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Intrinsic Structure of The Supermassive Black Hole at The Center of Our Galaxy
February 23, 2022
Astronomers have found that underlying structure of the supermassive black hole at the center of Milky Way, named Sagittarius A* (Sgr A*), is almost circular from the very long baseline interferometry (VLBI) observations at 1.3 cm and 7 mm wavelengths. Its size may indicate the accretion flow around the Sgr A* contains the nonthermal electrons, and the shape tells us the accretion flow’s rotation axis (or a possible radio jet) may be almost pointing at the Earth. This study is published in the Astrophysical Journal, led by Ilje Cho who is a researcher at IAA-CSIC, and co-authored, among others, by Juan Carlos Algaba, senior lecturer in the radio cosmology group of the department of physics, Universiti Malaya.
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Sgr A* is the best target to investigate what happens around a black hole, since it is the closest supermassive black hole from the Earth. In 2020, the Nobel Prize in Physics has also awarded to the studies of Sgr A*. To resolve the fine structure of Sgr A*, astronomers have used the VLBI technique which consists of many radio telescopes at different geographic locations so that it makes a virtual telescope the size of the distance between telescopes. Toward Sgr A*, however, there is another challenge which is the light scattering by the gas clouds in our Galaxy. To overcome this, Ilje Cho and international colleagues applied the scattering model from recent studies based on the historical observations to their observations with the East Asian VLBI Network (EAVN). As a result, the team has found that the intrinsic structure of Sgr A* is almost circular at 1.3 cm and 7 mm wavelengths. “Before correcting the effect of interstellar scattering medium, the structure of Sgr A* is more elongated towards the East-West direction. Through our study, we show that the elongation is mostly coming from the scattering effect”, explains CHO who has confirmed it with three different methods independently. “The EAVN enabled us to make it all”.
The EAVN consists of 21 radio telescopes: 4 in Korea, 11 in Japan, and 6 in China. For this study, 10 and 8 of them have been used to observe at 1.3 cm and 7 mm wavelengths, respectively. The observations were conducted in 2017 April, as a part of the large program of the Active Galactic Nuclei Science Working Group under the EAVN consortium. In addition, they are a part of the multi-wavelength campaign of the Event Horizon Telescope (EHT) which has unveiled the image of black hole shadow for the first time ever from the M87*.
("Malaysian participation in EAVN observations and involvement with the EAVN consortium is part of a long term project with the vision of including the country as a VLBI node for the network and training a new generation of Malaysian VLBI astronomers, and is currently supported by an FRGS grant", says ALGABA.)
(SOHN & JUNG, researchers at the Korean Astronomical and Space Science Institute (KASI), say, “The joint array of Korean VLBI Network (KVN) and the VLBI Exploration of Radio Astrometry (VERA), called the KVN and VERA Array (KaVA), is the precursor of the EAVN, which has been operated by the Korea Astronomy and Space Science Institute (KASI) and the National Astronomical Observatory of Japan (NAOJ) since 2010.”)
(SHEN, director of the Shanghai Astronomical Observatory (SHAO), says “The participation of the Tianma-65m telescope through the EAVN has dramatically improved the sensitivity of the observations so that the faint signal could be better detected.”)
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Compared with the theoretical simulation of an accretion flow, the circular shape of Sgr A* may imply that the rotation axis of the accretion flow is almost pointing at us. In addition, the size of Sgr A* implies that the accretion flow around the supermassive black hole has to contain the nonthermal electrons.
(As KINO and KAWASHIMA, researchers at the Kogakuin University of Technology & Engineering, and the University of Tokyo, respectively indicate, “If we consider only thermal electrons, the predicted size is much smaller than the observed value. We found that this size-mismatch problem can be solved by including nonthermal electron components”.)
Together with the result at 3 mm by Sara Issaoun and the colleagues, the relation of the Sgr A*’s size & brightness and observing wavelength have been found. This is the first analysis with the VLBI data within a 2 days timescale. Assuming the same relation at shorter wavelengths, the size & brightness of the Sgr A* at 1.3 mm wavelength (230 GHz frequency) is predicted.
