Galaxy Clusters

Galaxy clusters are the largest gravitationally bound structures in the universe today, and contain hundreds to thousands of galaxies orbiting around within a massive dark matter halo. They also contain a large amount of high temperature plasma (ionized gases, mostly Hydrogen) that we call the intracluster medium (ICM). 

Why study galaxy clusters?

Clusters are continuously undergoing hierarchical mergers (eating smaller clusters and groups) and accreting from their environments as they grow. Their dynamical state is very sensitive to cosmological models, so by studying clusters we can learn a lot about both the early history of the universe. The motions of the ICM is also (very sensitive) to plasma physics that is hard to study in laboratory environments, at high temperatures (10s of millions of degrees Kelvin) that would melt any container and at lower densities (less than 1000 atoms per cubic meter) than the best vacuum on Earth. However, even at these low densities, the ICM has a lot more mass than all the galaxies in the cluster put together, since it fills such a huge volume.

Radio Galaxies

Radio galaxies are galaxies that have a powerful jet emanating from their cores where a supermassive black hole accelerates material to velocities near the speed of light. This jet contains both magnetic fields and high energy electrons which radiate radio waves that we can observe.

Why study radio galaxies?

Radio galaxies inject a huge amount of energy into the ICM and help regulate the thermodynamics of the cluster. Without this energy, the ICM would cool and form many more stars than we see today (less than 5% of the cluster’s total “normal matter,” i.e. non-dark-matter, is in stars). Also, we see many diffuse radio structures in clusters, some connected to radio galaxy jets and others that remain mysterious. These radio structures can help us learn about the dynamics of the ICM. Some of them trace shocks in the environment, while others show evidence of turbulence. Some of the radio emissions that are not obviously connected to radio galaxies may come from magnetic fields and high energy particles that were initially injected into the ICM by the jets from radio galaxies. Studying radio galaxies can help us better understand the clusters in which they are embedded. 

Shocked Radio Galaxy Jets:

We studied what happens when a radio galaxy jet is hit by a shock wave traveling through the ICM. These shocks occur all the time in cluster, but particularly when two clusters merge together. We found that the shocks would collapse the “cocoons” that the low density jets excavate within the denser ICM. As the cocoons collapse, vorticity is generated and the cocoon plasma would get wrapped up in vortex rings by similar physics to blowing smoke rings or bubbles underwater. Additionally, the propagation of the jet would be affected depending on the angle at which the jet and shock met. If the shock pointed directly along the jet axis, the jet propagating against the shock would be slowed, stopped, or even reversed depending on the strength of the shock. If the jet was perpendicular to the shock direction, it would be bent backward into a pair of tails pointing in the direction the shock was traveling. If the jets were at some intermediate angle, the one pointing partially against the shock would travel forward, slow, bend, then reverse backward. Interestingly, whenever the jets were bent backward, the jet trajectory would become unstable and the tip of the jet would ‘flap’ around due to small pressure waves that induced instabilities in the jet. This flapping helps explain how some jets transition from well collimated, highly ordered flows to more turbulent flows that I refer to as tails, trailing behind the jets but no longer driving forward with significant momentum.

Papers: Jones et al. 2017, Nolting et al. 2019a, Nolting et al. 2019b 

Precessing Jets:

Some radio galaxies show side lobes and/or strong curvature in the jets. There have been multiple attempts to explain these, including multiple AGN, sudden reorientation of the jets, backflow of the jet material or precession of the jets. This precession could happen for a number of reasons, including a binary supermassive black hole system or that the disk of gas surrounding the supermassive black hole is tilted with respect to the ‘spin axis’ of the black hole, which can cause it to slowly rotate due to general relativistic effects. We performed some simulations to study how a radio galaxy jet would evolve if its launch direction changed over time, effectively inducing precession. We were able to recreate radio galaxy morphologies that resemble both those with curves and side lobes, depending on the precession angle (how wide a region the jet rotates around), the amount of time it took the jet to make one full rotation, and the angle of observation toward the source. We also found a curious detail in the jet’s propagation. When the jet had gone around about one full revolution, the jet would suddenly transition from being bent backward as it rotated to straitening out before bending again. Looking more closely, we realized the jet was encountering some previous jet material left behind earlier in its evolution. This meant that the force that bent it backwards, which depends on the density of the material the jet is running into, was greatly reduced as it ran into the low density material that the jet had left behind on its last passage. During this sudden straightening, the direction of the jet changed abruptly, something that would normally require some significant outside influence from its surroundings. However, in this case, the jet was able to self-induce this change. Check out this blog post for more details!

