Too many supernovae can slow star formation in a galaxy

Interstellar winds are powerful agents of change. First, they can interrupt or completely shut down the star birth process. That’s what a team of astronomers using the Karl Jansky Very Large Array in New Mexico discovered when they studied the galaxy M33. They also learned that fast cosmic rays play a huge role in driving these winds into interstellar space.

The idea that winds from supernova explosions and jets from galactic nuclei could “quench” star formation is not new. They essentially rob protostars of the gas and dust they need to form.

Here’s an interesting twist. When these supernovae occur, they shoot out large numbers of cosmic rays. The more supernovae “explode”, the more cosmic rays are emitted. They then exert a greater influence on the interstellar winds that eventually end up destroying the star nurseries.

“We have seen galactic winds driven by cosmic rays in our own Galaxy and in the Andromeda Galaxy, which have much slower star formation rates, but not before in a galaxy like M33,” said Fatemah Tabatabaei of the Institute for Fundamental Research 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

Star formation and galaxy building are closely related. In general, galaxies begin as small collections of stars, formed in hydrogen-rich clouds. Galaxies grow by merging with each other. Smaller ones mix to create bigger ones. The larger ones also collide and merge. Quite often, these mergers trigger new episodes of star birth. Our own Milky Way developed this way, as did M33.

Stars continue to form in clouds of hydrogen gas with dust scattered throughout the galaxies. Star formation bursts eat up the available gas and dust and this affects the galactic shape (or morphology). Additionally, as stars age, they contribute the elements they create in their cores back into the interstellar medium (ISM). These materials end up in the next generations of stars and planets. And, as the team led by Tabatabaei found, the most massive stars produce cosmic rays when they die. These fast-moving particles push winds through interstellar space and interact with magnetic fields.

Large amounts of cosmic rays create a pressure front that slams into stellar nurseries filled with gas and dust. The action of the wind breaks up the clouds and carries away the necessary stellar building blocks. Essentially, cosmic rays drive winds that extinguish star formation. This can be quite detrimental to a growing galaxy, which should be rich in star-forming regions. This is why it is important to study the ISM and detect the creation and emission of cosmic rays during the death of stars.

What the VLA observed in M33

The VLA observations allowed Tabatabaei’s team to study the ISM in M33 in regions as small as 30 parsecs (just under a hundred light-years) in width. They could look at regions of star formation as well as regions where no stars were being born. Both areas are important for understanding what processes and events can affect star formation.

“VLA observations have shown that cosmic rays in M33 are escaping from the regions where they are born, making them capable of driving more extended winds,” said William Cotton of the National Radio Astronomy Observatory. Based on these observations, astronomers suspect that many supernova explosions and supernova remnants in M33’s highly active star-forming regions made such cosmic-ray-driven winds more likely.

“This means that cosmic rays are probably a more general cause of galactic winds, particularly at earlier times in the universe’s history when star formation occurred at a much higher rate,” Tabatabaei said. He added: “This mechanism thus becomes a more important factor in understanding the evolution of galaxies over time.”

The team hopes that similar studies of large samples of galaxies beyond M33 will provide more information about the ray-driven cosmic winds that can disrupt star formation. In particular, facilities such as the Square Kilometer Array (SKA) and ngVLA (next generation VLA) should be suitable for the survey of other galaxies both in the modern universe and galaxies earlier.

For more information

VLA Finds Cosmic Rays Driving Galactic Winds
Cloud-scale radio surveys of star formation and feedback in the triangular galaxy M 33: VLA observations