Title: On the Origin of the Ancient, Large-scale Cold Front in the Perseus Cluster of Galaxies
Authors: Elena Bellomi, John ZuHone, Rainer Weinberger, Stephen Walker, Irina Zhuravleva, Mateusz Ruszkowski, Maxim Markevitch
First Author’s Institution: Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
Status: Published in ApJ [open access]
To quote the first sentence of every paper about galaxy clusters: galaxy clusters are the largest gravitationally bound objects in the universe (Basically Everyone et. al, 1930s – present). The main components of galaxy clusters are galaxies, dark matter, and the intracluster medium (ICM). The ICM is a hot, low density gas primarily observed in the X-ray band. Clusters grow through mergers and accretion, which causes significant disturbances to the ICM. These disturbances include shock fronts (the sharp boundary between gasses with different properties), cold fronts, and turbulence, all of which lead to observable effects on the surface brightness of the cluster in X-ray observations.
Cold fronts (CFs) are omnipresent in clusters, and were first observed in the late 90’s / early 00’s. CFs are similar to shock fronts in that they mark out the boundary between two regions of gas with different properties. The difference from a shock front is that the two sides of a CF are in pressure equilibrium, and the plasma on the denser (and therefore brighter) side is actually colder. CFs are easily formed when the cold dense gas at the center of the cluster comes into contact with the warmer, less dense regions of the ICM.
CFs are thought to form from gas sloshing in the cluster’s gravity well. This leads to oscillatory gas motions and spiral shaped CFs emerging from the cluster center. CFs can be very long lasting if they aren’t interfered with, and can propagate out to large distances. The sloshing is assumed to be initiated by the passage of a smaller gravitationally bound galaxy subcluster near the center of the larger central galaxy cluster.
The authors of today’s paper use simulations to study the formation of CFs in a cluster similar to the Perseus Cluster. The Perseus Cluster is a well studied local bright cluster. Recent observations have found CFs in the inner region (~200kpc) and an older feature out at 700kpc. There is also tenuous evidence for CFs at 1.2 and 1.7 Mpc, but these could also be due to the interaction of shocks from material accreting into the cluster and shocks from previous cluster mergers that are traveling outwards. The authors are looking for cluster-subcluster interactions that could produce this set of CF features.
To study the formation of a pattern of CFs like those in the Perseus Cluster, the authors used the AREPO magnetohydrodynamic (MHD) simulation code. They set up one main large cluster with parameters similar to the Perseus Cluster, and a smaller subcluster that falls in from 3,000 kpc away on a trajectory to interact with the cluster’s gravitational center (see Figure 2). The mass ratio of the two clusters, the initial infall speed and angle, the gas content, and the background magnetic field of the ICM are varied to try to reproduce both the larger 700kpc CF and the inner smaller CFs in the same cluster interaction.
They found that small mass ratios between the cluster and subcluster do not produce observable CFs,while the mass and gas content of the (sub)cluster have a large impact on the resulting CF structure. The initial infall speed and angle are also crucial, as they determine the number of close passes the subcluster will make by the gravitational center, each pass creating new wavefronts. Parameters that lead to a single pass by the center produce smooth CFs that could produce the 700kpc feature, while multiple passes create more unstable CFs that could produce both the 700kpc and the ~200kpc features. This narrows the parameter range that could explain the observed structure in the Perseus Cluster – a mass ratio of 1:5, an infall velocity of 1500 km/s, and an angle between 25 and 30 degrees. An example of comparing the observations of Perseus and a simulation close to the best match is shown in Figure 3.
This work demonstrates the possibility that the CF structure in the Perseus Cluster can be formed by the same gravitational interaction between a cluster and subcluster. Understanding the structure of CFs can help us trace galaxy cluster evolution and better understand how these massive structures came into their current states.
Astrobite edited by Pranav Satheesh
Featured image credit: Bellami+24 Fig 16 Top Left Panel; Clem Onojeghuo via Pexels
Discover more from astrobites
Subscribe to get the latest posts to your email.