Papers: Nolting et al. 2023

Minkowski’s Object & Jet—Cloud interactions:

Minkowski’s Object (MO) is a dwarf galaxy in the galaxy cluster Abell 194. It is well studied because it shows a burst of star formation that is uncommon in this kind of galaxy, but also because of its closeness to a radio jet from a larger nearby cluster member NGC 541. In fact, it looks as though the jet is hitting MO. This system is the poster-child of so called “jet-induced star formation.” Normally, the presence of a jet from a supermassive black hole is associated with an absence of star formation, since these systems produce a massive amount of harsh radiation that prevents cool gas from collapsing to form new stars. However, in certain circumstances, these jets may be able to help stars form by pushing directly onto the gas and forcing it to collapse. That seems to be the case in MO. Some previous simulations tested this idea with a hydrodynamic jet running into a cloud of cool gas and they were able to show radiative collapse which lead to high enough densities to form stars. With some new radio data from some observer colleagues, we wanted to reexamine this system and its radio properties. Our simulations lead to refining the dynamical scenario of the interaction, including introducing a "wind" that bent the jets and putting the cloud in motion with respect to the jet. We found good morphological agreement as well as matching many details of the radio spectrum and polarization properties of the jet emissions. Read in more detail in this blog post.

Papers: Nolting et al. 2022

Jet Backflows:

Again looking at radio galaxies with odd side lobes, I wanted to test the idea that some observers have that they might form as jet material backflows and rebounds off the medium of the host galaxy. We embedded our jets within a density profile and tested a few scenarios, including precession, backflow, and sudden reorientation. We found that backflow is difficult to show in stratified environments

Papers: Nolting et al. 2023 (in progress)

Jet Injection Models:

We do not understand the energy content of radio galaxy jets on kpc scales very well. Are they still relativistic on these scales? Are their magnetic fields dynamically significant? How much mass is in the jet? How much do cosmic rays contribute to the pressure in the jets? There are various models for how to inject these jets on these large scales in galaxy cluster simulations. We are working to test various jet models and make comparisons to observed radio galaxies by generating realistic radio images from our simulations. We can look at morphology, brightness distributions, and polarization properties of these jets to differentiate these jet models.

Papers: Nolting et al. 2023 (in progress)

Magnetohydrodynamics (MHD)

I use computer simulations to model these types of interactions. We often model the ICM and the jets from radio galaxies as fluids. Since these fluids are so hot (10's of millions of degrees Kelvin) they are completely ionized plasmas. Due to this, the ICM contains magnetic fields and we have to use an extension to the equations of hydrodynamics to accurately model its evolution. These magnetohydodynamic equations couple Maxwell's equations of electromagnetism with the conservation of mass, momentum, and energy.

Publications

Nolting, Chris; Lesoine, C. J.; Mendygral, P. J.; Simulating the formation of Radio Galaxy Wings, (in prep - 2022)

Nolting, Chris; Ball, J. C.; Nguyen, T.; Precessing AGN Jets and the Formation of X-shape Radio Galaxies, arxiv # 2301.04343 (2023)

Nolting Chris; Lacy, M.; Croft,S.; Fragile,P.C.; Linden,S.T.; Nyland,K.; Patil,P., Observations and Simulations of Radio Emission and Magnetic Fields in Minkowski’s Object, The Astrophysical Journal, Volume 936, Issue 2, id.130, 12 pp. (2022)

Rudnick, L.; Bruggen, M.; Brunetti, G.; Cotton, W.; Forman,W.; Jones, T. W.;Nolting, Chris; Schellenberger, G.; Sebokolodi, L.; van Weeren, R., Intracluster Magnetic Filaments and an Encounter with a Radio Jet, The Astrophysical Journal, Volume 935, Issue 2, id.168, 24 pp. (2022)

