For the first time, astronomers have observed, in the same image, the shadow of the black hole at the centre of a galaxy and the powerful jet expelled from it
The observations of the galaxy Messier 87 (M87) were done in 2018 with telescopes from the Global Millimetre VLBI Array (GMVA), the Atacama Large Millimeter/ submillimeter Array (ALMA), of which ESO is a partner, and the Greenland Telescope (GLT).
Most galaxies harbour a supermassive black hole at their centre. While black holes are known for engulfing matter in their immediate vicinity, they can also launch powerful jets of matter that extend beyond the galaxies that they live in.
Understanding how black holes create such enormous jets has been a long-standing problem in astronomy.
Ru-Sen Lu, from the Shanghai Astronomical Observatory in China, said: “We know that jets are ejected from the region surrounding black holes, but we still do not fully understand how this actually happens.
“To study this directly we need to observe the origin of the jet as close as possible to the black hole.”
The new image shows precisely this for the first time: how the base of a jet connects with the matter swirling around a supermassive black hole.
The target is the galaxy M87, located 55 million light-years away in our cosmic neighbourhood, and home to a black hole 6.5 billion times more massive than the sun.
Previous observations had managed to separately image the region close to the black hole and the jet, but this is the first time both features have been observed together.
The image was obtained with the GMVA, ALMA and the GLT, forming a network of radio-telescopes around the globe working together as a virtual Earth-sized telescope.
Such a large network can discern very small details in the region around M87’s black hole.
Shadow of a black hole
The new image shows the jet emerging near the black hole, as well as what scientists call the ‘shadow’ of the black hole.
As matter orbits the black hole, it heats up and emits light. The black hole bends and captures some of this light, creating a ring-like structure around the black hole as seen from Earth.
The darkness at the centre of the ring is the black hole shadow, which was first imaged by the Event Horizon Telescope (EHT) in 2017.
Both this new image and the EHT one combine data taken with several radio-telescopes worldwide, but the new image shows radio light emitted at a longer wavelength than the EHT one: 3.5mm instead of 1.3mm.
Thomas Krichbaum, of the Max Planck Institute for Radio Astronomy in Germany, said: “At this wavelength, we can see how the jet emerges from the ring of emission around the central supermassive black hole.”
The size of the ring observed by the GMVA network is roughly 50% larger in comparison to the Event Horizon Telescope image.
Keiichi Asada, from the Academia Sinica in Taiwan, explained: “To understand the physical origin of the bigger and thicker ring, we had to use computer simulations to test different scenarios.”
The results suggest the new image reveals more of the material that is falling towards the black hole than what could be observed with the EHT.
Interferometry
These new observations of M87’s black hole were conducted in 2018 with the GMVA, which consists of 14 radio-telescopes in Europe and North America.
In addition, two other facilities were linked to the GMVA: the Greenland Telescope and ALMA, of which ESO is a partner.
ALMA consists of 66 antennas in the Chilean Atacama desert, and it played a key role in these observations.
The data collected by all these telescopes worldwide are combined using a technique called interferometry, which synchronises the signals taken by each individual facility.
But to properly capture the actual shape of an astronomical object it’s important that the telescopes are spread all over the Earth.
The GMVA telescopes are mostly aligned east-to-west, so the addition of ALMA in the Southern Hemisphere proved essential to capture this image of the jet and shadow of M87’s black hole.
Lu said: “Thanks to ALMA’s location and sensitivity, we could reveal the black hole shadow and see deeper into the emission of the jet at the same time.”
Image 1: This image shows the jet and shadow of the black hole at the centre of the M87 galaxy together for the first time. The observations were obtained with telescopes from the Global Millimetre VLBI Array (GMVA), the Atacama Large Millimeter/ submillimeter Array (ALMA), of which ESO is a partner, and the Greenland Telescope. This image gives scientists the context needed to understand how the powerful jet is formed. The new observations also revealed that the black hole’s ring, shown here in the inset, is 50% larger than the ring observed at shorter radio wavelengths by the Event Horizon Telescope (EHT). This suggests that in the new image, we see more of the material that is falling towards the black hole than what we could see with the EHT. Credit: R-S Lu (SHAO), E Ros (MPIfR), S Dagnello (NRAO/ AUI/ NSF).
Image 2: Scientists observing the compact radio core of M87 have discovered new details about the galaxy’s supermassive black hole. In this artist’s conception, the black hole’s massive jet is seen rising up from the centre of the black hole. The observations on which this illustration is based represent the first time that the jet and the black hole shadow have been imaged together, giving scientists new insights into how black holes can launch these powerful jets. Credit: S Dagnello (NRAO/ AUI/ NSF).