(ZHAO from IAA-CSIC in Spain, who is one of the coordinators of the EHT scattering working group), says, “This could be of great help to the EHT 2017 data analysis aiming to make the first image of the black hole shadow of Sgr A*.”)
There is a long-standing debate on the dominant radio emission scenario for Sgr A*, whether an accretion flow or a jet. This study has improved the accretion flow model prediction, but the jet scenario is still available to explain the results. To constrain the possible scenarios, future studies will be of great importance such as the long-term monitoring observations with multiple wavelength receivers of EAVN. This will provide the finer structure of Sgr A* and accurate position of the radio emission region at different wavelengths so that the possibility of each emission model can be further narrowed down.
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December 7, 2021
Congratulations to Dr Juan Carlos and 'way to go' to the Astrophysics and Radio Astronomy Research Group! It is exciting to read the notable mention of Dr Juan Carlos Algaba's work on the shape of black hole jets in the Physics Today Oct 2021 (Research & Technology) commentary by Eleni Petrakou.
https://physicstoday.
An international astronomy team, in which the Universiti Malaya participates, has just published in Nature Astronomy the image of the heart of the Centaurus A galaxy in the finest detail ever. The work specifies the exact point where the supermassive black hole of this galaxy is located and and reveals how a gigantic jet is being born. Most remarkably, only the outer edges of the jet seem to emit radiation, which challenges our theoretical models of jets. This work, led by Michael Janssen from the Max Planck Institute for Radio Astronomy in Bonn and Radboud University Nijmegen is published in Nature Astronomy on July 19th.
At radio wavelengths, Centaurus A emerges as one of the largest and brightest objects in the night sky. After it was identified as one of the first known extragalactic radio sources in 1949, Centaurus A has been studied extensively across the entire electromagnetic spectrum by a variety of radio, infrared, optical, X-ray, and gamma-ray observatories. At the center of Centaurus A lies a black hole with the mass of 55 million suns, which is right between the mass scales of the Messier 87 black hole (six and a half billion suns) and the one in the center of our own galaxy (about four million suns).
In a new paper in Nature Astronomy, data from the 2017 EHT observations have been analyzed to image Centaurus A in unprecedented detail. An astronomer from Universiti Malaya, senior lecturer Juan Carlos Algaba, in the Radio Cosmology Laboratory of the Department of Physics, Faculty of Science, is part of this international research team based on the Event Horizon Telescope (EHT) Collaboration, known for having obtained the first image of a black hole.
“This allows us for the first time to see and study an extragalactic radio jet on scales smaller than the distance light travels in one day. We see up close and personally how a monstrously gigantic jet launched by a supermassive black hole is being born”, says astronomer Michael Janssen, lead author of the work.
Compared to all previous high-resolution observations, the jet launched in Centaurus A is imaged at a tenfold higher frequency and sixteen times sharper resolution. With the resolving power of the EHT, we can now link the vast scales of the source, which are as big as 16 times the angular diameter of the Moon on the sky, to their origin near the black hole in a region of merely the width of an apple on the Moon when projected on the sky. That is a magnification factor of one billion.
Understanding jets
Supermassive black holes residing in the center of galaxies like Centaurus A are feeding off gas and dust that is attracted by their enormous gravitational pull. This process releases massive amounts of energy and the galaxy is said to become ‘active’. Most matter lying close to the edge of the black hole falls in. However, some of the surrounding particles escape moments before capture and are blown far out into space: Jets – one of the most mysterious and energetic features of galaxies – are born.
Astronomers have relied on different models of how matter behaves near the black hole to better understand this process. But they still do not know exactly how jets are launched from its central region and how they can extend over scales that are larger than their host galaxies without dispersing out. The EHT aims to resolve this mystery.
The new image shows that the jet launched by Centaurus A is brighter at the edges compared to the center. This phenomenon is known from other jets, but has never been seen so pronouncedly before. “Now we are able to rule out theoretical jet models that are unable to reproduce this edge-brightening. It’s a striking feature that will help us better understand jets produced by black holes”, says Matthias Kadler, TANAMI leader and professor for astrophysics at the University of Würzburg in Germany.
