ALMA's Record-Breaking Mosaic Reveals the Hidden Chemistry and Chaos at the Milky Way's Core

A global collaboration of 160 scientists has produced the largest-ever radio image of our galaxy's turbulent center, uncovering a web of cold gas filaments, exotic molecules, and clues to how stars are born in extreme environments.

On February 25, 2026, a massive international team of astronomers unveiled something no one had seen before: a sweeping, 650-light-year-wide portrait of the Milky Way's innermost sanctum, captured in radio wavelengths by the world's most powerful millimeter-wave telescope [1]. The image, produced by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile's Atacama Desert, is the largest ever created by the observatory -- and it is rewriting what scientists thought they knew about the violent, chemically exotic region surrounding our galaxy's supermassive black hole.

The result of the ALMA Central Molecular Zone Exploration Survey, known as ACES, the mosaic spans a region of sky equivalent to three full Moons placed side by side [2]. It is the first time such a vast swath of the galactic center has been mapped in this level of detail at millimeter wavelengths, and the data it contains could keep researchers busy for years.

"It is the first time the cold gas across this whole region has been explored in such detail," said Ashley Barnes, an astronomer at the European Southern Observatory and a member of the ACES team [3].

What the Image Shows

The Central Molecular Zone, or CMZ, is the innermost few hundred light-years of the Milky Way. It is a dense, turbulent region of cold molecular gas and dust that surrounds Sagittarius A*, the supermassive black hole at our galaxy's heart -- a gravitational monster with the mass of roughly four million suns, located approximately 26,000 light-years from Earth [4].

What ALMA has revealed is a complex, filamentary network of cold gas threading through this region like a cosmic web. These elongated filaments funnel material into dense clumps where the raw ingredients for star formation accumulate [5]. The image captures structures at every scale -- from massive gas formations stretching dozens of light-years across to compact clouds enveloping individual newborn stars [6].

The survey's composite image encodes different molecular species in distinct colors: sulfur monoxide appears in cyan, silicon monoxide in green, isocyanic acid in red, cyanoacetylene in blue, and carbon monosulfide in magenta [7]. The result is a vivid chemical map of a region that, until now, had been largely hidden from view.

A Chemical Treasure Trove

Perhaps the most striking finding from ACES is the sheer chemical complexity lurking at the galaxy's core. The survey detected dozens of different molecules -- from simple compounds like silicon monoxide to surprisingly complex organic molecules including methanol, acetone, and ethanol [1][8].

This molecular diversity was unexpected. The center of the Milky Way is one of the most extreme environments in the galaxy -- bombarded by intense radiation, wracked by powerful magnetic fields, and subject to gravitational forces that would tear apart less robust structures. That such a rich chemical environment persists under these conditions challenges astronomers' understanding of astrochemistry in extreme settings.

"Our group will be unraveling this rich dataset for years," said Cara Battersby, an associate professor of physics at the University of Connecticut and a co-principal investigator on ACES [9].

The detected molecules include not just simple inorganic species but also compounds like HCO+, HNCO (isocyanic acid), and SiO (silicon monoxide), alongside more complex organic molecules [10]. This chemical inventory offers a fingerprint of the physical processes at work -- shock waves from supernova explosions, ultraviolet radiation from massive young stars, and the slow gravitational collapse of gas into new stellar nurseries.

Star Formation at the Edge of Theory

One of the central scientific questions driving ACES is whether existing theories of star formation hold up under the extreme conditions at the galaxy's center. The CMZ hosts some of the most massive stars known in the Milky Way -- stars that "live fast and die young," as principal investigator Steven Longmore of Liverpool John Moores University put it, ending their lives in powerful supernova explosions and even hypernovae [1][6].

Source: GDELT Project

Data as of Mar 9, 2026CSV

The conditions in the CMZ differ drastically from the quieter star-forming regions in the galaxy's spiral arms, where most of our models of stellar birth were developed. Gas temperatures are higher, turbulence is more intense, and the tidal forces from the central black hole add an additional layer of disruption.

"Understanding how material is transported from large to small scales is important for star formation," noted Jennifer Wallace, a Ph.D. student at UConn working on the physical and kinematic properties revealed by ACES [9].

Through the survey, researchers hope to determine how these violent conditions influence the birth of stars -- and whether the same fundamental physics that governs star formation in calmer galactic neighborhoods applies in the CMZ's extreme environment.

A Window Into the Early Universe

The significance of the ACES data extends far beyond our own galaxy. Longmore and his colleagues have noted that the CMZ shares many features with the dense, turbulent galaxies that existed in the early universe, shortly after the Big Bang [3][5]. In those ancient galaxies, star formation was prodigious and chaotic, driven by the same kinds of high-density, high-turbulence conditions that characterize our galactic center today.

This makes the CMZ a uniquely valuable laboratory. While early-universe galaxies are billions of light-years away and can only be studied in broad strokes, the Milky Way's center is close enough -- a mere 26,000 light-years -- that ALMA can resolve individual gas clouds and trace the specific chemical and physical processes at work.

