Cosmic rays drive the winds of the galaxy

Astronomers using the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) have discovered an important new clue to how galaxies dampen vigorous episodes of star formation. Their new study of nearby galaxy M33 indicates that fast-moving cosmic ray electrons can drive winds that carry away the gas needed to form new stars.

These winds are responsible for slowing the rate of star formation as galaxies evolve over time. However, shock waves from supernova explosions and black hole-powered jets of energetic matter from galactic nuclei have been thought to be the main drivers of these winds. Cosmic rays were thought to be minor contributors, especially in galaxies like M33 that have regions of prolific star formation.

“We have seen galactic winds driven by cosmic rays in our own Milky Way and in the Andromeda Galaxy, which have much lower star formation rates, but not before in a galaxy such as M33,” said said Fatemah Tabatabaei, of the Research Institute of Basic Sciences in Iran.

Tabatabaei and an international team of scientists have made detailed multi-wavelength VLA observations of M33, a spiral galaxy nearly 3 million light-years away and part of the Local Group of galaxies that includes the Milky Way. They also used data from previous observations with the VLA, the Effelsberg radio telescope in Germany, and millimeter wave, visible light and infrared telescopes.

Stars much more massive than our Sun go through their life cycle to eventually explode as supernovae. Explosive shock waves can accelerate particles to nearly the speed of light, creating cosmic rays. Enough of these cosmic rays can create pressure that causes the winds to carry away the gas needed to keep forming stars.

“VLA observations indicated that cosmic rays in M33 escape from the regions where they originated, making them capable of conducting more extensive winds,” said William Cotton of the National Radio Astronomy Observatory.

Based on their observations, the astronomers concluded that the numerous supernova explosions and supernova remnants in the prolific star-forming giant complexes of M33 made such cosmic-ray driven winds more likely.

“This means cosmic rays are likely a more general cause of galactic winds, especially at earlier times in the history of the universe when star formation was occurring at a much higher rate,” he said. Tabatabaei. She added: “This mechanism thus becomes a more important factor in understanding how galaxies evolve over time.”

Tabatabaei, Cotton and their colleagues report their findings in the October 25 issue of the Royal Astronomical Society Monthly Notices.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc.

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