Revealing the Dust Around an Active Galactic Nucleus with the JWST

Supermassive Black Holes (SMBHs) are located in the centers of large galaxies like ours. When they’re actively feeding, they produce more light and are called active galactic nuclei (AGN). But their details are difficult to observe clearly because large clouds of gas block our view.

The JWST was built just for circumstances like these.

New research published in the Monthly Notices of the Royal Astronomical Society (MNRAS) presents JWST observations of an SMBH in a galaxy about 70 million light-years away. The telescope found polar dust surrounding the SMBH. It was outside the expected torus of dust that directly accretes onto the black hole that researchers call the accretion disk. The polar dust is heated, but rather than being heated by the radiation coming from the heated accretion disk, the gas is heated when by energetic shock waves that come from relativistic jets.

The research is titled “Dust beyond the torus: revealing the mid-infrared heart of local Seyfert ESO 428-G14 with JWST/MIRI.” The lead author is Houda Haidar, a PhD student in the School of Mathematics, Statistics, and Physics at Newcastle University in the UK. Houda and her co-researchers are members of GATOS, the Galactic Activity, Torus, and Outflow Survey. According to the GATOS website, GATOS is an international team using the JWST to “crack the enigma that is active galactic nuclei.”

“Having the opportunity to work with exclusive JWST data and access these stunning images before anyone else is beyond thrilling,” said Houda. “I feel incredibly lucky to be part of the GATOS team. Working closely with leading experts in the field is truly a privilege.”

This is the JWST’s first look at the galaxy in question, ESO 428-G14, but it’s not astronomers’ first look at it. They’ve been observing the galaxy—called a Seyfert galaxy because of its high luminosity—for decades. The astronomy community has used several telescopes to examine the galaxy, including ALMA and the Hubble, and that data forms part of this work.

The challenge in observing this AGN, and many others like it, is dust. The thick, extensive clouds of dust and gas that eventually feed the black hole block our view of it. The JWST’s job description is to pierce dust like this and get a clearer look into these obscured regions.

The JWST has four primary science themes, one of which is Galaxies Over Time. A combination of processes drives galaxy evolution, and active galactic nuclei are part of the picture.

Active galactic nuclei can emit relativistic jets of material from their poles that, in some cases, can extend hundreds of millions of light-years into space. ESO 428-G14 is no different; it emits radio jets from its poles. Astrophysicists know that gravitational and magnetic forces are behind these jets, but the exact mechanism behind them is unknown and is an active area of research.

The jets could be the key to understanding SMBHs, how they recycle material in galaxies, and the dust that accumulates around them in a torus. “For decades, the dusty torus has been held responsible for the dichotomy between type?1 and type?2 active galactic nuclei (AGN), forming the keystone of AGN unification,” the authors write.

The unified AGN model states that types 1 and 2 AGN are differentiated by their viewing angles rather than by any fundamental differences between the two. Type 1 is viewed more face-on relative to the dust torus, displays broad emission lines in its spectra, and has visible accretion disks. Type 2 is viewed more edge-on relative to the torus, has narrow emission lines, and has obscured accretion disks.

Some AGN have polar dust that’s separate from their torus dust. Many models predict that this dust is energized by the jets that come out of its poles. “However, little is known about its characteristics, spatial extent, or connection to the larger scale outflows,” the researchers write in their paper. This is “the first JWST/MIRI study aimed at imaging polar dust by zooming on to the centre of ESO 428-G14.”

The JWST found extended Mid-infrared emissions that extended to 650 light-years from the AGN. The structure of this polar dust is co-linear with a radio jet emitted by the AGN. But the dust is perpendicular to a molecular gas lane that’s feeding the AGN and obscuring it. This is important evidence for the presence of polar dust. “Its morphology bears a striking resemblance to that of gas ionized by the AGN,” the authors write.

This brings us back to the ongoing debate about how AGN energize the gas and dust in their environment. What role do the jets play vs what role does electromagnetic radiation from the AGN play? One line of evidence shows that dust absorbs electromagnetic radiation emitted by the heated dust in the accretion disk.

However, the new JWST images show that much of the polar dust emission is extended and spread out along the jets’ paths. This clearly implies that the jets are responsible for heating and shaping the dust, and radiation from the AGN plays a lesser role. The accretion dust and the polar dust have different temperatures, and that provides clues about how different parts of the AGN are heated differently. Jet-induced shocks could be responsible for the heat differences between the polar dust and the accretion dust.

“There is a lot of debate as to how AGN transfer energy into their surroundings. We did not expect to see radio jets do this sort of damage. And yet here it is!” said Dr David Rosario, Senior Lecturer at Newcastle University and co-author of the study.