Milky Way galaxy is not ejecting stars, and astronomers know why

The center of our galaxy is quiet in a very specific way. A new paper describes a search for stars flung from the middle of the Milky Way and translates its silence into a quantifiable measurement.

At first glance, the galaxy’s silence might seem unremarkable. But to astronomers, what they do not find can be just as revealing as what they do.

If the black hole at the center of the Milky Way had recently merged with another massive object, the aftermath should have left a visible trail: stars tossed outward at unusual speeds, spread across the galactic halo.

Their absence paints a picture of a calm and stable core, one that has not seen a major collision for billions of years.

This new work uses some of the world’s most advanced sky surveys to search for those missing messengers, and in doing so, helps scientists refine the story of how our galaxy grew and evolved.

Why the Milky Way’s center matters

At the Milky Way’s core sits a supermassive black hole called Sgr A*, which weighs about four million Suns and dominates the Galactic Center environment, a dense, turbulent region packed with stars, gas, and powerful magnetic fields.

Smaller black holes are scattered throughout the Milky Way. Astronomers know there are countless stellar-mass black holes.

Each formed from the collapse of a massive star, and they suspect a few elusive intermediate-mass black holes bridging the gap between the small and the supermassive.

Near Sgr A*, these compact objects can sling nearby stars into space at astonishing speeds.

One of the best examples is S5-HVS1. It’s a hypervelocity star racing outward at about 1,100 miles per second, a speed that traces directly back to the galactic center and serves as a benchmark for studies of stellar ejection. 

That kind of launch has a name. The Hills mechanism is when a binary pair passes close to a massive black hole, one star gets captured, and the other flies off.

How to spot a castaway star

This search did not chase the fastest stars. It looked for slower castaways that stayed bound to the galaxy and have had billions of years to drift into the halo.

The team predicted what these stars should look like in the data. They would show high metallicity, which is a higher fraction of elements heavier than hydrogen and helium.

In addition, they should see very small vertical angular momentum, called LZ, because a star shot almost straight out of the center starts with little spin around the disk.

That creates a neat filter. The researchers explained that they used the first data release from the Dark Energy Spectroscopic Instrument (DESI) to look for slower stars ejected from the Galactic Center.

Castaway stars should stand out from the rest of the halo because of their unusually high metallicity and very low vertical angular momentum, values that tend to cluster near zero.

Discoveries at the Milky Way’s center

The DESI surveys the sky by capturing millions of detailed light spectra from stars and galaxies.

Mounted on the Mayall Telescope in Arizona, it is designed to map the universe’s expansion and study the role of dark energy by precisely measuring the distances and motions of galaxies and quasars.

This project made creative use of that powerful resource. By combining the first DESI data release with precise Gaia measurements, the researchers were able to calculate detailed orbits and chemical compositions for hundreds of thousands of stars across the Milky Way.

The analysis revealed no evidence of a true population of bound ejected stars. That absence became the key finding of the study.

The team used this lack of evidence to estimate how rarely the Milky Way’s center ejects stars.

Their analysis suggests that, over the past five billion years, such events have been extremely uncommon, occurring no more than a few times every thousand years.

The researchers noted that metallicity is a powerful clue because no other known stellar populations in the halo share the same chemical richness as stars from the Galactic Center.

That’s why the Milky Way’s center it a key marker for identifying potential ejected stars.

Black holes at the Milky Way center

If Sgr A* had collided with another massive black hole in the past few billion years, theory says it would have tossed many more stars into the halo.

A binary at the center presents a bigger target for slingshots and can fire stars at a higher rate.

A burst of ejecta would leave a long lived fingerprint. It should show up as a cluster of metal rich, low LZ stars far from the center, and that is the kind of signal this search was tuned to catch.

The absence of that signal narrows the options.

In their conclusion, the authors explain that a detection could have pointed to either Hills disruptions or past mergers, and the lack of bound ejecta constrains the merger history on billion year timescales. 

These rare objects give us a way to probe the deep potential at the center without fighting through clouds and dust. They also act as tracers of the Milky Way’s gravity field on large scales.

A broad review explains how their speeds and paths test both the launch mechanism near the center and the mass of the galaxy itself. That is one reason the clean case of S5 HVS1 is so important.

Reading the halo without confusion

The halo is messy. Streams from ancient dwarf galaxies, crisscross the sky and overlap in speed and chemistry. That overlap can hide a weak signal.

The team’s strategy of selecting by metallicity and angular momentum is designed to cut through that clutter without making strong assumptions.

It is important that the threshold is conservative. The work is careful not to label unrelated stars as ejecta, which keeps the limit meaningful.

DESI will keep releasing larger samples. Gaia’s next catalog will sharpen proper motions and distances, which tightens angular momentum and metallicity cuts.

Future surveys will be able to build on this approach. New studies could eventually reveal whether such stars were launched by the Hills mechanism, when a binary system is torn apart by Sgr A*,  or by past mergers between the central black hole and other massive companions.

The study is published in Astronomy and Astrophysics.

Image credits: ESO.

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