A Mysterious Barrier Is Blocking Cosmic Rays at the Heart of the Milky Way

New observations of the center of our galaxy, the Milky Way, have revealed that something is blocking cosmic rays from passing into it.

As well as suggesting the Central Molecular Zone (CMZ)—that rings the center of the Milky Way—is somehow blocking cosmic rays, the findings could indicate the presence of a massive high-energy natural particle accelerator at the heart of our galaxy.

The research, published in the journal Nature Communications, could help researchers explain the origins of cosmic rays, high-energy particles that originate outside the solar system and bombard Earth causing high-energy particle collisions in the upper atmosphere.

Despite being discovered over a century ago, researchers still don't know exactly what causes cosmic rays, that create cascades of secondary particles which rain down upon the surface of our planet.

"The short answer to why we can't trace cosmic rays back to their source: magnetic fields. A prediction of the exact path of a cosmic-ray particle is impossible," professor at the School of Physics and Astronomy at Ruhr University in Bochum, Germany, Julia Tjus, told Astronomy.

"It's as if it wandered through the universe like a ball in a pinball machine."

The CMZ is an area of our galaxy already shrouded in mystery. In fact, the CMZ, located 27,000 light-years away from Earth, is unlike any other place in our galaxy.

The region, which stretches for 700 light-years, is home to dense clouds of gas and is shrouded by dust that prevents astronomers from taking a good look at the region and the center of the Milky Way which it surrounds, in detail.

As this gas is the raw material needed for star formation, the CMZ is an ideal place to study star-birth.

Yet, we know that the CMZ, with a mass 60 million times that of the sun and home to most is the source of unexplained radio signals and glowing gamma-ray emissions

The authors of this study looked toward the CMZ to study cosmic rays, particles that are accelerated to high-energy states, and near light-speed by powerful cosmic events such as supernovas and stellar winds from massive stars.

Previous research has suggested that cosmic rays are fairly evenly distributed throughout the Milky Way, but what the authors of this paper discovered from data collected by Fermi Large Area Telescope was that there were less cosmic rays at the center of our galaxy than expected.

This means that something is blocking these rays at the Milky Way's center.

The researchers believe that it could be the magnetic fields associated with the molecular clouds of gas in the CMZ that is acting as a barrier against the transmission of cosmic rays.

They write: "Several mechanisms can impede cosmic ray penetration into molecular clouds, such as the effect of magnetic field compression and the self-excited magnetohydrodynamics turbulence."

The authors compare this effect to how magnetic fields associated with solar winds from the sun can block low-energy cosmic rays from entering the solar system.

The blocking effect could, alternatively, be caused by the material blown in galactic winds by the supermassive black hole that sits at the center of the Milky Way, Sagittarius A*.

The researchers also suggest that Sagittarius A* could be the powerful cosmic particle accelerator that they detected, or that this effect may be created by pulsars—rapidly rotating neutron stars—or supernovas, the explosive demise of dying stars.

The authors of the paper concede that the current research is limited by projection effects in the modeling of the gas cloud and the cosmic ray density, common to 3D modeling.

The researchers conclude their paper by suggesting that further investigation of the CMZ and the center of the Milky Way and more 3D precise models might help us finally crack the mysteries of cosmic rays.

Central Molecular Zone
An infrared image of the Central Molecular Zone of the Milky Way. New research indicates that something with the dense clouds of gas and dust is acting as a barrier preventing high-energy cosmic rays from reaching the centre of our galaxy. Susan Stolovy SSC/Caltech et al./ SPitzer/IRAC/NASA