Astronomers working with the Atacama Large Millimeter/submillimeter Array have produced what they describe as the largest image the observatory has ever created, mapping a vast web of cold gas and dust stretching roughly 650 light-years across the center of the Milky Way. The data release, part of the ALMA Central Molecular Zone Exploration Survey known as ACES, captures the region surrounding the supermassive black hole Sagittarius A* in unprecedented detail, revealing compact sources, bright emission zones, and filamentary structures that trace how gas flows through the galactic nucleus. The findings were detailed in a suite of technical papers posted on February 27, 2026, and were accompanied by public summaries highlighting the survey’s scope and early scientific payoffs.
The Largest ALMA Image Ever Made
The Central Molecular Zone, or CMZ, is the dense concentration of molecular clouds that surrounds Sagittarius A* at the Milky Way’s core. It measures about 650 light-years across, and capturing its full extent at high angular resolution has been challenging for previous observations. ACES changed that by stitching together many individual ALMA pointings into a continuous mosaic at 3 mm wavelengths, using the observatory’s Band 3 receivers. The result is the largest contiguous image ALMA has produced, covering the entire CMZ longitude range at spatial scales fine enough to pick out individual compact sources embedded in the gas and to trace the intricate filaments that weave around the galactic center.
The continuum map alone reveals striking features. Bright emission clusters near both Sgr A* and the massive star-forming complex Sgr B2 stand out against a web of fainter structures, according to the ACES continuum analysis, which details the mosaic construction, sensitivity benchmarks, and noise characteristics of the delivered images. Those compact sources, many of which likely correspond to dense clumps of gas or embedded protostars, had been difficult to catalog at this scale before ACES because earlier surveys either lacked the resolution or covered only fragments of the CMZ. A complementary public overview on hidden chemistry at the galactic center emphasizes how the new image transforms isolated snapshots into a coherent portrait of the inner Milky Way.
What the Molecular Lines Reveal
A continuum map shows where matter is concentrated, but it cannot explain what that matter is doing. That is where ACES’ spectral-line data becomes essential. The collaboration released high-resolution data cubes for molecules including HNCO and HCO+, two tracers that respond to very different physical conditions. HNCO tends to light up in warm, shocked gas, while HCO+ traces denser material more closely tied to star formation. Together, they sketch out the velocity field of the CMZ, exposing how gas streams along filaments, collides at intersections, and feeds or avoids the gravitational well of Sgr A*. The molecular line study documents the full mosaic cubes and derived kinematic maps delivered for every survey field, providing a template for future work on cloud dynamics and star-forming environments near supermassive black holes.
A separate set of intermediate-width spectral windows adds SiO and related shock tracers to the picture. SiO is released into the gas phase when dust grains are destroyed by violent collisions, making it a direct marker of energetic events such as cloud-cloud impacts or outflows from young stars. The intermediate-width data paper describes standard derived products including integrated brightness, peak brightness, centroid velocity, and longitude-velocity diagrams. Those products let researchers map not just where shocks are happening but how fast the disturbed gas is moving, a measurement that bears directly on estimates of energy injection into the CMZ. Combined with the continuum image, the line data reveal a complex interplay between relatively quiescent gas streams and violently stirred regions, suggesting that the galactic center cycles between calm and active phases on timescales that astronomers are only beginning to constrain.
Why Mass Flows Matter for the Galaxy
The scientific motivation behind ACES, laid out in the survey overview, centers on a deceptively simple question: how does gas get from the outer edges of the CMZ down to the black hole, and what happens to it along the way? Mass flows and feedback processes in galactic nuclei determine whether gas turns into stars, gets expelled by radiation and winds, or spirals inward to feed the central engine. In the Milky Way, Sgr A* is currently quiet compared to active galactic nuclei in distant galaxies, yet the CMZ still shows signs of turbulence, shocks, and episodic bursts of star formation that suggest the region is far from static. By delineating the web of filaments and clouds that thread the inner few hundred light-years, ACES offers the most direct observational test yet of how gas is funneled toward or kept away from the black hole.
ACES provides the observational backbone to test competing models of that activity. By combining continuum detections of compact sources with velocity-resolved molecular maps, researchers can now track how individual gas streams accelerate, decelerate, or fragment as they approach the galactic center. The filamentary structures visible in HNCO and HCO+ data hint that gas does not flow smoothly inward but instead follows narrow channels, potentially influenced by factors such as magnetic fields or past outburst events. If future multi-epoch ALMA monitoring confirms that velocity dispersions in those filaments change over time, it could support a picture in which episodic activity near the galactic center shapes the surrounding gas more effectively than steady, low-level accretion. That distinction matters well beyond the Milky Way, because the same physics governs nuclear regions in galaxies across cosmic time and influences how rapidly galaxies grow their central black holes.
A Legacy Dataset for Galactic Center Science
ACES is designed as a legacy survey, meaning its data products are intended to serve the broader astronomical community for years. The collaboration has released cubes for all observed fields, full CMZ mosaics, and a suite of derived maps that include moment analyses and kinematic summaries. For context, the region observed as part of the project was explored specifically to understand how gas behaves under the extreme conditions near a supermassive black hole, conditions that include intense radiation fields, strong tidal forces, and magnetic pressures far exceeding those in the solar neighborhood. The combination of high spatial resolution, broad spatial coverage, and sensitive molecular-line spectroscopy makes the ACES dataset a reference point for any future study of the Milky Way’s nucleus.
One gap in the current release is the absence of detailed mass and temperature estimates for the newly cataloged compact sources. The continuum and line papers describe data products and calibration strategies but leave the physical characterization of individual objects to follow-up work that will combine ALMA measurements with infrared and X-ray observations. That separation reflects a deliberate choice: by first establishing a robust, homogeneous survey of the CMZ, the ACES team ensures that subsequent analyses of star formation efficiency, cloud lifetimes, and feedback energetics rest on a consistent observational foundation. As additional teams mine the public cubes for new structures and kinematic patterns, the survey is expected to seed a broad range of studies, from the demographics of nascent star clusters to the signatures of past outbursts from Sgr A*.
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*This article was researched with the help of AI, with human editors creating the final content.
