Astronomers Spot Magnetic Filaments in Accretion Disk around Messier 77’s Central Black Hole | Sci.News

Messier 77 is a well-known, relatively nearby, bright spiral galaxy with a supermassive black hole at its center.

Messier 77 is a barred spiral galaxy located 62 million light-years away in the constellation of Cetus.

Also known as NGC 1068, LEDA 10266 and Cetus A, it has an apparent magnitude of 9.6.

Messier 77 was discovered by the French astronomer Pierre Méchain in 1780, who originally identified it as a nebula. Méchain then communicated the discovery to his colleague, the French astronomer Charles Messier.

Messier believed that the highly luminous object he saw was a cluster of stars, but as technology progressed its true status as a galaxy was realized.

At 100,000 light-years across, Messier 77 is one of largest galaxies in the Messier catalogue — so massive that its gravity causes other nearby galaxies to twist and become warped.

It is also one of the closest galaxies with an active galactic nucleus (AGN).

Such active galaxies are among the brightest objects in the Universe and emit light at most, if not all, wavelengths, from gamma rays and X-rays all the way to microwaves and radiowaves.

Despite its status as a popular target for astronomers, however, Messier 77’s accretion disk is obscured by thick clouds of dust and gas.

A few light-years in diameter, the outer accretion disk is dotted by hundreds of distinct water maser sources that hinted for decades at deeper structures.

Masers are distinct beacons of electromagnetic radiation that shine in microwave or radio wavelengths; in radio astronomy, water masers observed at a frequency of 22 GHz are particularly useful because they can shine through much of the dust and gas that obscures optical wavelengths.

Bucknell University astronomer Jack Gallimore and colleagues set out to observe Messier 77 with twin goals in mind: astrometric mapping of the galaxy’s radio continuum and measurements of polarization for its water masers.

“Messier 77 is a bit of a VIP among active galaxies,” said Dr. C.M. Violette Impellizzeri, an astronomer at Leiden Observatory.

“It is unusually powerful, with a black hole and an edge-on accretion disk. And because it is so nearby, it has been really, really well-studied in detail.”

The study authors looked at Messier 77 in a completely new way, however.

Their observations relied upon the recently upgraded High Sensitivity Array (HSA), which consists of NSF’s NRAO telescopes at the Karl G. Jansky Very Large Array, the Very Long Baseline Array, and the Green Bank Telescope.

By measuring the polarization of water masers as well as the continuum of radio emissions from Messier 77, they generated a map revealing the compact radio source now known as NGC 1068* as well as mysterious extended structures of more faint emissions.

Mapping the astrometric distribution of the galaxy and its water masers revealed that they are spread along filaments of structure.

“It really came out in these new observations, that these filaments of maser spots line up like beads on a string,” Dr. Gallimore said.

“We were stunned to see that there’s a clear offset — a displacement angle — between the radio continuum showing the structures at the galaxy’s core and the locations of the masers themselves.”

“The configuration is unstable, so we are probably observing the source of a magnetically-launched outflow.”

HSA measurements of the polarization of these water masers revealed striking evidence of magnetic fields.

“No one has ever seen polarization in water masers outside of our Galaxy,” Dr. Gallimore said.

“Similar to the looping structures seen on our Sun’s surface as prominences, the polarization pattern of these water masers clearly indicates that magnetic fields are also at the root of these light-year-scale structures as well.”

“Looking at the filaments, and seeing that the polarization vectors are perpendicular to them, that’s the key to confirming that they are magnetically driven structures. It’s exactly what you’d expect to see.”

Previous studies of the region hinted at patterns usually associated with magnetic fields, but such conclusions remained beyond the reach of observing technology until recently.

The findings reveal evidence of a compact central radio source (galaxy’s supermassive black hole), clear polarization of the water masers indicating structure within Messier 77’s magnetic fields, and spectacular extended features across the continuum of radio frequencies.

Together, these findings indicate that magnetic fields are the underlying drivers of these phenomena.

Plenty of mysteries remain, however. Within the radio continuum map, for instance, there is a diffuse, faint protrusion that the team nicknamed the foxtail, which extends northward from the central region.

“We said to ourselves, when we set out to do this, ‘let’s see if we can really push the limits and get a good continuum as well as polarization data.’ And both of those goals succeeded,” Dr. Gallimore said.

“With the NSF NRAO High Sensitivity Array, we detected water megamaser polarization for the first time, and we also made a really amazing continuum map that we’re still trying to wrap our minds around.”

A paper describing the resutls was published today in the Astrophysical Journal Letters.

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Jack F. Gallimore et al. 2024. The Discovery of Polarized Water Vapor Megamaser Emission in a Molecular Accretion Disk. ApJL 975, L9; doi: 10.3847/2041-8213/ad864f