The odd radio sphere (ORC), a type of extragalactic astronomical source discovered in 2019, is actually a shell formed by galactic winds spilling from massive supernovae, according to a new study.
ASKAP radio continuum image (contours) of ORC 1 superimposed on the DES tricolor composite image. Two galaxies of interest: 'C' is near the center of ORC 1, and 'S' coincides with the southern radio peak. Image credit: Norris otherarXiv: 2006.14805.
The first three ORCs were discovered during the Cosmic Evolution Map pilot survey using the Australian Square Kilometer Array Pathfinder (ASKAP) telescope.
A fourth ORC, called ORC4, was discovered in archival data taken with the Giant Meterwave radio telescope, and additional ORCs were discovered in subsequent ASKAP and MeerKAT data.
These sources are huge, hundreds of kiloparsecs across, with one kiloparsec equal to 3,260 light-years.
Multiple theories have been proposed to explain its origin, including planetary nebulae and black hole mergers, but it was not possible to distinguish between the theories based on radio data alone.
Intrigued, Professor Alison Coyle of the University of California, San Diego, and colleagues thought that the radio rings could have originated from the later stages of the starburst galaxies they were studying.
Until then, ORCs had only been observed by radio emission, and no optical data were available.
Astronomers observed ORC 4 using the Integrating Field Spectrometer at W.M. Keck Observatory on Mauna Kea, Hawaii, and found that it contained a huge amount of high-brightness, much more than is seen in the average galaxy. A heated compressed gas was revealed.
With more questions than answers, the team set to work doing some detective work.
Using optical and infrared imaging data, they determined that the stars in the ORC 4 galaxy are about 6 billion years old.
“This galaxy experienced an explosion of star formation that ended about a billion years ago,” Professor Coyle said.
The authors also ran a series of numerical computer simulations to recreate the size and properties of the massive radio ring containing large amounts of shocked cold gas in the central galaxy.
Simulations show that the outflowing galactic winds will continue to blow for 200 million years before stopping.
When the winds stopped, forward shocks continued to push hot gas out of the galaxy, forming a radio ring, while reverse shocks caused cooler gas to fall into the galaxy.
The simulation ran for 750 million years. This is within ORC 4's estimated stellar age of 1 billion years.
“For this to work, you need a high mass egress rate, which means you're releasing a lot of material very quickly,” Professor Coyle says.
“And the surrounding gas just outside the galaxy has to be low density, otherwise the shock stalls. Those are the two key factors.”
“We found that the galaxies we have studied have high rates of mass outflow. Although rare, they do exist. This points to ORC originating from some type of outflow galactic wind. I really think so.”
Outflow winds not only help astronomers understand ORCs, but ORCs also help astronomers understand outflow winds.
“ORC provides a way to 'see' the wind through radio data and spectroscopy,” Professor Coyle said.
“This will help us determine how common extreme outflow winds from galaxies are and what the life cycle of winds is like.”
“These can also help us learn more about galaxy evolution. Do all giant galaxies go through an ORC phase?”
“Do spiral galaxies become elliptical when they stop forming stars? I think there's a lot we can learn about and from the ORC.”
of study It was published in the magazine Nature.
_____
AL coil other. Ionized gas extends beyond 40 kpc within odd radio host galaxies. Nature, published on January 8, 2024. doi: 10.1038/s41586-023-06752-8
Source: www.sci.news
