Astronomers have unveiled the most extensive high-resolution view of the Milky Way’s inner region ever produced with the Atacama Large Millimeter/submillimeter Array (ALMA). The new mosaic, released by the European Southern Observatory (ESO), captures the cold molecular gas that fills the Central Molecular Zone (CMZ) around our galaxy’s core. The image comes from the ALMA Central Molecular Zone Exploration Survey, widely known as ACES.

The dataset traces cold molecular gas with uniform sensitivity and fine angular resolution. As a result, it provides an essential framework for understanding star formation and gas dynamics near the Galactic Centre. For decades, the CMZ has challenged observers. Optical light cannot penetrate the thick dust that blankets the region. Earlier radio surveys revealed fragments of its structure, but they lacked either coverage or resolution. The new ALMA mosaic sheds new light on the situation.

The galactic centre demands millimeter observations

The Milky Way’s centre lies roughly 26,000 light-years away in the direction of Sagittarius. Dense interstellar dust obscures the region at optical wavelengths. As a result, visible-light telescopes cannot provide a clear view of the cold gas that fuels star formation there.

Millimeter and submillimeter observations offer a solution. These longer wavelengths pass through dust and trace molecular emission lines. They allow astronomers to map the cold, dense phase of the interstellar medium. This phase is especially important because stars form from molecular gas.

The Central Molecular Zone occupies the inner few hundred light-years of the Galaxy. It contains massive gas clouds, strong turbulence, elevated temperatures, and intense radiation fields. In addition, the gravitational influence of the central supermassive black hole shapes gas motions throughout the region.

These extreme conditions make the CMZ very different from typical star-forming regions in the Galactic disk. However, until recently, astronomers lacked a uniform, high-resolution map of the entire zone. Most previous studies focused on selected targets rather than the full environment. ACES was designed to overcome that limitation.

Inside the ACES survey

The ACES program represents one of the most ambitious Galactic mapping efforts conducted with ALMA. The team aimed to image the CMZ with both high spatial resolution and continuous coverage. Achieving both goals required careful coordination of multiple array configurations.

The survey used the 12-meter array to capture fine structural detail. It also employed the 7-meter array and total power antennas to recover extended emission. By combining these datasets, researchers ensured sensitivity to structures spanning a wide range of spatial scales.

Observations took place between late 2021 and 2023. The team selected ALMA Band 3, centered near the 3-millimeter wavelength range. This band provides strong molecular line coverage while maintaining good angular resolution.

The final mosaic spans more than 650 light-years across the Galactic Centre. In the sky, the mapped region spans roughly three full moons. The dataset comprises the largest ALMA mosaic to date.

Equally important, the survey maintains nearly uniform sensitivity across the entire field. This consistency allows reliable comparisons between different parts of the CMZ. Earlier patchwork surveys could not provide that level of uniformity.

A web of molecular filaments

One of the most striking outcomes of the ACES survey is the clear filamentary structure of the CMZ. The dense gas does not appear as isolated clouds. Instead, it forms an interconnected web of elongated features that thread through the Galactic Centre.

Some filaments extend for roughly ten parsecs. Others measure closer to one parsec in length. Both size scales appear repeatedly across the region. In many cases, the filaments remain coherent in both position and velocity space. This behavior suggests organized gas flows rather than random turbulence alone.

The larger filaments likely trace global orbital streams that carry material around the Galactic Centre. Meanwhile, the smaller filaments probably arise from local shear and turbulent compression within the molecular clouds.

This structural hierarchy has implications for star formation. Dense filaments often act as the sites where gas accumulates and fragments into gravitationally bound cores. By mapping these structures across the entire CMZ, ACES allows astronomers to follow the pathways that funnel gas into potential star-forming regions. Earlier observations hinted at such networks but covered only limited areas. The new mosaic finally shows the filament system in its full spatial context.

Building the largest ALMA mosaic

Producing the ACES image required extensive observing time and careful data processing. The team combined more than forty individual sub-mosaics into one continuous map. Each pointing captured a small patch of the CMZ. Researchers then calibrated and stitched the data together with consistent weighting.

The survey targeted key molecular transitions such as HCO⁺ and HNCO lines. These transitions provide sensitive tracers of dense gas and allow precise velocity measurements. High spectral resolution ensures that even subtle gas motions remain detectable.

The multi-array approach proved essential. The 12-meter antennas supplied high angular resolution. The 7-meter array filled in intermediate spatial scales. Total power observations recovered the most extended emission. Without this layered strategy, the final map would miss critical structural information.

The completed mosaic preserves both compact clumps and large-scale filaments. That combination makes the dataset uniquely valuable for studying the full gas ecosystem of the Galactic Centre.

Clear skies!

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