A Delicious Recipe for the Donut in NGC 6109

Title:

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 [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. As recently as 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 mysterious donut. Previously such ring- or donut-like structures in low power (< 10⁴⁶ erg/s) had not been observed. Today we know about more of these strange structures, which form the small and mysterious class of . 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.

Ingredients

NGC 6109, like most galaxies, has a () at its center. This SMBH is pulling so much galactic material into its that it has become an Active Galactic Nucleus (). 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 (ICM) where they produce radio emissions via . Galaxies with these large radio emitting jets are, unsurprisingly, known as . 

The north-west main jet is a mostly straight outflow extending for 250 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 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 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 , 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 () 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.

The authors also evaluated the (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 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 . 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.

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: , Paper Figure 1