Nolting, Chris; Jones, T. W.; O’Neill, Brian J.; Mendygral, P. J., Simulated Interactions between Radio Galaxies and Cluster Shocks. II. Jet Axes Orthogonal to Shock Normals, The Astrophysical Journal, Volume 885, Issue 1, article id. 80, 14 pp. (2019)

O’Neill, Brian J.; Jones, T. W.; Nolting, Chris; Mendygral, P. J., A Fresh Look at Narrow-Angle Tail Radio Galaxy Dynamics, Evolution and Emissions, The Astrophysical Journal, Volume 884, Issue 1, article id. 12, 21 pp. (2019)

O’Neill, Brian J.; Jones, T. W.; Nolting, Chris ; Mendygral, P. J., Shocked Narrow-Angle Tail Radio Galxies: Simulations and Emissions, The Astrophysical Journal, Volume 887, Issue 1, article id. 26, 18 pp. (2019)

Nolting, Chris ; Jones, T. W.; O’Neill, Brian J.; Mendygral, P. J., Interactions Between Radio Galaxies and Cluster Shocks - 1: Jet Axes Aligned with Shock Normals , The Astrophysical Journal, Volume 876, Issue 2, article id. 154, 16 pp. (2019)

Mendygral, P. J.; Radcliffe, N.; Kandalla, K.; Porter, D.; O’Neill, B. J.; Nolting, C. ; Edmon, P.; Donnert, J. M. F.; Jones, T. W., WOMBAT: A Scalable and High-performance Astrophysical Magnetohydrodynamics Code , The Astrophysical Journal Supplement Series, Volume 228, Issue 2, article id. 23, 23 pp. (2017)

Tom Jones, Chris Nolting , Brian O’Neill, Peter Mendygral, Using Collisions of AGN Outflows with ICM Shocks as Dynamical Probes, Physics of Plasmas, Volume 24, Issue 4, id.041402

Chris Nolting, Liliya L. R. Williams, Michael Boylan-Kolchin, Jens Hjorth, Testing DARKexp against energy and density distributions of Millennium-II halos, Journal of Cosmology and Astroparticle Physics, Issue 09, article id. 042 (2016)

Presentations

Chris Nolting, Precessing Radio Galaxy Jets: Dynamics and Observable Properties, Talk, 38th New Mexico Symposium, Socorro, NM, February 2023

Chris Nolting, Simulations of Precessing Jets and the Formation of X-shaped Radio Galaxies, Oral Session Talk, American Astronomical Society Meeting 241, January 2023

Chris Nolting; Mark Lacy; Steve Croft; P. Chris Fragile; Sean Linden; Kristina Nyland; and Pallavi Patil, Observations and Simulations of Radio Emission and Magnetic Fields in Minkowski’s Object, iPoster Presentation, American Astronomical Society Meeting 240, June 2022

Chris Nolting, Precessing Radio Galaxy Jets: Simulations and Observable Signatures, Poster Presentation, Extragalactic jets on all scales - launching, propagation, termination, June 2021

Chris Nolting; Tom Jones; Chika Onubogu; Alex Reineck; Peter Mendygral, Magnetic Field Configurations in Dynamic Radio Galaxy Environments, Oral Session Talk, American Astronomical Society Meeting 237, January 2021

Chris Nolting; Tom Jones, Shocked Radio Galaxies, Invited talk, 10th Korean Astrophysics Workshop, Busan, Korea, July 2019

Chris Nolting; Tom Jones,Shocked Radio Galaxy Jets: Clues to Cluster Weather, Invited talk, Wombat User Group Meeting, Bologna, Italy, July 2018

Chris Nolting; Tom Jones, Shocked Radio Galaxy Jets: Clues to Cluster Weather, Snowcluster 2018 Conference Talk, March 2018

Chris Nolting, Tom Jones, Brian O’Neill, Peter Mendygral, Shocked Radio Jets: Emerging Complex Structures, Physics of the Instracluster Medium: Theory and Computation Workshop, Poster Presentation, August 2016

Chris Nolting, Tom Jones,Radio Galaxy Dynamics: A Mpc long tail in Abell 2256, Minnesota Supercomputing Institute Research Exhibition, Poster Presentation, April 2016