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The first holistic view of minor mergers of galaxy clusters

Sheardown, Alexander James


Alexander James Sheardown


Brad K. Gibson


Utilising hydrodynamic + N-body simulations, we present the first holistic view of minor mergers of galaxy clusters. A minor merger describes the merging of a main cluster and a less massive subhalo, this could be a subcluster, group or an elliptical galaxy. Throughout the minor merger, we consider simultaneously both the evolution of the subhalo atmosphere via gas stripping and other processes, along with the evolution of gas sloshing and shocks in the main cluster atmosphere. These merger features, and others, are recorded in the intra-cluster medium (ICM) of the cluster and are thus embedded in X-ray observations. The X-ray band is the focus of our analysis although other wavelengths are discussed. We direct our attention on the Fornax Cluster and the infall of its elliptical galaxy NGC 1404, constructing a tailored simulation of this system. By comparing our simulated data to observations of the cluster, we are able to constrain it’s previous 1.1 – 1.3 Gyr history, showing that a second or third infall of NGC 1404 into Fornax reproduces all the main observed merger features in both objects, ruling out the possibility of a first infall. Additionally, we are further able to make predictions for other possible observed merger features in Fornax. We also provide an outlook on a side project based on this work which analyses the globular cluster content observed in NGC 1404 and the central BCG galaxy in Fornax. Analysing these and further minor cluster merger simulations in a broader context, we identify a new class of gas tail, which we term slingshot tail. Gas tails of galaxies and subhalo’s are generally accredited to ram pressure stripping which produces an orderly head-tail like morphology with the tail pointing in the direction of the recent orbital path. In contrast, slingshot tails form as a subhalo moves away from pericentre toward apocentre during a merger. Here, the gas in the tail slingshots as the subhalo rapidly decelerates, resulting in a tail that can point perpendicular or even opposite to the subhalo’s direction of motion. We also find that the flow patterns associated with slingshot tails are highly irregular. Therefore naively applying the ram pressure scenario to a gas tail should be cautioned, as the tail direction can be misleading and can also lead to incorrect conclusions regarding properties of the subhalo and the surrounding ICM. Applying this new understanding, we attempt to explain some examples of gas tails reported in literature. Finally, building on the detection of a stolen atmosphere feature of NGC 1404 in the Fornax merger simulations, we present a description of work being conducted by Dr Elke Roediger and Thomas Fish which provides the theory behind the feature and also provides a comparison between stagnation point methods which are used to estimate the infall velocity of NGC 1404.


Sheardown, A. J. (2019). The first holistic view of minor mergers of galaxy clusters. (Thesis). University of Hull. Retrieved from

Thesis Type Thesis
Deposit Date Oct 15, 2019
Publicly Available Date Feb 23, 2023
Keywords Physics
Public URL
Additional Information Department of Physics and Mathematics, The University of Hull
Award Date Sep 1, 2019


Thesis (130.3 Mb)

Copyright Statement
© 2019 Sheardown Alexander James. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.

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