A Delicious Recipe for the Donut in NGC 6109

Title: Extreme jet bending on kiloparsec scales: the `doughnut’ in NGC 6109

Authors: Josie Rawes, Mark Birkinshaw, Diana M. Worrall

First Author’s Institution: HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK

Status: Published in Monthly Notices of the Royal Astronomical Society [open access]

In a delicious piece of line-of-sight luck, this paper explores recipes for the donut in NGC 6109. The system is shown in Figure 1, and has three main ingredients: one central galaxy, one main jet, and one donut. Previously such ring- or donut-like structures in low power (< 1046 erg/s) radio galaxies had not been observed. Today we know about more of these strange structures, which form the small and mysterious class of Odd Radio Circles. Understanding the production of the donut can tell us more about how galaxies interact with their external environments and about this new kind of radio phenomenon.

A diagram showing VLA data of the source. There is a main figure, which has a donut-shaped feature in the bottom left and the extended streak of the jet leading up and to the right. There is a pop-out box highlighting the donut feature.
Figure 1: VLA radio data showing all three components of the system. The image is color scaled by the radio surface brightness. The ‘main jet’ extends up and to the right (north-west) from the galaxy (‘core’). The ‘donut’ is in the left bottom (SE) corner, with a slightly dimmer center. (Figure 2 in the paper.)

Ingredients

NGC 6109, like most galaxies, has a supermassive black hole (SMBH) at its center. This SMBH is pulling so much galactic material into its accretion disk that it has become an Active Galactic Nucleus (AGN). Along its axis of rotation, the AGN produced two strong outflows of plasma (hot, magnetized gas), called jets. These jets transport enormous amounts of mass, momentum, and magnetic fields from the AGN into the surrounding intracluster medium (ICM) where they produce radio emissions via synchrotron radiation. Galaxies with these large radio emitting jets are, unsurprisingly, known as radio galaxies

The north-west main jet is a mostly straight outflow extending for 250 kpc from the central ‘core’ galaxy. Most jets come in pairs, so the lack of a south-east facing jet in existing data was odd. Updated VLA radio observations found something stranger: instead of a symmetric jet, they found the donut shaped structure. 

The donut in NGC 6109 has a diameter of about 6 kpc and is very faintly connected to the core. Assuming that the two jets began at the same time with the same speed, they should have matching features. The placement of the donut matches the bright knot 12 arcseconds along the north-west jet and the two have similar energy densities. This suggests a common origin and that the donut is actually the tail end of the south-east jet.

Mise en Place

In order to characterize the donut feature in the observations, the authors considered two important measurements. First, the fractional polarization, which is a measurement of how organized the light’s orientations are. The donut has an increase in its fractional polarization around the edges (see Fig. 2, top panel). This is similar to what’s seen in the ends (lobes) of radio jets that aren’t bent, adding support to the idea that the donut is the result of the second jet. However, the edges are not significantly more polarized than the center, so there likely is no large-scale ordered magnetic field in the area.

A top and bottom paneled image. The two images show the donut shape and the extended jet colored in by the polarization (top) and rotation measure (bottom) on white backgrounds.
Figure 2:  The top panel shows a map of the fractional polarization of the system. The bottom panel shows the RM of the donut and main jet. Areas in green/yellow correspond to the highest fractional polarization (top) or highest RM (bottom).(Figure 3 in the paper.)

The authors also evaluated the rotation measure (RM), which allows them to estimate the magnetic fields in the region based on how those fields affected the light’s polarization. The southern part of the donut has a higher RM and a higher radio surface brightness. This might be due to an interaction with a magnetized plasma in the local environment that could create compressive shocks and trigger the radio emission. The fact that the RM is not constant across the donut suggests that there is some amount of magnetic material in the area, even if the polarization map suggests that the magnetic fields aren’t widespread and organized.

Donut Recipes

The authors do-not (🍩) know for sure how nature cooked up this interstellar pastry, but they narrowed it down to two possibilities: either an intrinsically precessing jet or an externally bent jet. In both scenarios the donut is the radio emission from the south-east jet as viewed end-on. 

In a precessing AGN the jets flow along a straight line while the direction of jet ejection changes over time. This precession could come from a number of sources, including a binary central black hole or the Lense-Thirring Effect. Precession simulations are able to produce a donut-like structure (see Fig. 3), but would cause the other jet to also be looped, which is not observed (see Fig. 1). The precessing jet model also cannot explain the observed RM or magnetic field structures, as those are attributed to environmental interactions.

A simulation output of a central AGN and one red and one blue jet emerging from it. The two jets both loop around, and the blue jet has a circular projection towards the viewer, suggesting a donut shape.
Figure 3: A simulation of the precessing jet model. In blue the opening of the donut exists, but there is an additional loop not observed in the data. (Figure 8 in the paper.) 

The other possibility is that the jet is less dense than its surroundings and was deflected by some external influence. However, there isn’t observational evidence for a collision with the kind of large gas cloud needed to create the bend. While the southern RM of the donut suggests some interaction with magnetized plasma, this interaction alone could not produce the observed degree of bending. The amount of fractional polarization observed is also lower than in known bent jet sources. External fluid pressure could only bend the jet if the jet is extremely low-density, over a hundred-thousand  times less dense than the ICM. The bend could be caused by a bulk flow with a magnetic field, but this would likely also impact the north-west jet. 

While there are still empty pages in the cookbook for this type of radio ring, today’s authors have used NGC 6109 to narrow down the recipe. 

Astrobite edited by Amaya Sinha and Konstantin Gerbig.

Featured image credit: cottonbro studio, Paper Figure 1

About Lindsey Gordon

Lindsey Gordon is a fourth year Ph.D. candidate at the University of Minnesota. She works on AGN jets, radio relics, MHD simulations, and how to use AI to study all those things better.

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