"Studying star formation in the CMZ illuminates how galaxies grew and evolved," Longmore explained, framing the research as not just a local investigation but a probe into the fundamental mechanisms that shaped the cosmos [3].

The ALMA Machine

The observatory that made this possible is itself a feat of engineering. ALMA consists of 66 high-precision dish antennas -- 54 measuring 12 meters in diameter and 12 measuring 7 meters -- arrayed across the Chajnantor plateau at an altitude of 5,000 meters (16,500 feet) in the Chilean Atacama Desert [11]. The site was chosen for its extreme aridity, which minimizes atmospheric interference with the faint millimeter-wavelength signals that ALMA is designed to detect.

The antennas can be repositioned across distances ranging from 150 meters to 16 kilometers, giving the array a variable "zoom" capability [11]. Operating at wavelengths between 0.32 and 3.6 millimeters, ALMA can achieve resolution up to ten times sharper than the Hubble Space Telescope at its operating frequencies.

For the ACES survey, this formidable instrument was pointed at the galactic center repeatedly, building up a mosaic from numerous individual observations stitched together like the tiles of an enormous puzzle [2]. The result is the most detailed millimeter-wavelength view of the CMZ ever achieved.

A Massive Collaboration

The ACES project is notable not just for its scientific output but for the scale of its human effort. More than 160 scientists from over 70 institutions across Europe, North and South America, Asia, and Australia contributed to the work [1]. The collaboration spans career stages from Master's students to retirees, reflecting the broad appeal and fundamental importance of the questions being addressed.

Dani Lipman, a Ph.D. candidate at UConn, is using advanced supercomputer simulations to create theoretical models of the CMZ that can be compared directly with the ACES observations. "Simulations are necessary to look at the overall large scope of the CMZ and do a one-to-one comparison," Lipman said [9].

The results are being published in a series of papers in the Monthly Notices of the Royal Astronomical Society, with five papers accepted and a sixth under final review as of the announcement date [1][10]. The science-ready data has been made publicly available through the ALMA Science Archive, ensuring that the broader astronomical community can build on the findings.

Source: ESO / ALMA Observatory / ACES Survey

Data as of Mar 9, 2026CSV

Building on a Legacy of Galactic Exploration

The ACES survey represents the latest milestone in a decades-long campaign to map the Milky Way's heart. In 2012, ESO's VISTA telescope -- the world's largest dedicated infrared survey telescope, with a 4.1-meter mirror -- produced a nine-gigapixel mosaic of the galactic center containing over 84 million stars [12]. That survey was later expanded through the VVV and VVVX projects, which over 13 years accumulated 500 terabytes of data from 200,000 images covering an area equivalent to 8,600 full Moons [13].

More recently, the James Webb Space Telescope has contributed its own infrared observations of the galactic center. In 2023, JWST revealed approximately 500,000 stars in the Sagittarius C region, including a dense cluster of newborn giant stars roughly 300 light-years from Sagittarius A* [14]. JWST has also tracked the supermassive black hole itself, observing five to six major flares per day during a 48-hour monitoring campaign [15].

Each of these observations has probed the galactic center at different wavelengths, revealing different aspects of the same complex environment. VISTA mapped the stars. JWST peered through the dust in infrared. And now ALMA has unveiled the cold gas -- the raw material from which all those stars were born.

What Comes Next

The ACES dataset is expected to fuel research for years to come, but the story of galactic center exploration is far from over. ALMA itself is undergoing a transformative Wideband Sensitivity Upgrade (WSU), which will double the observatory's instantaneous bandwidth in a first stage and ultimately quadruple it [16]. When completed in the early 2030s, the upgrade will make ALMA observations dramatically faster and more sensitive, enabling astronomers to detect fainter objects and map even larger regions of the sky.

Meanwhile, ESO's Extremely Large Telescope (ELT), currently under construction on Cerro Armazones in the Atacama Desert, is expected to achieve first light in 2029 and begin scientific operations by 2030 [17]. With a 39-meter primary mirror and adaptive optics that will produce images 16 times sharper than Hubble, the ELT will be capable of resolving individual stars and compact structures at the galactic center with unprecedented clarity.

Together, these next-generation facilities promise to push the boundaries of what we can learn about the most extreme environment in our cosmic neighborhood -- and, by extension, about the fundamental processes that shaped galaxies across the universe.

The Bigger Picture

The unveiling of ALMA's record-breaking mosaic arrives at a moment when astronomy is experiencing a golden age of multiwavelength observation. From the Event Horizon Telescope's first image of Sagittarius A* in 2022 to JWST's infrared revelations and now ACES's millimeter-wave chemical map, each new dataset adds another layer to our understanding of the galaxy we call home.

What emerges from the ACES data is a picture of a galactic center that is far more chemically rich, structurally complex, and dynamically active than earlier observations suggested. It is a place where organic molecules persist amid extreme radiation, where gas filaments channel material toward stellar nurseries under impossible conditions, and where the physics of star formation may operate under rules that scientists are only beginning to understand.

As Cara Battersby of UConn put it, the team will be "unraveling this rich dataset for years" [9]. For a region that has been hidden from our eyes for all of human history, that prospect alone represents a remarkable achievement.