Scientific Paper

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 sub-structure 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. 

Additional information 

The results of the study are published in Astronomy & Astrophysics in the following paper: A multifrequency study of sub-parsec jets with the Event Horizon Telescope. 

The National Radioastronomy Observatory of United States (NRAO), an ALMA partner on behalf of North America, published the original article. 

The EHT collaboration involves more than 400 researchers from Africa, Asia, Europe, and North and South America, with close to 300 participating in this paper. The international collaboration set out to capture the most detailed images of black holes using a virtual Earth-sized telescope, and in the process, it also produced unprecedented results on AGN. Supported by considerable international efforts, the EHT links existing telescopes using novel techniques, creating a fundamentally new instrument reaching extreme angular resolution. 

The EHT consortium consists of 13 stakeholder institutes: the Academia Sinica Institute of‬ Astronomy and Astrophysics, the University of Arizona, the Center for Astrophysics | Harvard &‬ Smithsonian, the University of Chicago, the East Asian Observatory, the Goethe University‬ Frankfurt am Main, the Institut de Radioastronomie Millimétrique, the Large Millimeter Telescope, the Max Planck‬ Institute for Radio Astronomy, the MIT Haystack Observatory, the National Astronomical Observatory of‬ Japan, the Perimeter Institute for Theoretical Physics, and the Radboud University. 

The AGN analyzed in this study were observed with subsets of the eight telescope stations present in the EHT 2017 array: the Atacama Large Millimeter/submillimeter Array (ALMA), the Atacama Pathfinder Experiment (APEX), the Institut de Radioastronomie Millimétrique (IRAM) 30-meter Telescope, the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope Alfonso Serrano (LMT), the Submillimeter Array (SMA), the UArizona Submillimeter Telescope (SMT), and the South Pole Telescope (SPT).  

The data were processed at the MPI for Radio Astronomy correlator facilities in Bonn, Germany, and MIT/Haystack Observatory in Massachusetts, USA. Since 2017, the EHT has added the Greenland Telescope (GLT), the IRAM Northern Extended Millimeter Array (NOEMA), and the UArizona 12-meter Telescope on Kitt Peak to its network. 

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF), and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan, and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of ALMA's construction, commissioning, and operation

Images 

1. Brightness temperature is a way for astronomers to quantify how much radiation a source emits in a given direction. It is not an actual physical temperature, but a comparative measure of how “bright” an object would appear in radio waves if it behaved like a blackbody (i.e., an ideal object that emits radiation uniformly). ↩︎