‘Wasteful’ habits of star-forming galaxies highlighted in new study

The tendency of galaxies to eject oxygen, carbon and iron atoms produced by star formation into their surrounding halos and even into deep space leave them devoid of the raw materials required to build additional stars and planets, according to a recently-published study.

While these heavy elements are essential to such processes, Benjamin Oppenheimer, a research associate working in the Center for Astrophysics & Space Astronomy (CASA) at the University of Colorado-Boulder, and his colleagues found that galaxies tend to “waste” large quantities of them by discharging them distances up to one million light years away.

As Oppenheimer, whose team published their findings online in a recent edition of the Monthly Notices of the Royal Astronomical Society, said Monday in a statement, “Previously, we thought that these heavier elements would be recycled in to future generations of stars and contribute to building planetary systems. As it turns out, galaxies aren’t very good at recycling.”

Using data from the Hubble Space Telescope’s Cosmic Origin Spectrograph (COS) instrument, the researchers analyzed the nearly-invisible reservoir of gases surrounding a galaxy, known as the circumgalactic medium (CGM), around both spiral and elliptical galaxies, each of which are typically home to several billion heavy element producing stars.

Findings also reveals why different galaxies have different oxygen levels

While a typical galaxy is between 30,000 to 100,000 light years in size, the CGM can be up to one million light years big. While researchers believe that the CGM plays a key role in cycling elements into and out of the galaxy, the exact processes are still not well understood.

As part of their study, Oppenheimer’s team used the COS instrument’s ultraviolet spectroscopy capabilities to study the galaxies and the CGM surrounding them, as well as several simulations, and determined that the CGM’s in both types of galaxies contained more than half of a galaxy’s heavier elements, indicating that they are much less efficient at maintaining these raw materials than experts had previously believed.

“The remarkable similarity of the galaxies in our simulations to those targeted by the COS team enables us to interpret the observations with greater confidence,” study co-author Robert Crain, a Royal Society University Research Fellow at Liverpool John Moores University, said. The study authors also noted that their simulations helped explain why COS observations appeared to show that elliptical galaxies had less oxygen surrounding them than spiral ones.

“The CGM of the elliptical galaxies is hotter,” explained Joop Schaye, a professor at Leiden University in the Netherlands and another co-author of the new study. “The high temperatures, topping over one million degrees Kelvin, reduce the fraction of the oxygen that is five times ionized, which is the ion observed by COS.”

In contrast, the temperature of CGM gases in spiral galaxies is 300,000 degrees Kelvin, or about 50 times hotter than the sun’s surface. Oppenheimer explained that ejecting heavy elements into the CGM requires “massive amounts of energy from exploding supernovae and supermassive black holes… This is a violent and long-lasting process that can take over 10 billion years, which means that in a galaxy like the Milky Way, this highly ionized oxygen we’re observing has been there since before the Sun was born.”

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Image credit: Adrien Thob, LJMU

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