"Studying the shape of the jet as it forms near the black hole and how its geometry changes as it expands away from it can be the key to one of the mysteries of modern astrophysics: how the black holes and the plasma around them interact to create these jets. The observations on Centaurus A are very unique to help us find another piece of this puzzle", highlights Juan Carlos Algaba.
Future observations
With the new EHT observations of the Centaurus A jet, the likely location of the black hole has been identified at the launching point of the jet. Based on this location, the researchers predict that future observations at an even shorter wavelength and higher resolution would be able to photograph the central black hole of Centaurus A. This will require the use of space-based satellite observatories.
“These data are from the same observing campaign that delivered the famous image of the black hole in M87. The new results show that the EHT provides a treasure trove of data on the rich variety of black holes and there is still more to come”, says Heino Falcke, EHT board member and professor for Astrophysics at Radboud University.
Additional Information
To observe the Centaurus A galaxy with this unprecedentedly sharp resolution at a wavelength of 1.3 mm, the EHT collaboration used Very Long Baseline Interferometry (VLBI), the same technique with which the famous image of the black hole in M87 was made. An alliance of eight telescopes around the world, of which ALMA is the most sensitive element, joined together to create the virtual Earth-sized Event Horizon Telescope. The EHT collaboration involves more than 300 researchers from Africa, Asia, Europe, North and South America.
The EHT consortium consists of 13 stakeholder institutes: the Academia Sinica Institute of Astronomy and Astrophysics, the University of Arizona, the University of Chicago, the East Asian Observatory, Goethe University Frankfurt, Institut de Radioastronomie Millimétrique (MPG/CNRS/IGN), Large Millimeter Telescope, Max Planck Institute for Radio Astronomy, MIT Haystack Observatory, National Astronomical Observatory of Japan, Perimeter Institute for Theoretical Physics, Radboud University and the Center for Astrophysics | Harvard & Smithsonian.
TANAMI (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) is a multiwavelength program to monitor relativistic jets in active galactic nuclei of the Southern Sky. This program has been monitoring Centaurus A with VLBI at centimeter-wavelengths since the mid 2000s. The TANAMI array consists of nine radio telescopes located on four continents observing at wavelengths of 4 cm and 1.3 cm.
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (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 Ministry of Science and Technology (MOST) 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 the construction, commissioning and operation of ALMA
Publication:
Event Horizon Telescope observations of the jet launching and collimation zone in Centaurus A, by M. Janssen, H. Falcke, M. Kadler, E. Ros, M. Wielgus et al. (EHT Collaboration), Nature Astronomy, DOI: https://doi.org/10.1038/
PHOTO Credit:
Radboud University; ESO/WFI; MPIfR/ESO/APEX/A. Weiss et al.; NASA/CXC/CfA/R. Kraft et al.; EHT/M. Janssen et al.
Distance scales uncovered in the Centaurus A jet. The top left image shows how the jet disperses into gas clouds that emit radio waves, captured by the ATCA and Parkes observatories. The top right panel displays a color composite image, with a 40⨉ zoom compared to the first panel to match the size of the galaxy itself. Submillimeter emission from the jet and dust in the galaxy measured by the LABOCA/APEX instrument is shown in orange. X-ray emission from the jet measured by the Chandra spacecraft is shown in blue. Visible white light from the stars in the galaxy has been captured by the MPG/ESO 2.2-metre telescope. The next panel below shows a 165 000⨉ zoom image of the inner radio jet obtained with the TANAMI telescopes. The bottom panel depicts the new highest resolution image of the jet launching region obtained with the EHT at millimeter wavelengths with a 60 000 000⨉ zoom in telescope resolution. Indicated scale bars are shown in light years and light days. One light year is equal to the distance that light travels within one year: about nine trillion kilometers. In comparison, the distance to the nearest-known star from our Sun is approximately four light years. One light day is equal to the distance that light travels within one day: about six times the distance between the Sun and Neptune.
PHOTO Credit:
Credit: R. Bors; CSIRO/ATNF/I. Feain et al., R. Morganti et al., N. Junkes et al.; ESO/WFI; MPIfR/ESO/APEX/A. Weiß et al.; NASA/CXC/CfA/R. Kraft et al.; TANAMI/C. Müller et al.; EHT/M. Janßen et al.
Last Update: 15/07/2022