astronomers understand how this process works in the outer disk of the Milky Way, the conditions in the center are far more extreme. How new stars form and evolve under such conditions is still a mystery to astronomers. With this new dataset, astronomers hope to test whether theories of star formation still apply in extreme environments. Said Longmore:

The CMZ hosts some of the most massive stars known in our galaxy, many of which live fast and die young, ending their lives in powerful supernova explosions, and even hypernovae. By studying how stars are born in the CMZ, we can also gain a clearer picture of how galaxies grew and evolved. We believe the region shares many features with galaxies in the early Universe, where stars were forming in chaotic, extreme environments.

The observations also provided a few surprises. Whereas the team anticipated that their observations would yield a high level of detail, they were still awestruck by the complexity and richness revealed in the final mosaic. This detailed survey is likely to be followed up with even more detailed observations once ALMA is upgraded, and when next-generation telescopes become operational.

“The upcoming ALMA Wideband Sensitivity Upgrade, along with ESO’s Extremely Large Telescope, will soon allow us to push even deeper into this region — resolving finer structures, tracing more complex chemistry, and exploring the interplay between stars, gas, and black holes with unprecedented clarity,” says Barnes. “In many ways, this is just the beginning.”

Further Reading: ESO