The Event Horizon Telescope observes the magnetic fields around the black hole of galaxy M87

A new landmark has been reached in astronomic observation by analysing the supermassive black hole of M87 in polarised light.Black hole

The Event Horizon Telescope (EHT) collaboration, which produced the first image of a black hole, recently revealed a new image of the massive object at the heart of galaxy M87: how it is viewed in polarised light. This is the first time that astronomers have been able to measure the polarisation (the “signature” of magnetic fields) so close to the event horizon of a black hole. These observations are key to explain how galaxy M87, located 55 million light years away, can shoot highly-energetic material from its nucleus.

Among the specialists from different countries that have contributed to this project are astronomers Iván Martí-Vidal and Alejandro Mus, from the University of Valencia.

“This is unique proof to understand how magnetic fields around black holes behave, and how the activity in this compact region of space can launch powerful streams that go beyond the galaxy”, explains Monika Mościbrodzka, coordinator of the Polarimetry work group of the EHT and assistant lecturer at the University of Radbout (Holland).

The first image of a black hole was published on 10 April 2019, revealing a bright ring-shaped structure with a dark central region: the shadow of the black hole. Since then, the EHT collaboration has looked into the data of the supermassive item at the heart of the M87 galaxy compiled in 2017, and they have discovered that a significant fraction of light around black hole M87 is polarised.

“This study is a key landmark: the polarisation of light carries information that allows us to better understand the physics behind the image that we saw in April 2019, which was not possible before,” says Iván Martí-Vidal, fellow coordinator of the Polarimetry work group of the EHT collaboration and honourable GenT researcher of the University of Valencia. “Revealing this new image in polarised light has required years of work due to the complex techniques involved in obtaining and analysing the data,” adds the researcher.

The light becomes polarised when it passes through certain filters, such as the lenses of polarised sunglasses, or when beamed in hot and magnetised regions of space. The same way as polarised sunglasses only let light through when the electric field is pointing in a specific direction, astronomers can detect the polarisation of light from space using polarisers installed in telescopes. In the case of EHT, studying the polarisation of light allows astronomers to map the magnetic field lines that are very close to the event horizon of the black hole of M87.

“The recently published polarised images are key to understand how the magnetic field allows the black hole to “eat” matter and shoot powerful streams,” says Andrew Chael, member of the EHT and researcher at the Princeton Center for Theoretical Science (USA).

The bright streams of energy and matter that emerge from the nucleus of M87 and reach up to five thousand light years from the centre are one of the most mysterious and energetic characteristics of the Galaxy. A majority of the matter close to the border of a black hole falls inside. However, some of the surrounding particles escape moments before being captured and are expelled into space in the form of streams.

The research team has drawn from different models of how matter behaves near the black hole to better understand this process. But they still do not know exactly how streams that are larger than the galaxy are propelled from its central region (as small in size as the solar system), nor exactly how the matter falls in the black hole. With this new image of the EHT, astronomers have managed to envisage for the first time the confines of the black hole where this interaction between the matter that flows inwards and the matter that is expelled takes place.

The observations provide new information on the structure of the magnetic fields at the borders of the black hole. The team discovered that only theoretical models with strongly magnetised gas can explain what they are seeing in the event horizon. “The observations suggest that the magnetic fields at the border of the black hole are sufficiently intense as to retain the hot gas and help it resist the pull of gravity. Only the gas that slides through the field can spiral towards the event horizon”, says Jason Dexter, assistant lecturer at the University of Colorado Boulder (USA) and coordinator of the theoretical work group of the EHT.

To observe the heart of the M87 galaxy, the collaboration connected eight telescopes around the world to create a virtual telescope the size of the Earth, the EHT. The impressive resolution obtained with the EHT is equivalent to the resolution needed to measure the length of a credit card on the surface of the Moon.

This allowed the team to directly observe the shadow of the black hole and the ring of polarised light around it, which clearly shows that the material that surrounds the black hole is magnetised. The results were published recently in two separate articles in The Astrophysical Journal Letters by the EHT collaboration, which involves over 300 researchers from numerous organisations and universities around the world.

“The EHT is making fast strides, with technological updates that are being conducted on the network and adding new observatories. We hope that future observations of the EHT reveal with greater accuracy the structure of the magnetic field around the black hole and tells us more about the physics of the hot gas in this area,” concludes Jongho Park, member of the EHT cooperation and researcher at the Sinica Academy (Astronomy and Astrophysics Institute of Taipei).

Alejandro Mus, researcher in training affiliated to the GenT project of the University of Valencia, stresses that: “As well as the coordination work co-led by Iván Martí-Vidal, at the university we have also contributed to the development of several algorithms to overcome the instrumental limitations of the EHT, as well as to ensure the reproducibility of our analyses by any other researcher”.

Iván Martí-Vidal also assigns great value the efficiency of the Valencian group dedicated to analyse the polarisation in M87. “Despite being a small group, with just two people, our contribution has reached the same level as the ones from larger groups within the EHT collaboration,” says the researcher.

In parallel to these results, Iván Martí-Vidal also co-leads another official article from EHT, which shows a detailed study on the polarised emission of several black holes observed with the ALMA telescope. According to Martí-Vidal, “this is an example of how programmes such as GenT are helping put Valencian science and innovation on the world map.”

Other Valencian members of the EHT cooperation are Juan Carlos Algaba (University of Malaya), Rebecca Azulay and Eduardo Ros (both from the University of Valencia and the Max-Planck Institute of Radioastronomy, Germany).


Video by Iván Martí-Vidal:

– Paper VII (The Astrophysical Journal Letters, Vol. 910, L12):

– Paper VIII (The Astrophysical Journal Letters, Vol. 910, L13):