Artist concept of a radio-loud quasar. Image: ESO/ M. Kornmesser
Astronomers have discovered the most distant radio signals yet, which voyaged more than 13 billion light years across the universe before they were captured by multiple observatories here on Earth. The signals come from a tremendously powerful “quasar,” a special type of galactic core that radiates enormous amounts of light and energy. Scientists observed this particular quasar, known as P172+18, as it was when the universe was only 780 million years old, or about five percent of its current age.
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A team co-led by Eduardo Bañados, a staff astronomer of the Max Planck Institute for Astronomy in Germany, and Chiara Mazzucchelli, a research fellow at the European Southern Observatory (ESO) in Chile, spent years chasing these extremely rare radio signals, hoping to confirm that they were indeed the most distant ever measured. Finally, on the night of January 12, 2019, the researchers captured clear observations of the far-flung quasar using a spectrograph on the Magellan Baade telescope at Las Campanas Observatory in Chile, according to a study published on Monday in The Astrophysical Journal.“It was an exciting night,” Bañados said in an email, noting that both he and Mazzucchelli were present for the discovery. “We were extremely happy. Minutes after we got the data from the telescope we knew we had made an important discovery: The first human beings recognizing this object as a quasar and the most distant source of strong radio emission known so far.” In addition to their observations that night, the team confirmed their findings using a variety of telescopes such as ESO's Very Large Telescope, the National Radio Astronomy Observatory's Very Large Array, and the Keck Telescope. The sum total of these observations revealed that P172+18 contains a supermassive black hole that is about 300 million times as massive as the Sun, which is gobbling up material at one of the highest rates ever observed at such distances.
P172+18 is part of an extremely rare group of radio-loud quasars that exist at the edge of space and time. While it is not the most distant quasar ever observed—that honor goes to J0313- 1806, which appears about 100 million years earlier in time—it is the farthest quasar that shows a strong radio signal. “Finding quasars at these early epochs is already like finding a needle in a haystack,” Bañados explained. “But only 10 percent of quasars show strong radio emission so those objects are even rarer.”Quasars are the powerful and luminous nuclei of enormous galaxies that were plentiful in the first few billion years of the universe. Despite the remoteness of these objects in space and time, they are often observable thanks to bright disks of superheated gas that surround the supermassive black holes at the galactic cores, as well as twin jets of material that shoot out of their poles at relativistic speeds. These features are created by the extreme interactions between the gravitational forces of the huge black holes and the gassy material that falls into them.Scientists have long been puzzled by the fact that these gargantuan objects exist so early in the universe, because models suggest it should take much longer for such massive monsters to evolve.
One explanation posits that radio quasar jets, like those seen at P172+18, agitate the gassy disk in the galactic core, accelerating the rate at which the black hole devours material and grows. For this reason, radio-loud quasars could provide a special observational model of this enigmatic process in action. The reason why these radio-bright quasars are so unusual is not yet fully understood, but it could be related to the exotic environment that characterized the early universe.Radio jets observed in later epochs are generated by the interactions of electrons within the jets with the strong magnetic field of the quasar: the electrons cool down due to contact with this field, a process that emits radio waves.In the early universe, however, the surrounding environment was hotter because of the proximity of photons from the cosmic microwave background (CMB), the oldest detectable light in the universe, which permeated this ancient era. Bañados noted that quasar jet electrons in such an environment may have interacted more with CMB photons as opposed to magnetic fields. In this case, models suggest that the radiation of these interactions would show up as X-rays, not radio emission, a possibility that Bañados and Mazzucchelli will explore in a forthcoming paper. Meanwhile, the team hopes to find even more of these radio-loud quasars in the distant reaches of the universe, especially since they could fill in some of the missing pieces in our understanding of this bizarre period of the cosmos, when the first massive galaxies emerged.“We are now on the hunt of more similar objects,” Bañados said. “This is a very ‘normal’ radio-loud quasar (except that it's very distant) so we think there should be more out there, even at larger cosmological distances.”“In the meantime we are also trying to understand why some quasars show strong radio emission while others do not, and their connection to CMB/X-ray emission,” he concluded.