Title: Orbital motion of NGC 6166 (3C 338) and its impact on the jet morphology at kiloparsec scales
Authors: A. S. R. Antas, A. Caproni, R. E. G. Machado, T. F. Laganá, G. S. Souza
First Author’s Institution: Núcleo de Astrofísica, Universidade Cidade de São Paulo R. Galvão Bueno 868, Liberdade, São Paulo, SP, 01506-000, Brazil
Status: Published in MNRAS [open access]
AGN jets are massive plasma outflows from the central black hole and accretion disk of a galaxy. The jets stretch out for up to hundreds of kiloparsecs, and typically terminate in “lobes” of radio-bright material. Hercules A (Fig. 1) is a classic example of what a radio galaxy with two jets looks like, but there are many cases of jets with unusual morphologies. Jets may bend, be one-sided, or even form a donut-like structure. The cause of these strange shapes is thought to come from a variety of different effects, including jet precession, density and temperature gradients, and the relative motions of the intracluster medium (ICM) and galaxy, making identifying the exact cause of specific shapes difficult.
[via NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA)]
In this paper, the authors investigate the radio structure in NGC 6166 (optical designation) / 3C 338 (radio designation). NGC 6166 is the brightest central galaxy (BCG) in the Abell 2199 (A 2199) cluster. It is a cD galaxy, a type of very large elliptical galaxy with an extended stellar halo and signatures of AGN activity at its center.
The full system of 3C 338 is shown in the radio band in Figure 2 below. The inner green rectangle highlights the compact radio core region, which coincides with the position of the optical nucleus in optical data of NGC 6166. There are two small symmetric “new” radio jets emerging from the core, and two “old” jets that are fainter and extend out to much larger scales along the same east-west direction. These “old” jets end in the east-west lobes indicated on the diagram. There is also a large peculiar “ridge” feature. No other object that could have produced this ridge feature has been observed, and determining its formation is one of the key objectives of this paper. The shape of the ridge and lobes 1 and 2 in the diagram are reminiscent of a wide angle tail (WAT) radio galaxy, which are typically the BCG of a cluster, like NGC 6166.
The distinction between the old and new material is based on the estimates of the spectral indexes of the radio observations. The indexes are steeper in the radio lobes and the ridge, suggesting that they are older than the new jets and core. This is due to the higher rate of synchrotron energy loss from high energy electrons compared to lower energy electrons. The extended features, including the ridge, could be extremely distorted jets or due to previous jet activity originating from NGC 6166 over the past tens of millions of years.
Two major hypotheses for this system’s formation are that the ridge is due to either (1) orbital motion of NGC 6166 in the cluster or (2) ram pressure stripping from an accretion flow. In this work, the authors explore the first possibility by modeling the orbital motion of NGC 6166 around the presumed center of the A 2199 cluster. The cluster center is estimated based on the position of peak X-ray brightness, which is due to the ICM and does not necessarily coincide with the optical center of the BCG. There is a 1 kpc offset between the optical center of NGC 6166 and the X-ray center of A 2199, which may indicate that there is significant motion by the galaxy within the cluster due to previous mergers and sloshing effects. This motion could be causing bent and unusual jet structures to form.
Orbital Modeling + Hydrodynamic Simulations
To test this hypothesis, the authors first generated a set of possible orbits for NGC 6166 around the cluster center. They used the python package galpy to numerically integrate the orbit for a given dark matter potential. The positions and velocities of the galaxy and cluster were estimated from the data, and were used to estimate limits on the orbital conditions that would allow NGC 6166 to have produced the ridge feature in a previous epoch of jet emission.
These sets of parameters were then fed into hydrodynamical simulations using the PLUTO code to try to reproduce the system morphology. In these simulations, the jet emitting region (“inlet region”) moves along the orbits defined by their previous calculations. In this type of simulation, jet parameters are chosen and manually injected; the central accretion disk system is assumed to have produced those values and is not directly simulated. The jet was manually turned on and off near the ridge region and its current position to try and produce the ridge and new jet features.
Resulting Jet Morphologies
For simulations with constant jet power, the authors found the brightest region of the simulated galaxy to correspond to the present day position of NGC 6166 and the new jets. In models with an increased orbital speed, the relative motion between the AGN and ICM increases. This leads to ram-pressure forces that deflect the jets and induce curvature. Some of the simulations are able to produce the main features of 3C 338 (Figure 3), including a ridge feature. However, the feature is not exactly the size expected, opening up the idea of jets with non-constant powers.
Once the constraint of constant jet power was relaxed, the authors were able to produce simulations (Figure 4) that developed the shape of the extended structure and ridge in the right positions. There is a slight inclination not observed in reality, but it is minor.
These simulations are not absolute evidence that the observed structures are caused by orbital motion of NGC 6166, but they do demonstrate the feasibility of this formation method. The orbital motion of the galaxy can plausibly be explained by previous cluster mergers causing major sloshing motions and disturbances to the gravitational potential. Future work will need to investigate the alternative models of ram pressure stripping and the roles of sloshing and magnetic fields to make definitive conclusions about the formation of this unique radio system.
Astrobite edited by Alexandra Masegian
Featured image credit: Antas+24 Figure 1
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