In the galaxy cluster Abell 194, something incredibly unlikely seems to be taking place. A radio jet originating from the lenticular galaxy NGC 541 may be impacting directly onto another galaxy in the cluster, the nearby dwarf galaxy known as “Minkowski’s Object,” which we will refer to as “MO” here to avoid some repetition. The coincidence in location is backed up by intense star formation in the supposed jet interaction region, detected through observations of Hα and CO.
1.5 GHz radio observation from the VLA with overlayed contours of neutral hydrogen (cyan) and molecular CO (magenta) - Adapted from Lacy et al. (2017). NGC 541 lies in the bottom right corner with one of its two jets pointed toward the upper left where it impacts onto MO, indicated by the cyan contours. At the interaction site, CO is observed, which is a tracer of star formation.
Some previous simulation work was done which showed that a jet impacting onto a dense cloud of gas can drive radiative shocks into the gas cloud lead to the collapse that eventually forms new stars.
Simulation of a jet—cloud collision. (Left) Temperature map from the simulation, showing the hot jet material (red) impacting onto the cool gas in the cloud (blue) and causing radiative cooling and collapse to form molecular gas (green). (Right) Zoomed in to the interaction region in the simulation, shows star formation locations at various times throughout the simulation from 10Myr (yellow) to 40 Myr (orange). Adapted from Fragile et al. (2017).
To better understand this type of dynamical interaction, and to help explain new radio observations of this system, we began a simulation study with Wombat of jet—cloud interactions. We began similar to the simulations above from Fragile et al. 2017, but adding in the magnetic fields (full Magnetohydrodynamics, not just Hydro) as well as tracking and evolving a population of cosmic ray electrons injected with the jet. This allowed us to calculate the radio emissions from the jet directly. We quickly realized that the geometry of the jet directly impacting onto the cloud would not work since the jet material rebounds laterally away from the interaction site for head-on impacts. This caused radio bright material to extend perpendicular away from the collision region, which is not observed in the case of MO.
Simulated radio emissions from the direct collision simulation 40Myr from the beginning of the simulation. Radio brightness contours (6GHz - Black) are shown on top of radio spectral index (6-15GHz). Cyan contours show the projected surface density of the dense cloud. The jet rebounds off the cloud and quickly leaves the simulated domain, without time to radiatively cool significantly, and leading to transverse radio emission that is not observed near MO.
Knowing that this simple dynamical picture was insufficient, we explored a range of other options, including offsetting the jet from the cloud center, putting the cloud in motion, and finally adding a “wind” or relative motion between the jet source and the surrounding medium. This wind causes the jet to bend, but also importantly pushes the older jet plasma away so it does not build up as a “cocoon” of low density, radio emitting plasma surrounding the jet. This better matched our target, since we don’t observe such a cocoon surrounding the jet near MO. Lastly, the morphology of NGC 541’s jet looks very much like a bent jet source (a so-called “Narrow Angle Tail” radio galaxy) that is highly projected away from our line of sight. With all these things in mind, we ran a final simulation:
3D rendering of the density from the simulated bent jet—cloud interaction. The low density, high temperature jets (in blue) are bent backward by pressure exerted by the higher density wind (density of wind not visible). The cloud (in orange - much cooler and denser than the wind) moves along with the wind, but also down and into the path of the jet. The jet is deflected as it rebounds off of the cloud.
With the simulation in hand, we were able to calculate the radio emissions from the magnetic fields and cosmic ray electrons we injected in the jets. This calculation can be done at any given frequency and from any viewing angle. Combining multiple radio maps at different frequencies allows us to create maps of the radio spectral index, which is related to the energetics of the cosmic ray electrons and the magnetic field strength in the jet. In absence of additional acceleration mechanisms or amplification of the magnetic field, the radio spectral index is a good indicator of the age of the cosmic ray electrons (i.e., how long they have existed since they were initially accelerated to high energy). This is because the electrons are continuously losing energy as they interact with the magnetic fields and produce the radio emissions we use to observe them as well as interacting with photons in the cosmic microwave background, another energy loss mechanism called inverse Compton. Here is a movie showing the evolution of the radio spectral index in this simulation (as well as a companion simulation, exactly the same but with no dense cloud):
Radio spectral index of the bent jet—cloud interaction simulation (top and middle left panels) and the companion simulation with no dense cloud (top and middle right panels). The lower panel shows the observed properties of NGC 541 and MO. Radio brightness contours (6GHz - Black) are shown on top of radio spectral index. Surface density contours (cyan) show the location of the dense cloud. The jet source is labeled with a blue circle. The wind direction in the top panels points from left to right, while in the middle panels, the view is rotated so the wind goes mostly into the page, but also partially up and to the left. In the top left panel, the dense cloud comes in from the upper left and begins to interact with the jet around 45 Myr. The cloud has fully entered into the path of the jet by 65 Myr, causing the jet material to rebound laterally.
