A view of the M87 supermassive black hole in polarised light, credits: EHT collaboration
“We are now seeing the next crucial piece of evidence to understand how magnetic fields behave around black holes, and how activity in this very compact region of space can drive powerful jets that extend far beyond the galaxy,” says Monika Mościbrodzka, astrophysicist at Radboud University and coordinator of the EHT Polarimetry Working Group.
On 10 April 2019, scientists released the first ever image of a black hole, revealing a bright ring-like structure with a dark central region — the black hole’s shadow. Since then, the EHT collaboration has delved deeper into the data on the supermassive object at the heart of the M87 galaxy collected in 2017. They have discovered that a significant fraction of the light around the M87 black hole is polarised.
Polarisation of light
“This work is a major milestone: the polarisation of light carries information that allows us to better understand the physics behind the image we saw in April 2019, which was not possible before,” explains Iván Martí-Vidal, also coordinator of the EHT Polarimetry Working Group and GenT Distinguished Researcher at the University of Valencia, Spain. He adds that “unveiling this new polarised-light image required years of work due to the complex techniques involved in obtaining and analysing the data.”
Light becomes polarised when it goes through certain filters, like the lenses of polarised sunglasses, or when it is emitted in hot regions of space that are magnetised. In the same way polarised sunglasses help us see better by reducing reflections and glare from bright surfaces, astronomers can sharpen their vision of the region around the black hole by looking at how the light originating from there is polarised. Specifically, polarisation allows astronomers to map the magnetic field lines present at the inner edge of the black hole.
Black hole launches jets
"Our new polarised images map out magnetic fields in extreme gravity. They help us better understand how our bright ring of emission connects to the powerful jets of energy and matter that emerge from M87’s core and extend at least 5000 light-years from its centre”, says co-author Sara Issaoun, astrophysicist at Radboud University.
These jets are one of the galaxy’s most mysterious and energetic features. Most matter lying close to the edge of a black hole falls in. However, some of the surrounding particles escape moments before capture and are blown far out into space in the form of jets.
Astronomers have relied on different models of how matter behaves near the black hole to better understand this process. But they still don’t know exactly how jets larger than the galaxy are launched from its central region, which is as small in size as the Solar System, nor how exactly matter falls into the black hole. With the new EHT image of the black hole and its shadow in polarised light, astronomers
managed for the first time to look into the region just outside the black hole where this interplay between matter flowing in and being ejected out is happening.
The observations provide new information about the structure of the magnetic fields just outside the black hole. The team found that only theoretical models featuring strongly magnetised gas can explain what they are seeing at the event horizon.
“The polarimetric images suggest that the magnetic fields near the black hole could be strong enough to prevent matter from falling under the event horizon. The gas surrounding the black hole can be easily turned into a jetted outflow”, says Mościbrodzka.
Event Horizon Telescope and ALMA
To observe the heart of the M87 galaxy, the collaboration linked eight telescopes around the world – including the northern Chile-based Atacama Large Millimeter/submillimeter Array (ALMA) - to create a virtual Earth-sized telescope, the EHT. The impressive resolution obtained with the EHT is equivalent to that needed to measure the length of a credit card on the surface of the Moon.
"ALMA data were, among others, crucial to calibrate, image and interpret the EHT observations, providing tight constraints on the theoretical models that explain how matter behaves near the black hole event horizon," adds Ciriaco Goddi, a scientist at Radboud University and Leiden Observatory, the Netherlands, who lead an accompanying study that relied only on ALMA observations.
The EHT collaboration involves more than 300 researchers from Africa, Asia, Europe, North and South America. In The Netherlands, astronomers from Radboud University, the University of Amsterdam, Leiden University, and JIVE are involved in the EHT, as well as the NOVA-technical submm-group of the University of Groningen.
The BlackHoleCam research group was awarded the European Research Council €14 million Synergy Grant in 2013. Principle Investigators are Heino Falcke, Luciano Rezzolla and Michael Kramer. Partner institutes are JIVE, IRAM, MPE Garching, IRA/INAF Bologna, SKA and ESO. BlackHoleCam is part of the Event Horizon Telescope collaboration.
The EHT Collaboration et al., First M87 Event Horizon Telescope Results. VII. Polarization of the Ring, The Astrophysical Journal Letters, Vol. 910, L12, http://doi.org/10.3847/2041-8213/abe71d
- The EHT Collaboration et al., First M87 Event Horizon Telescope Results. VIII. Magnetic Field Structure near The Event Horizon, The Astrophysical Journal Letters, Vol. 910, L13, http://doi.org/10.3847/2041-8213/abe4de
- Goddi, Martí-Vidal, Messias, et al. (EHT Collaboration): Polarimetric properties of Event Horizon Telescope targets from ALMA, The Astrophysical Journal Letters, Vol. 910, L14, http://doi.org/10.3847/2041-8213/abee6a