In September, a group of physicists published their findings on what is now the largest known pair of black hole jets in the universe—Porphyrion, named after the giant from Greek mythology (Oei et al., 2024). In a galaxy 10 times more massive than our own, Porphyrion stretches 23 million light-years across, the equivalent length of 140 Milky Ways. Porphyrion’s discovery brings new insights into the physics of black holes and their potential to transport energy and magnetism throughout the cosmos.
Black Hole Mechanisms
At the center of nearly every large galaxy in the universe is a supermassive black hole (SMBH). Though black holes are famous for not letting light escape, they can become the brightest objects in the universe. This phenomenon is called a quasar, an extremely luminous type of active galactic nuclei (AGN; a galaxy with an SMBH actively “accreting” matter). According to processes defined by the mathematician Roy Kerr in the 1960s, as an SMBH spins around its axis, it draws in matter and charged particles which are heated by friction into a plasma. This plasma accretion disk carries with it an electrical current that produces a magnetic field (Orf, 2024).
Black hole jets, also known as astrophysical jets, are the consequence of this powerful magnetic field. The Blandford-Znajek process describes the motion of the fields and their influences on the black hole. When the magnetic field lines fall onto the black hole, its rotation twists the field into a helix along its axis. Just on Earth as moving magnetic fields generate voltage, the voltage created by an SMBH is powerful enough to draw up and eject some of the charged particles in two galaxy-sized jets (Wolchover, 2021).
Coincidently, the particles funneled into the jets are accelerated to speeds close to the speed of light. When looking through radio telescopes on Earth, these particles are observed as synchrotron radiation, visible across a wide range of wavelengths. The first recorded image of a black hole, the SMBH at the center of the Messier 87 galaxy, confirmed the presence of the warping magnetic field and further solidified the existence of astrophysical jets (Klesman, 2023).
Finding Porphyrion
The discovery of Porphyrion happened as an accident. Back in 2018, Marjin Oei, the lead author of the study, was initially searching for a radio signature of the cosmic web. The cosmic web is what defines the large-scale structure of the universe—it is a network of voids and filaments of matter that connect the galaxies to one another. Using Europe’s International LOFAR Telescope (ILT), Oei and his team incidentally found thousands of new streaks that seemed to originate from nearly every large galaxy with an AGN (Clavin, 2024). By using machine learning, and with the help of citizen scientists, the team pored over radio imagings to search for more hidden jets, increasing “the number of known mpc-scale outflows from a few hundred to more than 11,000” (Oei et al., 2024, p. 537). Porphyrion was the largest.
Bordering the detection limits of current leading telescopes, Porphyrion is 7.5 billion years from Earth and seen at the age of 6.3 billion years after the Big Bang. About 7 mpc in length, for Porphyrion to be so huge, the “black hole responsible would have needed to ingest about a sun’s worth of matter each year for a billion years” (Wilkins, 2024). To make and confirm these measurements, the team initially used ILT to capture the radio image due to its sensitivity to synchrotron radiation. Once Porphyrion was identified, they used the upgraded Giant Metrewave Radio Telescope (uGMRT) to determine the host galaxy. In measuring spectroscopic redshift and other factors that could influence its distance measurement, the team used the W. M. Keck Observatory in Hawaii. After accounting for all these measurements and data reductions, the team can say with certainty that Porphyrion originates from a cosmic web filament (Lea, 2024).
Figure 1
Deep radio images of Porphyrion, a) image taken with ILT, b) image taken with uGMRT. Note the blob of light near Porphyrion’s origin is a separate AGN system.
(Oei et al., 2024, p. 538)
Besides its record-breaking size, the Keck telescope also observed something unexpected about Porphyrion. The observations reveal that Porphyrion emerges from a radiatively efficient AGN (RE AGN; also known as radiative active or quasar mode) as opposed to a radiatively inefficient AGN (RI AGN; jet, kinetic, or radio-jet mode). RE and RI AGN are both feedback mechanisms that influence the accretion disk and thus jet structure. The difference between the two comes from the activity of their host galaxies and the accretion rates of their SMBHs. Generally found in blue star-forming galaxies, RE AGNs “efficiently convert the gravitational potential energy of infalling matter into radiation,” whereas RI AGNs are found in massive elliptical galaxies that accrete hot gas at a slower rate (Oei et al., 2024, p. 537). RE AGNs were typical of the early universe, while RI AGNs are more common in the later universe. Additionally, all previously discovered jet outflows of record length were fueled by RI processes (Lea, 2024). It is thus surprising to see Porphyion as a large RE AGN so late in the development of the universe.
What’s Next?
In an interview with Caltech, Oei says “If distant jets like these can reach the scale of the cosmic web, then every place in the universe may have been affected by black hole activity at some point in cosmic time” (Clavin, 2024). It is clear from the study that jets like Porphyrion potentially have a much more significant role in heating and spreading energy and magnetism throughout the universe than previously thought. But it is difficult to measure the extent of this influence. At the present, there are no computers capable of running such simulations. According to Laura Olivera-Nieto from the Max Planck Institute in Germany, “It’s truly a challenge to try to understand how this is physically possible. We cannot put it in a computer, it’s too big” (Wilkins, 2024).
Porphyrion carries energy into the IGM comparable to that released during galaxy cluster mergers. Given the jets are from a younger and denser universe, it’s a mystery how Porphyrion is able to extend so far beyond its host galaxy without destabilizing (Orf, 2024). The researchers are also interested in whether these jets have an impact on magnetizing the intergalactic medium (the voids of space that fill the cosmic web), and the magnetism of the universe as a whole.
The survey only covered about 15% of the sky, meaning that Porphyrion’s discovery is only just the beginning. As better instruments become more widely available, and with the successive development of artificial intelligence and quantum computing, scientists will learn more about these immense jets and their place in the universe.
References
cover image...
(Wolchover, 2021)
cool animation of Porphyrion...
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