Gamma rays from a dwarf galaxy solve an astronomical puzzle

Gamma rays from a dwarf galaxy solve an astronomical puzzle

Credit: NASA Goddard Space Flight Center

A bright spot known as the ‘cocoon’, which appears to be inside one of the huge gamma-ray emanations from the center of our galaxy dubbed the ‘Fermi bubbles’, has puzzled astronomers since its discovery in 2012.

In new research published in natural astronomy, we show that the cocoon is caused by gamma rays emitted by fast-rotating extreme stars called “millisecond pulsars” located in the Sagittarius dwarf galaxy, which orbits the Milky Way. While our results clarify the mystery of the cocoon, they also cast a shadow over attempts to search for dark matter in any gamma glow it may be emitting.

See with gamma rays

Fortunately for life on Earth, our atmosphere blocks out gamma rays. They are particles of light with energies more than a million times greater than the photons we detect with our eyes.

Because our view at ground level is obscured, scientists had no idea how rich the sky was in gamma rays until the instruments were blasted into space. But, beginning with the chance discoveries of the Vela satellites (put into orbit in the 1960s to monitor the nuclear test ban), more and more of this wealth has been revealed.

The state-of-the-art gamma-ray instrument in operation today is the Fermi Gamma Ray Space Telescope, a major NASA mission that has been in orbit for more than a decade. Fermi’s ability to resolve fine detail and detect faint sources has revealed a number of surprises about our Milky Way and the wider cosmos.

mysterious bubbles

One such surprise appeared in 2010, shortly after the launch of Fermi: something at the center of the Milky Way is blowing out what looks like a pair of giant bubbles emitting gamma rays. These completely unexpected “Fermi bubbles” cover 10% of the sky.

A prime suspect for the source of the bubbles is the galaxy’s resident supermassive black hole. This mastodon, 4 million times more massive than the sun, hides in the galactic core, the region from which the bubbles emanate.

Most galaxies host such giant black holes at their center. In some, these black holes are actively swallowing up matter. Thus fed, they simultaneously spit out giant, outgoing “jets” visible across the electromagnetic spectrum.

So, a question researchers posed after the bubbles were discovered: can we find an irrefutable gun linking them to our galaxy’s supermassive black hole? Soon, tentative evidence emerged: there was a hint, inside each bubble, of a fine jet of gamma rays pointing towards the galactic center.

With time and other data, however, this image has become blurred. While the jet-like feature in one of the bubbles was confirmed, the apparent jet in the other appeared to evaporate under scrutiny.

The bubbles seemed strangely unbalanced: one contained an elongated bright spot – the “cocoon” – with no counterpart in the other bubble.

The cocoon and where it came from

Our recent work in natural astronomy is an in-depth examination of the nature of the “cocoon”. Remarkably, we discovered that this structure had nothing to do with Fermi bubbles or, indeed, the galaxy’s supermassive black hole.

Rather, we discovered that the cocoon is actually something entirely different: gamma rays from the Sagittarius dwarf galaxy, which lies behind the southern bubble as seen from Earth’s position.

The Sagittarius Dwarf, so called because its position in the sky is in the constellation Sagittarius, is a “satellite” galaxy orbiting the Milky Way. It’s the remnant of a much larger galaxy that the strong gravitational field of the Milky Way literally tore apart. Indeed, the stars torn from the dwarf of Sagittarius are found in “tails” that wrap around the sky.

Gamma rays from a dwarf galaxy solve an astronomical puzzle

Diagram showing the Milky Way, the gamma-emitting Fermi Bubbles (pink) and the Sagittarius dwarf galaxy and its tails (yellow/green). From the sun’s position, we see the Sagittarius dwarf through the southern Fermi Bubble. Credit: Aya Tsuboi, Kavli IPMU, author provided

What do gamma rays do?

In the Milky Way, the main source of gamma rays is when high-energy particles, called cosmic rays, collide with the very tenuous gas between stars.

However, this process cannot explain the gamma rays emitted by the Sagittarius dwarf. Long ago, it lost its gas to the same gravitational stalking that drove so many of its stars away.

So where do gamma rays come from?

We considered several possibilities, including the exciting prospect that they are a signature of dark matter, the invisible stuff known only by its gravitational effects that astronomers say makes up much of the universe. Unfortunately, the shape of the cocoon closely matches the distribution of visible stars, which rules out dark matter as the origin.

Somehow the stars were responsible for the gamma rays. And yet: the stars of the Sagittarius dwarf are old and at rest. What type of source among such a population produces gamma rays?

Millisecond Pulsars

We are convinced that there is only one possibility: rapidly rotating objects called “millisecond pulsars”. These are the remnants of particular stars, significantly more massive than the sun, which also orbit closely around another star.

Under the right circumstances, such binary systems produce a neutron star – an object about as heavy as the sun but about 20 km in diameter – which spins hundreds of times per second.

Due to their rapid rotation and strong magnetic field, these neutron stars act as natural particle accelerators: they launch very high-energy particles into space.

These particles then emit gamma rays. The Sagittarius dwarf’s millisecond pulsars were the ultimate source of the mysterious cocoon, we’ve discovered.

The hunt for dark matter

Our findings have shed new light on millisecond pulsars as sources of gamma rays in other ancient star systems.

At the same time, they also cast a veil over efforts to find evidence of dark matter via observations of other Milky Way satellite galaxies; unfortunately, there is a stronger “background” of gamma rays from millisecond pulsars in these systems than previously thought.

Thus, any signal they produce might not be unambiguously interpreted as being due to dark matter.

The hunt for dark matter signals continues.


Spinning stars shed new light on strange signal from galactic center


More information:
Roland M. Crocker et al, Gamma-Ray Emission from the Sagittarius Dwarf Spheroidal Galaxy Due to Millisecond Pulsars, natural astronomy (2022). DOI: 10.1038/s41550-022-01777-x

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