Pōwehi: Astronomers Image Magnetic Fields at the Edge of M87’s Black Hole

Two Hawaiʻi-based telescopes, the James Clerk Maxwell Telescope, operated by the East Asian Observatory and the Submillimeter Array, operated by the Smithsonian Astrophysical Observatory and the Academia Sinica Institute for Astronomy and Astrophysics, have once again combined efforts with the global network of telescopes known as the Event Horizon Telescope.

Today the image of Pōwehi, the Black Hole at the center of M87, has been shown in new light – specifically polarized light. The polarized light has enabled astronomers for the first time in history to measure polarization, a signature of magnetic fields, this close to the edge of a black hole. The observations are key to explaining how the M87 galaxy, located 55 million light-years away, is able to launch energetic jets from its core. 

On April 10, 2019, scientists released the first ever image of a black hole, Pōwehi, 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 polarized. 

Light becomes polarized when it goes through certain filters. As an example many of us here in Hawaiʻi have polarized sunglasses, in space light can become polarized when it is emitted in hot regions of space that are magnetized. In the same way polarized 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 polarized. Specifically, polarization allows astronomers to map the magnetic field lines present at the inner edge of the black hole. 

The bright jets of energy and matter that emerge from M87’s core and extend at least 5000 light-years from its center 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 polarized 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 team found that only 0.1 percent of the theoretical models can explain what the astronomers are seeing at the event horizon. The new observations also revealed information about the structure and strength of the magnetic field just outside the black hole that astronomers didn’t have before. 

“Our first glimpse of Pōwehi – a snapshot of the total light intensity – was like seeing the movie poster. Now, with our polarized glasses on, we have front row seats as the film begins. The polarized images show us how black holes do what they do and why we see what we see,” JCMT Deputy Director, Dr Jessica Dempsey states. “Our worldwide and home team pushed every technical, theoretical and observational boundary to achieve this. And we are still in the first minutes of the story. We have so much more to see. Pass the popcorn.” 

To observe the heart of the M87 galaxy, the collaboration linked eight telescopes around the world, including the JCMT and SMA located on Maunakea, 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. 

This allowed the team to directly observe the black hole shadow and the ring of light around it, with the new polarized-light image clearly showing that the ring is magnetized. 

“The EHT is a one-of-a-kind facility to test the laws of physics in a region of extreme gravity. It gives us a unique chance to look at phenomena we have never studied before,” said EHT collaboration member Jongho Park, an East Asian Core Observatories Association Fellow at the Academia Sinica, Institute of Astronomy and Astrophysics in Taiwan. Future EHT observations will reveal even more information about the mysterious region of space near the event horizons of supermassive black holes.

The results are published today in two separate papers in The Astrophysical Journal Letters by the EHT collaboration. The research, which was coordinated by Mościbrodzka, involved over 300 researchers from multiple organisations and universities worldwide.

Simon Radford, Director of Hawaiʻi Operations, Submillimeter Array said, “This research showcases the close cooperation between observatories in Hawaiʻi and elsewhere. The SMA and the JCMT have participated in the EHT for more than a decade. They will continue to play a major role in future EHT observations because of their location, their technology, and the dedication of their talented staff.”