Star Formation Breaks Its Own Rules at the Centre of the Milky Way

A vast new radio telescope mosaic of the galaxy’s turbulent core is forcing astronomers to rethink where and why stars form — and has already thrown up one object nobody can explain.

Cold gas streams through the heart of the Milky Way in ribbons and filaments, feeding on itself, crashing into dense clumps, occasionally igniting a star — and mostly, for reasons nobody properly understands, not doing so. This is the Central Molecular Zone, the inner hundred light-years of our galaxy, packed with more raw star-forming material than almost anywhere else in the cosmos. It surrounds Sagittarius A*, the supermassive black hole four million times the mass of the sun. And until very recently, we had never mapped it properly.

That changes now. A team of more than 160 astronomers, working across 70 institutions on five continents, has stitched together the largest image the Atacama Large Millimeter/submillimeter Array has ever produced — a mosaic three full moons wide, charting the chemistry and motion of cold molecular gas across the entire inner core.

The project, called ACES — the ALMA CMZ Exploration Survey — has been years in the making, its results now appearing across five papers in Monthly Notices of the Royal Astronomical Society. What it reveals is a region of almost theatrical complexity: dense clouds threaded with filaments of silicon monoxide and isocyanic acid, expanding shells of shocked gas left by ancient explosions, nested streams of material orbiting the galactic centre on tilted, inclined paths. “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 in Germany who co-leads the survey. The data span scales from about 650 light-years down to individual gas clouds barely a tenth of a light-year across.

The CMZ is, in one sense, familiar territory — it’s been studied across the electromagnetic spectrum for decades, from X-ray to radio. But there’s a fundamental problem with almost everything we thought we knew about it.

Stars should be forming there at a furious rate. Standard models of star formation, calibrated on gas-rich regions throughout the Milky Way’s disc, predict the CMZ ought to be churning out roughly half a solar mass of new stars every year. The actual measured rate is closer to a tenth of that — stubbornly, persistently lower than theory says it should be.

One long-standing explanation is that the environment in the CMZ is simply too violent for stars to form efficiently: the gas is warmer, more turbulent, and bathed in a far stronger cosmic-ray flux than gas in the outer galaxy, all of which can suppress the gravitational collapse that creates stars. But it’s a hard hypothesis to test without a complete, uniform picture of how that gas is actually distributed and moving.

ACES was designed specifically to provide that picture. Using ALMA’s Band 3 receivers, the survey tracked more than 70 different spectral features, each a molecular fingerprint for a particular physical condition. Sulphur monoxide lights up in shocked gas; silicon monoxide traces violent collisions between clouds; isocyanic acid maps denser, quieter regions; hydrogen recombination lines reveal where gas has been ionised by young hot stars. “By studying how stars are born in the CMZ, we can also gain a clearer picture of how galaxies grew and evolved,” says Steve Longmore, professor of astrophysics at Liverpool John Moores University and the survey’s principal investigator. “We believe the region shares many features with galaxies in the early Universe, where stars were forming in chaotic, extreme environments.”

That early-universe comparison matters more than it might first appear. Conditions in the CMZ — high gas density, extreme turbulence, intense radiation — broadly resemble those found in actively star-forming galaxies at redshifts of one to three.

Early results have already thrown up some surprises. One is a nested system of six inclined spiral arms orbiting Sagittarius A*, identified in the ACES kinematic data by Yoshiaki Sofue of the University of Tokyo and colleagues.

Another is the M0.8−0.2 ring, a shell of shocked molecular gas six light-years across, containing nearly a million solar masses of material and expanding at roughly 21 kilometres per second. The energetics point to a single catastrophic explosion — not a supernova of the kind that ends most massive stars, but something considerably more violent, probably a hypernova from a runaway star. “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,” Longmore says. It’s a reminder that the galactic centre is a place where the upper end of stellar violence gets properly exercised.

And then there is the MUBLO. Discovered during routine data quality checks, the Millimeter Ultra-Broad Line Object sits near the 50 km/s cloud — compact, cold (around 13 degrees above absolute zero), and emitting molecular lines so extraordinarily broad, spanning 160 kilometres per second, that no known class of object quite fits. It contains roughly 50 solar masses of dust, yet shows no infrared emission, no X-rays, no hint of ongoing star formation. Whether it is a protostellar outflow, a stellar merger remnant, or gas bound to an intermediate-mass black hole remains genuinely unclear. It may be something new altogether.

“In many ways, this is just the beginning,” says Barnes, pointing to planned upgrades of ALMA and the imminent arrival of the Extremely Large Telescope, which between them should push the survey’s reach down to finer structures and stranger chemistry still. Whatever those instruments find lurking in the gas, the CMZ has already made clear it will not yield its secrets quietly.

Study link: https://almascience.nrao.edu/alma-data/lp/aces