Morphologically, this simulation matches the observations very well in the rotated view. We see the jet closer to MO (the eastern jet) going relatively straight toward MO until the interaction, then deflecting away. We also see the oppositely directed (western) jet is strongly bent backwards from our perspective and nearly overlapping the eastern jet in projection. Then, looking at the radio spectral index tells us a lot about this source. Particularly, we can see the radiative aging of the cosmic ray electrons as the spectral index becomes steeper (more negative) toward the back of the bent jets. This is due to the energy losses associated with radio emissions (synchrotron radiation) and inverse Compton. These details also match the observations well.
One detail that doesn’t match perfectly also tells us something about this system: the steepening of the spectral index in the jet in the close vicinity of MO. This is not observed in our simulation because at that location, a strong shock develops as the jet rebounds off the cloud. This does a number of things: (1) At a shock, electrons gain energy through a process called Diffusive Shock Acceleration, causing the observed spectral index to be flatter (less negative). (2) The magnetic fields are compressed, which increases surface brightness and would cause us to observe emissions from electrons which have undergone less extensive energy losses. This would also lead to observing a flatter spectrum. This means one of two things must likely be true: either the observed jet does not develop as strong of a shock here, or there are some other emission (or absorption) physics happening in MO that we do not include in the simulations that are altering the observed radio spectrum.
In addition to the spectral index, we also can learn a lot from the polarization information from our radio emissions. This is due to the fact that the radio emission from these sources comes from relativistic electrons spiraling around magnetic field lines. This type of emissions (called synchrotron) is highly polarized, meaning the electric (and magnetic) field orientation in the electromagnetic wave (light) is aligned in a certain direction. This direction is related to the direction of the magnetic field in the emitting region. We can plot these vectors on our image and show the (2D projected) orientation of the magnetic field for these sources:
Radio polarization for the two simulations (no cloud - top and with cloud - middle panels) and the observation (bottom panel). The polarization vector (projected magnetic field orientation) is shown with red vectors that have length proportional to their fractional polarization. The polarized intensity is shown in grayscale. Cyan contours show the projected surface density of the cloud as in the other figures.
These polarization images show the last important piece I wanted to highlight here. In the observation, near the source of the jet, the polarization vectors are perpendicular to the jet propagation direction. This implies that the magnetic field is “torroidal” or wraps around the jet rather than “poloidal” which would mean the field points along the direction the jet propagates. However, near the sides of the jet where the jet interacts with the surrounding medium, the vectors are rotated 90 degrees. This is likely due to stretching of the field lines along the jet direction in these locations due to velocity shear. This shear is a gradient in velocity due to the fact that the jet is moving very fast relative to its surroundings. We observe this transition from torroidal in the jet core to poloidal near the jet edge in the simulations as well. Most importantly, we see in both the observations and in the simulations a major transition in polarization angle at the location of the jet—cloud interaction region. This is the most striking proof of an interaction taking place, and our simulation reproduces the orientation change very well implying we are getting the dynamical scenario right (or at least close!). In our simulation, the jet rebounds off of the cloud but remains well collimated, which is why the magnetic fields remain well ordered, but change direction since the jet propagation direction is altered by the interaction. This may be the case in the observed jet as well.
To summarize, our simulations support the idea of a jet interaction is taking place between NGC541 and MO. They also help to constrain the dynamical scenario of this interaction. To previous simulation efforts, we add a wind to bend the jets, put the cloud in motion to pass through the jet, and most importantly for this work we have added magnetic fields and cosmic ray electrons so we could produce the radio images I’ve presented above. These type of comparisons to observations are where the Wombat code shows its unique capabilities. Few other codes have the cosmic ray physics to create self-consistent synthetic radio images.
If you are interested in learning more about this work, please read the paper in the Astrophysical Journal, or contact the author Chris Nolting with questions!