ALMA unveils hidden chemistry at the heart of the Milky Way in its largest image ever

Astronomers have captured the central region of our Milky Way in a striking new image using the Atacama Large Millimeter/submillimeter Array (ALMA), unveiling a complex network of cosmic gas filaments in unprecedented detail. The rich dataset — the largest ALMA image to date — will allow astronomers to probe the lives of stars in the most extreme region of our galaxy, next to the supermassive black hole at its center.

“It’s a place of extremes, invisible to our eyes, but now revealed in extraordinary detail,” says Ashley Barnes, an astronomer at the European Southern Observatory (ESO) in Germany who is part of the team that obtained the new data. 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 entire region has been explored in such detail.

The region featured in the new image spans more than 650 light-years. It harbors dense clouds of gas and dust surrounding the supermassive black hole at the center of our galaxy. “It is the only galactic nucleus close enough to Earth for us to study in such fine detail,” says Barnes. The dataset reveals the CMZ like never before, from gas structures dozens of light-years across down to small gas clouds around individual stars.

The gas that ACES — the ALMA CMZ Exploration Survey — specifically explores is cold molecular gas. The survey unpacks the intricate chemistry of the CMZ, detecting dozens of different molecules, from simple ones such as silicon monoxide to more complex organic molecules like methanol, acetone, and ethanol.

Cold molecular gas flows along filaments, feeding into clumps of matter from which stars can grow. In the outskirts of the Milky Way, we understand how this process occurs, but within the central region the events are much more extreme. “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,” says ACES leader Steve Longmore, a professor of astrophysics at Liverpool John Moores University, UK. With ACES, astronomers hope to better understand how these phenomena influence the birth of stars and whether our theories of star formation hold in extreme environments.

“By studying how stars are born in the CMZ, we can also gain a clearer picture of how galaxies grew and evolved,” Longmore adds. “We believe the region shares many features with galaxies in the early Universe, where stars were forming in chaotic, extreme environments.”

To collect this new dataset, astronomers used ALMA, the world’s most advanced millimeter/submillimeter observatory, located in Chile’s Atacama Desert. This is the first time such a large area has been scanned with ALMA, making this the largest ALMA image ever produced. Seen in the sky, the mosaic — obtained by stitching together many individual observations — spans an area equivalent to three full Moons placed side-by-side.

“We anticipated a high level of detail when designing the survey, but we were genuinely surprised by the complexity and richness revealed in the final mosaic,” says Katharina Immer, an ALMA astronomer at ESO who is also part of the project.

“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.”

More information

This research was presented in a series of papers presenting the ACES data, to appear in Monthly Notices of the Royal Astronomical Society:

• Paper I - ALMA Central Molecular Zone Exploration Survey (ACES) I: Overview paper https://arxiv.org/abs/2602.20340 
• Paper II - ALMA Central Molecular Zone Exploration Survey (ACES) II: 3mm continuum images https://arxiv.org/abs/2602.20240 
• Paper III - ALMA Central Molecular Zone Exploration Survey (ACES) III: Molecular line data reduction and HNCO & HCO+ data https://arxiv.org/abs/2602.20276 
• Paper IV - ALMA Central Molecular Zone Exploration Survey (ACES) IV: Data of the two intermediate-width spectral windows https://arxiv.org/abs/2602.20445 
• Paper V - ALMA Central Molecular Zone Exploration Survey (ACES) V: CS(2-1), SO 2_3-1_2, CH3CHO 5_(1,4)-4_(1,3), HC3N(11-10) and H40A lines data
• Paper VI - ALMA Central Molecular Zone Exploration Survey (ACES) VI: ALMA Large Program Reveals a Highly Filamentary Central Molecular Zone (undergoing minor revision) https://arxiv.org/abs/2602.20262 

The data itself will be available from the ALMA Science Portal at https://almascience.org/alma-data/lp/aces.

The international ACES team is composed of over 160 scientists ranging from Master’s students to retirees, working at more than 70 institutions across Europe, North and South America, Asia, and Australia. The project was instigated and led by Principal Investigator Steven Longmore (Liverpool John Moores University, UK), together with co-PIs Ashley Barnes (European Southern Observatory, Germany), Cara Battersby (University of Connecticut, USA [Connecticut]), John Bally (University of Colorado Boulder, USA), Laura Colzi (Centro de Astrobiología, Madrid, Spain [CdA]), Adam Ginsburg (University of Florida, USA [Florida]), Jonathan Henshaw (Max Planck Institute for Astronomy, Heidelberg, Germany), Paul Ho (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan), Izaskun Jiménez-Serra (CdA), J. M. Diederik Kruijssen (COOL Research DAO), Elisabeth Mills (University of Kansas, USA), Maya Petkova (Chalmers University of Technology, Sweden), Mattia Sormani (Dipartimento di Scienza e Alta Tecnologia (DiSAT), University of Insubria, Italy), Robin Tress (École Polytechnique Fédérale de Lausanne, Switzerland & Institut für Theoretische Astrophysik, Universität Heidelberg, Germany), Daniel Walker (UK ALMA Regional Centre Node, University of Manchester, UK), and Jennifer Wallace (Connecticut).

Within ACES, the ALMA data reduction working group is coordinated by Adam Ginsburg, Daniel Walker, and Ashley Barnes, and includes Nazar Budaiev (Florida), Laura Colzi (CdA), Savannah Gramze (Florida), Pei-Ying Hsieh (National Astronomical Observatory of Japan, Mitaka, Tokyo, Japan), Desmond Jeff (Florida), Xing Lu (Shanghai Astronomical Observatory, Chinese Academy of Sciences, China), Jaime Pineda (Max-Planck-Institut für extraterrestrische Physik, Germany), Marc Pound (University of Maryland, USA), and Álvaro Sánchez-Monge (Institut de Ciències de l’Espai, CSIC, Bellaterra, Spain; Institut d’Estudis Espacials de Catalunya, Castelldefels, Spain), together with more than 30 additional team members who contributed to the data reduction effort.

The original press release was issued by the European Southern Observatory (ESO), an ALMA partner on behalf of Europe.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF), and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan, and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of ALMA's construction, commissioning, and operation.

Images