A radio telescope just took this giant picture of the Milky Way's core

Gas streams gush into the Milky Way's center and pile up into thick clouds, but for some reason, those ingredients don't materialize into as many stars as astronomers would expect.

An international team has mapped nearly the entire 650 light-year span of the galaxy's core to try to figure out why. Using the Atacama Large Millimeter/submillimeter Array in Chile, the researchers took the most detailed image of its kind — and the facility's largest mosaic picture to date. The effort, called the ALMA Central Molecular Zone Exploration Survey, traces the cold gas and dust that fuel star birth in space.

The survey targets the so-called Central Molecular Zone, which contains tens of millions of times the sun's mass in dense material, surrounding the supermassive black hole, Sagittarius A*. The region runs warmer and far more chaotic than most of the Milky Way's disk. By standard estimates, that much dense gas should produce stars at a steady pace. Instead, the region forms them about 10 times slower than predicted.

That mismatch has puzzled astronomers for years. Getting answers would inform how researchers understand galaxies more broadly, and the heart of the Milky Way offers the only galactic center close enough for this type of intricate study.

"It's a place of extremes, invisible to our eyes, but now revealed in extraordinary detail," said Ashley Barnes, a European Southern Observatory astronomer who helped obtain the new data, in a statement.

The researchers' findings are laid out in five papers accepted for publication in Monthly Notices of the Royal Astronomical Society, with a sixth on the way. 

The new survey stands out because prior ones forced a tradeoff. Some covered wide areas but blurred the small structures that matter most. Others zoomed in on a handful of clouds but lost sight of how those clouds connect to the bigger picture.

But the ALMA survey, called ACES for short, accomplishes both in one fell swoop. It reveals structures small enough to pinpoint dense clumps that could host developing stars — while still spanning almost the entire star-forming gas supply in the core.

The team also measured more than 70 chemical fingerprints in the gas, such as silicon monoxide, methanol, and acetone. Each responds to different physical conditions, so by comparing them, researchers can assess density, temperature, and motion, and identify where shocks ripple through clouds or where young stars add energy.

Those measurements allow scientists to trace how gas flows inward, where it bunches up, and where it finally collapses into stars. Or, where it fails to.

"The [core] hosts some of the most massive stars known in our galaxy, many of which live fast and die young," said ACES leader Steve Longmore, an astrophysics professor at Liverpool John Moores University in the United Kingdom, in a statement. 

Several ALMA antennas in the Atacama Desert of northern Chile point toward the central regions of the Milky Way above. Credit: ESO / B. Tafreshi

Some of those enormous stars end their lives in hypernovas, sometimes called super-supernovas, Longmore said, releasing more than 10 times the energy of a normal supernova. Those colossal star explosions usually collapse into black holes and are thought to produce long gamma-ray bursts, the universe's most powerful explosions. 

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The researchers paired the observations with computer simulations that follow gas as it streams along the galactic bar, forms clouds, and reacts to radiation and explosions. They turn those simulations into mockups and compare them directly with the survey data to test which scenarios best match what they actually see.

Armed with the full data, the team will now probe where star formation switches on and off along the gas stream, whether certain orbital points trigger collapse, and how gravity and other factors compete to control the rate of star birth.

"We believe the region shares many features with galaxies in the early universe," Longmore said, "where stars were forming in chaotic, extreme environments."