Astronomers Discover Magnetic Loops Around Supermassive Black Hole

NGC 1068 is a well-known, relatively nearby, bright galaxy with a supermassive black hole at its center. Despite its status as a popular target for astronomers, however, its 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.

Led by astronomer Jack Gallimore of Bucknell University, an international team of astronomers and students set out to observe NGC 1068 with twin goals in mind: astrometric mapping of the galaxy’s radio continuum and measurements of polarization for its water masers. “NGC 1068 is a bit of a VIP among active galaxies,” says co-author C. M. Violette Impellizzeri. “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.” Gallimore and his team set out to look at NGC 1068 in a completely new way, however.

Observations of this nature relied upon the recently upgraded High Sensitivity Array (HSA), which consists of the NSF NRAO telescopes at the Karl G. Jansky Very Large Array, the Very Long Baseline Array, and the Green Bank Telescope networked together and fully supported by NSF NRAO. By measuring the polarization of water masers as well as the continuum of radio emissions from NGC 1068, the team 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 NGC 1068 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,” Gallimore summarized. The team was 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,” Gallimore emphasized. 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,” Gallimore summarizes. 

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. “Only the HSA has the combination of resolution and sensitivity needed to map out magnetic fields using polarized light,” says Gallimore. Impellizzeri adds, “There have been a lot of upgrades that the NSF NRAO facilities have undergone. All of the telescopes have had significant improvements. And so one of the reasons we decided that it was worth redoing these observations, instead of just going back to the archive, was that we knew we would get much better data now.”

Their findings reveal evidence of a compact central radio source (the galaxy’s supermassive black hole), clear polarization of the water masers indicating structure within NGC 1068’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. 

Gallimore says, “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. 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.” 

About NRAO 

The National Radio Astronomy Observatory (NRAO) and the Green Bank Observatory (GBO) are major facilities of the U.S. National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.