'Wasteful' galaxies discard raw ingredients

A new study reveals that galaxies are not as efficient at creating stars as originally thought.

A simulation of a spiral galaxy, seen in white at the centre, surrounded by the circumgalactic medium (CGM), which appears as black. The CGM also contains very hot gas, shown in red, orange and white.
Credit: Adrien Thob, LJMU

Galaxies are wasteful and do not efficiently recycle the elements they produce during the creation of stars, according to a new study.

Heavy elements such as oxygen, carbon and iron are produced as stars form, and are also used in the formation of new stars. But the process of recycling stellar ingredients is not as efficient as might be expected.

“Our findings indicate that most of the atoms forged throughout the formation of a galaxy are ejected in violent outflows of gas, driven by the growth of black holes and supernovae - the explosions that mark the death of massive stars,” says Dr Robert Crane of Liverpool John Moores University, who contributed to the study.

Galaxies are surrounded by what is known as the circumgalactic medium (CGM); massive clouds of gas thought to play a significant role in supplying their host galaxy with fresh elements. While a typical galaxy ranges from about 30,000 to 100,000 lightyears in size, CGMs typically span up to a million lightyears.

Using the Cosmic Origin Spectograph on the Hubble Space Telescope, the team studied the CGMs of two types of galaxies: spirals and ellipticals. Spiral galaxies form stars and glow bright blue, while elliptical galaxies form fewer stars and appear red. Both types contain tens to hundreds of billions of stars that themselves create heavy elements.

Inputting the Hubble data into a computer simulation, the team found that the CGMs in both types of galaxy contained more than half of that galaxy’s heavier elements. The result suggests that galaxies are not so efficient at retaining the raw materials for star formation.

“Previously, we thought that these heavier elements would be recycled in to future generations of stars and contribute to forming planetary systems and providing the building blocks of life,” says Benjamin Oppenheimer, lead author of the study. “As it turns out, galaxies aren’t very good at recycling.”


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