The Earth-spanning telescope network of the Event Horizon Collaboration has targeted another black hole: The center of the galaxy Centaurus A, which is around 13 million light years away, which is particularly “loud” in the radio wave range. instead, the images show the jet in a previously unattainable resolution. It can be seen, among other things, that its features match those of the streams of radiation and particles at the much larger black hole M87 *, but also that the edges of the jet emit most of the radio radiation. This calls into question a number of theoretical models of how the jets work.
When active black holes devour matter, they often create huge jets of highly accelerated particles and radiation. These race out into space at almost the speed of light and transport energy and matter over huge distances – often far beyond the original galaxy of the black hole. But how and where these streams of radiation and particles arise and where their enormous energies come from is unclear. The differences between different types of black holes with regard to their jets are also only partially known because mostly high-resolution images and data are missing. This is where the Event Horizon Telescope (EHT) comes into play: Because the resolution of its radio telescopes, which are coupled together by means of interferometry, corresponds to that of an earth-sized antenna, it can even image distant jets more precisely than all the telescopes in front of it.
Centaurus A jets in view
One of the objects that the EHT has set its sights on as part of its observations is the black hole in the center of the galaxy Centaurus A, around 13 million light years away. Its active galaxy core was identified as one of the first and most powerful cosmic radio sources as early as 1949. “Centaurus A is the closest loud radio source to earth,” explain Michael Janssen from the Max Planck Institute for Radio Astronomy in Bonn and his colleagues. That is why it has already been investigated in almost all electromagnetic wavelengths. From this data we know that the black hole in the center of the galaxy must be about 55 million solar masses. “In terms of mass and accretion rate, it lies between the supermassive black hole in the galaxy M 87 with six and a half billion solar masses and that in our galactic center with around four million solar masses,” say the astronomers. From earlier observations it was also known that Centaurus A * like M87 * produces a jet.
Janssen and his team observed this jet and its possible place of origin using the Event Horizon telescope. To do this, they used the data from eight radio telescopes, which are primarily located in the southern hemisphere and thus have a clear view of Centaurus A. Among them were the Atacama Large Millimeter / submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX) in Chile, the James Clerk Maxwell Telescope in Hawaii, and the South Pole Telescope in Antarctica. In a concerted action on April 10, 2017, the telescopes aimed their antennas at Centaurus A for six hours and captured radio radiation in the range around 1.3 millimeters wavelength. The processing of your data now results in the most precise radio recordings of Centaurus A’s jet to date.
Noticeably lighter edges
The new recordings show Centaurus A with a nominal resolution of 25 micro-arcseconds – 16 times higher than in previous recordings – and a frequency ten times higher. “This allows us, for the first time, to examine an extragalactic radio jet on scales smaller than the distance traveled by light in a day. We see up close how a tremendously huge jet is born from a supermassive black hole, ”says Janssen. The pictures show that one of the two jets is pointing northeast and is approaching us at an angle. The opposing jet directed to the southwest can be seen much less strongly. In its basic structure and the main features, the jet of Centaurus A corresponds to that of the much more massive black hole M87 *. “With our observations we are demonstrating that the fundamental relationships for the activity and the jets of black holes are valid for objects of both sizes,” the astronomers report.
One feature that both jets show is particularly interesting: the jet that shoots out from the center of Centaurus A is also brighter at the edges than at the center. This phenomenon has already been observed in other jets, but has never been shown in this detail. “Now we can exclude all theoretical jet models that cannot reproduce this edge brightening. It is a striking observation feature that will help us to better understand jets that are generated by black holes, ”says co-author Matthias Kadler from the University of Würzburg. The EHT images also provide a first closer look at the location of Centaurus A. Janssen’s black hole and his team were able to trace the origin of the jets back to a region that, from our point of view, is only the size of an apple on the moon. At the same time, their data suggest that the shadow of this supermassive black hole could become visible at radio frequencies of a few terahertz and with a telescope network that also includes an orbital radio telescope.
Source: Michael Janssen (Max Planck Institute for Radio Astronomy, Bonn) et al., Nature Astronomy, doi: 10.1038 / s41550-021-01417-w