An international scientific team observed 16 active galactic nuclei at multiple wavelengths to study how black holes launch relativistic jets.
The study was conducted in 2017, during the first Event Horizon Telescope (EHT) campaign, in which the Atacama Large Millimeter/submillimeter Array (ALMA) played a key role. The extreme resolution achieved by the EHT made it possible to study the jets of the central supermassive black holes in these galaxies with unprecedented resolution.
The team, led by members of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, and the Instituto de Astrofísica de Andalucía (IAA) in Granada, Spain, investigated the acceleration and magnetization of the jets by comparing the results obtained at different frequencies and angular scales. The results show deviations from established models of jets from supermassive black holes.
Cardiography of a Cosmic Monster
Active Galactic Nuclei (AGN) are the bright hearts of some galaxies powered by supermassive black holes. Powerful plasma jets emerge from some objects, reaching thousands of light-years into intergalactic space. Observations with extreme angular resolution are required to understand the complicated physics behind this phenomenon, allowing astronomers to peer into the realm close to the origin of the jet.
In the most common model, jets are assumed to be conical, containing plasma moving with constant velocity. Meanwhile, the jet plasma’s magnetic field strength and density decrease as it moves farther from the central engine. Based on these assumptions, predictions can be made about the observable properties of jets.
“This basic model cannot be a perfect description for all jets—most likely, only for a small fraction. The dynamics and substructure of jets are intricate, and observational results can suffer greatly from astrophysical degeneracies.” Says project leader Jan Röder (MPIfR and IAA-CSIC). “For example, we know that many jets appear to accelerate. Either the plasma accelerates, or it can be an effect of geometry: if the jet bends, it may point at us more directly, giving the impression of faster movement”, added.
To assess how accurate (or inaccurate) our understanding of the evolution of jets is, the researchers compared the EHT results with previous observations of the same sources. These had been carried out with the Very Long Baseline Array (VLBA) and the Global Millimeter VLBI Array (GMVA), probing much larger spatial scales than the EHT.
The ALMA data were considered the most reliable flux density measurements in the EHT data set. These measurements were used to calibrate the short baseline from the Very Long Baseline Interferometry (VLBI), a crucial factor for validating the survey results.
From this comparison, it was possible to infer the evolution of jets from close to their origins up to many light years into interstellar space. The radiative power per solid angle received from a given source (in our case, an AGN), measured by the brightness temperature1, gradually increases as the emitting jet plasma gets farther from the black hole.
What’s Next?
While alternative explanations exist for these new observations, such as a deviation from conical geometry, the basic theoretical model clearly cannot fully reproduce the properties of jets close to their origin. “More studies are needed to fully understand the acceleration mechanism, the flow of energy, the role of magnetic fields in AGN jets, and their geometries. The expanding EHT array will play an important role in the future discoveries of these fascinating objects,” concludes Jan.
