Interstellar winds are powerful agents of change. On the one hand, they can interrupt or completely stop the process of star birth. That’s what a team of astronomers using the Karl Jansky Very Large Array in New Mexico found when they studied the galaxy M33. They also learned that fast cosmic rays play a huge role in pushing these winds through interstellar space.
The idea that winds from supernova explosions and jets from galactic nuclei could “smother” star formation is not new. Essentially, they starve protostars of the gas and dust they need to form.
Now here’s an interesting twist. When these supernovae occur, they eject large numbers of cosmic rays. The more supernovae that “jump”, the more cosmic rays are emitted. Then they exert more influence on the interstellar winds which eventually destroy the birth nurseries of the stars.
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“We have seen galactic winds driven by cosmic rays in our own Milky Way and the Andromeda Galaxy, which have much lower star formation rates, but not before in a galaxy such as M33,” said Fatemah Tabatabaei, from the Research Institute of Basic Sciences in Iran. Tabatabaei and a team of scientists used the VLA, the Effelsberg radio telescope in Germany, and a selection of millimeter wave, visible light and infrared telescopes to observe this nearby galaxy.
Cosmic rays, star formation and galaxies
The formation of stars and the construction of galaxies are intimately linked. Generally speaking, galaxies start out as small collections of stars, which form in hydrogen-rich clouds. Galaxies grow by merging. The smaller ones mingle to create larger ones. Larger ones also collide and merge. Quite often, these mergers stimulate new episodes of star birth. Our own Milky Way grew this way, as did M33.
Stars continue to form in clouds of hydrogen gas mixed with scattered dust in galaxies. Bursts of star formation eat up available gas and dust and this affects the galactic shape (or morphology). Additionally, as stars age, they contribute the elements they manufacture in their cores to the interstellar medium (ISM). These materials are found in the next generations of stars and planets. And, as the team led by Tabatabaei discovered, the most massive stars generate cosmic rays when they die. These fast-moving particles push winds through interstellar space and interact with magnetic fields.
Large amounts of cosmic rays build up a pressure front that slams into stellar nurseries filled with gas and dust. The action of the wind causes the clouds to burst and carry away the necessary stellar building blocks. Essentially, cosmic rays drive winds that stifle star formation. This can be quite damaging for a growing galaxy, which should be rich in star birth regions. This is why it is important to study the ISM and to trace the creation and emission of cosmic rays during the death of stars.
What VLA observed at M33
VLA observations allowed Tabatabaei’s team to study ISM in M33 in regions as small as 30 parsecs (just under a hundred light-years). They could look at star forming regions as well as areas where no star births have taken place. Both of these areas are important for understanding what processes and events can affect star formation.
“VLA observations indicated that cosmic rays from M33 are escaping 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 these observations, astronomers suspect that numerous supernova explosions and supernova remnants in the highly active star-forming regions of M33 have made these 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.”
The team hopes that similar studies in large samples of galaxies beyond M33 will yield more information about cosmic-ray-driven winds that can disrupt star formation. In particular, facilities such as the Square Kilometer Array (SKA) and the ngVLA (next generation VLA) should be well suited to study other galaxies in the modern universe as well as older galaxies.
For more information
VLA finds cosmic rays driving galaxy winds
Cloud-scale radio records of star formation and feedback in the triangle galaxy M 33: VLA observations
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