Study Shows How Cosmic Rays Affect the Formation of Stars in Galaxies

Young massive stars located in galaxies control the triggering of star formation as well as its quenching. These massive stars inject momentum and energy into the interstellar medium. Feedback obtained from the supermassive black holes at the nuclei of galaxies plays an equally significant role.

Study Shows How Cosmic-Rays Affect the Formation of Stars in Galaxies.
An image of a galaxy seen face-on in a simulation. It shows the distribution of gas over the galaxy (red is higher density and blue is lower density); the clumpiness of the gas is apparent. When cosmic-ray transport is suppressed, the simulations show that this clumpiness is reduced, in turn reducing the star formation activity. Astronomers modeling cosmic-ray influences on star formation have motivated their simulations with gamma-ray observations to investigate cosmic ray transport. Image Credit: Semenov et al., 2021.

Such processes drive the large gas outflows seen in galaxies, for instance. The details, however, such as how they function and the relative functions of various feedback processes, are intensely debated. In particular, cosmic rays are accelerated in powerful shocks created by stellar winds (both aspects of the formation of stars) and supernova explosions and produce significant pressure in the interstellar medium.

Cosmic rays play a major role in controlling thermal balance in thick molecular clouds where the majority of stars form and may play a significant role in driving galactic winds, controlling star formation and even in establishing the character of the intergalactic medium.

According to astronomers, the main property restricting the influence of cosmic rays is the ability to propagate out of the places where they are created into the interstellar medium and further than the disk, but the details are yet to be fully understood.

Vadim Semenov, an astronomer from the Center for Astrophysics, and two other collaborators employed computer simulations to examine how this variation of cosmic-ray propagation can influence the formation of stars in galaxies — driven by new observations of gamma-ray emission from nearby cosmic rays sources, such as supernova remnants and star clusters.

Such observations examine the propagation of cosmic rays because a considerable fraction of gamma-ray emission is assumed to be generated when interactions occur between the cosmic rays and the interstellar gas.

The visualized gamma-ray fluxes indicate that the propagation of cosmic rays close to these sources can be locally inhibited by a considerable factor, up to many orders of magnitude. Theoretical studies indicate that such suppression can lead to nonlinear interactions of cosmic rays with turbulence and magnetic fields.

Using the simulations, the researchers probed the impacts of suppressing the transport of cosmic rays close to the sources. They discovered that suppression promotes the build-up of local pressure and generates intense pressure gradients that inhibit the development of large clumps of molecular gas that form new stars. Thus, it qualitatively alters the global distribution of the formation of stars, particularly in gas-rich, massive galaxies that are likely to form clumps.

The researchers concluded that this cosmic ray effect controls the development of the galaxy’s disk structure and is a crucial complement to other processes that are active in shaping the galaxy.

Journal Reference:

Semenov, V. A., et al. (2021) Cosmic-Ray Diffusion Suppression in Star-forming Regions Inhibits Clump Formation in Gas-rich Galaxies. The Astrophysical Journal.

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