The Context

The center of the Milky Way has always been one of the hardest places in the galaxy to study. Thick clouds of gas and dust block visible light, making optical telescopes essentially useless for probing the region around Sagittarius A*. Our galaxy's supermassive black hole.

For decades, astronomers relied on radio and infrared observations to piece together a fragmentary picture of this extreme environment. Actually, that understates it. Where temperatures run 10 times hotter than typical molecular clouds and magnetic fields are a thousand times stronger than those in the galactic disk.

What made the challenge especially frustrating was the sheer complexity.

The Central Molecular Zone, a region spanning a few hundred light-years around the galactic center, holds somewhere between 20 and 60 million solar masses of molecular gas (a pattern we keep seeing). That's an enormous reservoir of raw material for building stars. Yet the rate at which stars actually form there's roughly 10 percent of what models predict (a pattern we keep seeing). Something was suppressing star formation at the heart of our galaxy. And without a thorough chemical map, nobody could say exactly what. Previous surveys captured snapshots of individual clouds or narrow chemical windows. The missing piece was a panoramic view that could trace the full chemical diversity of the Central Molecular Zone at high resolution, connecting the physics of individual star-forming cores to the large-scale flows of gas feeding the galactic center.

The Development

In March 2026, an international collaboration released the results of the ALMA Central Molecular Zone Exploration Survey, known as ACES. Using the Atacama Large Millimeter/submillimeter Array in Chile, the team produced the largest ALMA image ever created, a contiguous mosaic stretching across more than 650 light-years of the Milky Way's core. In my reading of the research, stitched together from 45 individual sub-mosaics, the final product spans an area equivalent to three full Moons side by side in the night sky.

The survey captured over 70 distinct spectral features across Band 3 frequencies, identifying dozens of different molecules ranging from simple compounds like silicon monoxide and carbon monosulphide to complex organic molecules including methanol, acetone. And ethanol. Each molecule traces different physical conditions, whether shock waves from supernova remnants; the slow collapse of gas clouds into protostars. Or the outflows from newly born stellar systems. As we explored in The James Webb Space Telescope: Revolutionizing Our Understanding of the Universe, modern astronomy increasingly depends on these multi-wavelength, large-scale surveys to connect the dots across cosmic structures.

ACES achieved angular resolution between 1.5 and 2.5 arcseconds and spectral resolution of 0.2 to 3 km/s by combining data from all three ALMA arrays, the 12-meter, 7-meter. And Total Power antennas. That combination gave the team a multiscale view of gas structures from 100-parsec global flows down to 0.05-parsec dense cores where individual stars are beginning to form. Or rather, the findings were presented in five papers accepted for publication in Monthly Notices of the Royal Astronomical Society (a pattern we keep seeing).

What This Changes

The immediate payoff is a coherent explanation for why the galactic center forms stars so slowly despite having so much raw material.The chemical tracers reveal that non-thermal motions, essentially turbulence and magnetic pressure. Are far more dominant in the Central Molecular Zone than in the rest of the galactic disk. Line widths measured by ACES show full-width half-maximum values of 5 to 10 km/s, compared to the roughly 1 km/s typical of quieter molecular clouds. Combined with milligauss-strength magnetic fields, these forces effectively resist gravitational collapse in all but the densest pockets.

For astrochemistry, the survey is a goldmine. Mapping dozens of molecules simultaneously across a single contiguous region lets researchers study how chemistry varies with environment on scales that were never accessible before. Where gas is shocked by expanding supernova shells, silicon monoxide lights up. Where clouds are gently collapsing, nitrogen-bearing molecules dominate.This chemical fingerprinting gives astronomers a powerful diagnostic tool. One that will be applied to observations of galactic centers in other galaxies where individual clouds can't be resolved.

There's also a direct connection to understanding the early universe.

Conditions in the Central Molecular Zone, the high temperatures, strong turbulence. And intense radiation fields, resemble those found in starburst galaxies at high redshift. As The Science Behind the James Webb Telescope: How It Will Revolutionize Astronomy explored, combining data from JWST's infrared capabilities with ALMA's radio observations creates a more complete picture of how galaxies built their stars across cosmic time. ACES gives astronomers a local laboratory to test theories about star formation under extreme conditions before applying them to distant galaxies (at least for now).

Looking Forward

The ACES data release is just the beginning. At the same time, survey's spectral richness means that additional molecular lines not yet analyzed will yield further discoveries as researchers dig deeper into the dataset. Several follow-up proposals are already in the pipeline to target specific regions of interest at even higher resolution, particularly around the circumnuclear disk closest to Sagittarius A* (a pattern we keep seeing).

The broader lesson is that astronomy's next frontier isn't always about building bigger telescopes. Sometimes it's about building bigger mosaics, stitching together observations at scale to capture the connections that isolated snapshots miss. ACES mapped 650 light-years of our galaxy's most extreme environment in a single coherent survey. And in doing so, turned one of astronomy's longest-standing puzzles into something that finally has a clear chemical narrative.

Sources: ESO Press Release eso2603 (March 2026), NRAO Press Release (March 2026), ALMA Science Portal ACES Data Release, Monthly Notices of the Royal Astronomical Society (2026)

James Okafor

ALMA Maps the Hidden Chemistry at the Heart of the Milky Way