Observations from the National Astronomical Observatory and others provide evidence that the supermassive black hole in M87 is rotating.

Publication Date: September 28, 2023, 16:54

A joint announcement was made on September 28 by ten institutions, including the National Astronomical Observatory of Japan (NAOJ), the Graduate University for Advanced Studies, the University of Tokyo, Ibaraki University, Yamaguchi University, Osaka Public University, Kogakuin University, Komazawa University, Tsukuba University, and Toyo University. They revealed that they had discovered a jet emanating from the center of the elliptical galaxy "M87" performing precession (nodding) motion predicted by the general theory of relativity in an approximately 11-year cycle. This discovery was made by analyzing numerous images obtained from detailed observations over the past 20 years or more, primarily through the East Asia VLBI Network (EAVN).

The image below illustrates a precession motion of an accretion disk and a jet around a rotating supermassive black hole. The rotation axis of the black hole is fixed in the up and down direction of the figure. If the rotation axis of the accretion disk is tilted relative to the rotation axis of the black hole, such precession motion occurs due to the effects of the general theory of relativity. (c)Cui et al.(2023), Intouchable Lab@Openverse, Zhejiang Lab. (Source: NAOJ)

This achievement is the result of an international collaborative research team involving 79 researchers from 45 research institutions and universities worldwide, centered around Assistant Professor Kazuhiro Hada of NAOJ Mizusawa VLBI Observatory, Visiting Researcher Yoshiki Kino of Kogakuin University Education Promotion Organization, Special Researcher Tomonao Kawashima of the University of Tokyo Institute for Cosmic Ray Research, and Director/Professor Nozomu Tomita of NAOJ Mizusawa VLBI Observatory. The details were published in the British scientific journal "Nature".

It is believed that supermassive black holes with masses millions to billions of times that of the sun exist at the center of most galaxies in the universe. Some of these are very active, emitting beam-like gas called "jets" in the north-south direction and shining as active galactic nuclei or quasars.

A supermassive black hole, equivalent to about 6.5 billion solar masses, sits at the center of M87, located 55 million light-years from Earth. This black hole is known as one of the most representative active galactic nuclei. In 2019, it became famous for the first direct observation of a black hole by the Event Horizon Telescope (EHT). Observations have continued since then, and in April 2023, the Global Millimeter VLBI Array (GMVA) revealed the state of the accretion disk surrounding the supermassive black hole of M87.

These observations confirmed that the ultimate energy source of active galactic nuclei is a supermassive black hole and an accretion disk, and suggested that they are also involved in the formation of jets.

According to Einstein's general theory of relativity, rotation is one of the most important elements determining the basic properties of a black hole and the structure of space-time around it, along with mass. In recent years, it has been proposed that the energy of black hole rotation is necessary to drive powerful jets. However, unlike mass, which can be relatively easily measured from the size of the black hole and the motion of surrounding stars, it is not easy to determine the presence or absence of rotation from observations.

Therefore, the research team analyzed 170 radio images of M87 jets accumulated over the past 20 years or more, mainly from observational data obtained by EAVN and the US radio telescope network. They investigated how the shape of the jet changes. As a result, they discovered that the direction of the jet's ejection changes periodically in an approximately 11-year cycle.

The image above shows an example of a radio image of the M87 jet taken by EAVN and others. Numerous images taken in the 7mm wavelength band from 2013 to 2018 have been averaged into three images every two years. The arrows extending from the center of each image indicate the direction of the jet's ejection. The graph below shows the time variation of the jet's ejection direction measured between 2000 and 2022. The red sine curve is the model of the 11-year cycle precession motion that best matches the measurement results. (c)Cui et al. (Source: NAOJ)

The image below shows the time variation of the direction in which the jet is ejected. (c)Cui et al.(2023) (Source: NAOJ)

Previous studies suggested the existence of a "mysterious lateral swing" phenomenon in which the M87 jet swings laterally relative to the ejection direction, but the cause and whether there was a cycle were not well understood. To identify the cause, a theoretical simulation using the astronomical supercomputer "Aterui II" operated by NAOJ was conducted, and the observational results were considered. As a result, it became clear that the observed 11-year cycle of jet oscillation can be well explained by the "Lense-Thirring precession," a motion caused by a rotating black hole dragging the surrounding space-time.

The image below shows the state of precession motion of the accretion disk and jet shown in a general relativistic magnetohydrodynamic simulation by Aterui II. An accretion disk tilted relative to the rotation axis of the black hole was initially set up, and the time variation was followed. The color diagram represents the density in the meridian plane. (c)Tomonao Kawashima (Source: NAOJ)

Lense-Thirring precession is similar to the precession caused by a spinning top on Earth. In the case of a spinning top on Earth, the gravity of the Earth acting on the tilted top triggers the precession motion, while in the case of the supermassive black hole of M87, the force due to the rotation of the black hole acts on the tilted accretion disk, causing the Lense-Thirring precession motion. According to the simulation results, it was found that the accretion disk rotates around due to the Lense-Thirring precession at a cycle similar to the observation, and the simulation also showed that the jet also performs precession motion in the same way as the accretion disk is dragged.

According to the research team, this research result strongly supports that the supermassive black hole of M87 is rotating. At the same time, it strongly supports the theory that rotation is deeply involved in the formation of jets.

The research team continues to observe the M87 jet. According to Professor Tomita of NAOJ, who operates the VERA telescope, a major station of EAVN, they want to more accurately derive the connection between the black hole and the jet and the speed of rotation by comparing the shape change of the obtained jet with the black hole video obtained by EHT.

The image below shows the group of radio telescopes belonging to EAVN. (c)National Astronomical Observatory (Source: NAOJ)

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