South Africa: The combined power of two telescopes helps unravel the mystery of strange rings in the sky
When astronomers dream of their ideal telescopes, it’s not that different from what people expect from their televisions and computer monitors. The images they produce must be large in size and high definition, such as those from the Australian Square Kilometer Array Pathfinder (ASK), which are around 10k resolution (beyond the typical quality you get from digital TVs and digital cinematography). And they should have high dynamic range, indicating high-quality imaging with deep sensitivity to faint objects.
But not all telescopes can do everything. This is why complementary science – using some telescopes for some tasks, others for different but related tasks, and then combining the data – is so important in astronomy.
The value of complementary science is emphasized in our recent article. We have worked with ASKAP and South Africa MeerKAT Telescope to exploit their different abilities. In 2019, ASKAP discovered a type of rare and mysterious object, called “strange radio circle“(ORC). We didn’t know what those strange glowing rings in the sky were.
It took data from MeerKAT to help us conclude that the circles are most likely huge shells of gas, about a million light-years in diameter, emanating from the central galaxy.
Neither the discovery nor the detail would have been possible without the two telescopes. ASKAP’s unique field of vision allows the discovery of rare objects such as ORCs. This also allowed the discovery of many new Fast Radio Bursts; these are seemingly rare, extremely bright, short-lived flashes of radio waves.
Meanwhile, MeerKAT’s unique sensitivity and sampling capability, achieved through its large number of dishes (64, located in a remote part of the Northern Cape Province of South Africa), very low-noise amplifiers sensitive and wide bandwidth, allow to study these objects in more detail. . MeerKAT is the best imaging radio telescope in its class.
ASKAP and MeerKAT are both precursors to Square Kilometer Network (SKA). It is a global project to build the largest and most sensitive radio telescope in the world over the next decade, co-located in South Africa and Australia. As our new research clearly shows, complementary science will be at the heart of the SKA. This is an exciting prospect for African science, with South Africans presenting themselves as world leaders in radio astronomy.
The nature of ORC1
Our new article focuses on the first ORC discovered by ASKAP in 2019. We call it ORC1. MeerKAT provided something essential to deepen our understanding of what it could be and how it formed: beautiful detailed images.
The data we have collected from MeerKAT has been processed by a complex workflow. This was developed and provided by the Interuniversity Institute for Data-Intensive Astronomy (IDIA), a partnership of three South African universities. This specialized software made it possible to generate specific data products, such as images of the polarization and “radio color” of ORC1.
MeerKAT’s technology revealed three particularly important and previously uncertain details about ORC1. The first was the object’s internal structure, revealed for the first time thanks to MeerKAT’s deep sensitivity and high resolution. We can now see that ORC1 contains several arcs, a radio source where the central galaxy is located, and radio emission nodes associated with other nearby galaxies.
Our theory is that the central galaxy, a few billion light-years away, triggered the ORC in a particular event. It may have been the merger of supermassive black holes or a starburst event (the rapid formation of many stars in the galaxy) that occurred billions of years ago. It was during this event, we assume, that the ORC grew to its enormous size of about 1.6 million light-years.
The second detail revealed by the MeerKAT data concerns the polarization of the ORC, made possible by its high sensitivity.
All light in the electromagnetic spectrum is polarized: its magnetic and electric fields point in a certain direction. However, waves or photons from an unpolarized light source are randomly polarized – they do not tend to any particular orientation.
Certain physical processes, such as the presence of magnetic fields, can polarize light. This causes some or all of the waves to point in the same direction. We found that ORC1 is highly polarized along its outer ring.
The third detail was the structure of the spectral index or “radio color” of ORC1: how its brightness changes with frequency.
Typically, the spectral index is measured with several combined radio telescopes, each observing at a different frequency that can be compared to see how the brightness changes. For large resolved sources like ORC1, there is a huge margin of uncertainty. The large bandwidth of MeerKAT allowed us to measure an “in-band” spectral index map over the entire source. In this map, each pixel itself measures the spectral index on the many frequencies that we have combined. Our resulting map showed a steep spectral index on both the ring and its internal structure, suggesting that they may have been produced by the same mechanism.
These new details correspond to an explanation that synchrotron radiation (electrons whistling around magnetic fields) is the source of the radio emission, from a gas envelope in the form of a spherical shock wave. . However, internal arcs and rings require further explanation. We have hypothesized that these are caused by nearby galaxies moving through the shell and leaving trails in their wake.
New questions to explore
So what does this all mean? As with so much radio astronomy, we’re not sure: more data and information added to the mystery, with some clues provided.
However, we have three hypotheses to explain the nature of ORCs. A: It is a spherical shell of an expanding shock wave caused by a huge explosion, such as the merger of two supermassive black holes. Second: This is a spherical shell from the “terminal shock” of a previous “starburst” event – when many stars rapidly formed in the galaxy over a short period of time. Three: This may be an end view of powerful radio jets of highly energetic particles shooting out of a central supermassive black hole.
Not having definitive answers can seem frustrating to some. But that’s the nature of some science. What’s exciting is that there’s more to come: the SKA, which should become operational in the next decade, will probe faint, rare and mysterious objects even deeper. It almost guarantees the discovery of the unexpected, as we have seen throughout the history of science, and as we see now with ORCs. Future discoveries far above us may seem dim – but the possibilities point to a bright future.
Jordan Necklaceilifu Support Astronomer, Interuniversity Institute for Data-Intensive Astronomy