A look into the heart of the Centaurus A galaxy with EHT

An international team of scientists from the Event Horizon Telescope (EHT) collaboration combined observations from various radio telescopes around the world, using the same technique by which the famous image of the black hole at the center of the galaxy M87 was made, to photograph the heart of the nearby radiogalaxy Centaurus A in unprecedented detail at a wavelength of 1.3 mm.

The team, which includes researchers from the National Institute of Astrophysics (INAF), the National Institute of Nuclear Physics (INFN) and the University of Naples Federico II, pinpointed the location of the central supermassive black hole revealing the birth of a giant jet. Surprisingly, the experts found that only the outer edges of the jet appear to emit radiation, which defies the predictions of current theoretical models. The study, led by Michael Janssen of the Max Planck Institute for Radio Astronomy in Bonn and Radboud University in Nijmegen, the Netherlands, was published today in the journal Nature Astronomy.

Centaurus A (Ngc 5128) is one of the closest active radio galaxies to Earth and one of the brightest celestial objects in the southern hemisphere night sky in radio wavelengths. Located in the direction of the Centaurus constellation and having been identified as one of the first known extragalactic radiosources as early as 1949, Centaurus A has been extensively studied in virtually all bands of the electromagnetic spectrum by radio, infrared, optical, X-ray and gamma-ray observers. At the center of Centaurus A is a black hole with a mass of 55 million suns (the black hole in the galaxy M87 is 6.5 billion suns and the one at the center of the Milky Way is about 4 million suns).

"The EHT images show the jet of Centaurus A with a spectacular level of detail," explains Kazi Rygl, INAF researcher and member of the EHT science team at the Italian node of the Alma Regional Centre, housed at the INAF headquarters in Bologna. "The detection of the illuminated edges of both the jet and the counter-jet, the latter being much less luminous, allows us to study their collimation profile-practically the angle they subtend-as well as to empirically determine the approximate position of the jet apex and that of the black hole."

The EHT data date from the 2017 observing campaign. Compared with all previous high-resolution observations, the jet launched from Centaurus A was observed in the radio band at a frequency 10 times higher, resulting in images with 16 times sharper resolution than those available to date. Thanks to the resolving power of the EHT, researchers are able to locate the source of the radio signal extending far beyond the galaxy, in a portion of the sky equal to 16 times the Moon's apparent diameter. This allows them to link such large scales to the region near the black hole, whose apparent width in the sky is instead that of an apple observed at the distance of the Moon. In practice, EHT is a very powerful telescope with a magnification capacity of one billion times.

"The mechanisms of formation of these jets are not yet fully understood and are the subject of intense study," explains Mariafelicia De Laurentis, of INFN and Federico II University, a member of the EHT Collaboration. "What is certain is that the origin of these jets should not be sought inside the black hole but from what happens outside it, outside therefore the so-called event horizon," De Laurentis concludes.

Supermassive black holes that reside at the center of galaxies like the one in Centaurus A feed on gas and dust, attracted by their enormous gravitational force. This process releases enormous amounts of energy and makes the galaxy "active." Most of the matter near the edge of the black hole plummets into it; however, some of the surrounding particles escape a few moments before capture and are propelled out into space at speeds approaching that of light. This is how jets, one of the most mysterious and energetic features of galaxies, are born.

Several theoretical models are trying to describe the behavior of matter near the black hole to better understand the process of jet formation, but questions still remain about how the jets themselves are launched from the region near the black hole and how they can propagate to the very large distances observed without dissipating. The Event Horizon Telescope aims to solve this puzzle.

The new image shows that the jet launched from Centaurus A is brighter at the edges than at the center. This phenomenon is also known in other jets, but has never been seen so pronounced before.

"Although the black hole at the center of Centaurus A is too small for us to see its 'shadow,' its proximity to Earth has allowed us, for the first time, to see and study an extragalactic radio jet on scales of about six times the distance between the Sun and Neptune," adds Ciriaco Goddi, a researcher at the Dutch University of Nijmegen and INAF associate. "EHT provides a treasure trove of data on a wide range of black holes. And we are still at the beginning."

With the new EHT observations of the Centaurus A jet, the probable location of the black hole at the jet's launch point has been identified. Based on this location, the researchers predict that future observations at an even shorter wavelength and higher resolution could allow them to photograph the central black hole of Centaurus A. This will require the use of an on-orbit interferometric observatory, a concept for which there are already ideas such as THEZA (TeraHertz Exploration and Zooming-in for Astronomy), proposed for the European Space Agency's Voyage 2050 program.