Abstract
Abstract We show that there is a new class of gas tails—slingshot tails—that form as a subhalo (i.e., a subcluster or early-type cluster galaxy) moves away from the cluster center toward the apocenter of its orbit. These tails can point perpendicular or even opposite to the subhalo direction of motion, not tracing the recent orbital path. Thus, the observed tail direction can be misleading, and we caution against naive conclusions regarding the subhalo’s direction of motion based on the tail direction. A head-tail morphology of a galaxy’s or subcluster’s gaseous atmosphere is usually attributed to ram pressure stripping, and the widely applied conclusion is that gas stripped tail traces the most recent orbit. However, during the slingshot tail stage, the subhalo is not being ram pressure stripped (RPS) and the tail is shaped by tidal forces more than just the ram pressure. Thus, applying a classic RPS scenario to a slingshot tail leads not only to an incorrect conclusion regarding the direction of motion but also to incorrect conclusions regarding the subhalo velocity, expected locations of shear flows, instabilities, and mixing. We describe the genesis and morphology of slingshot tails using data from binary cluster merger simulations and discuss their observable features and how to distinguish them from classic RPS tails. We identify three examples from the literature that are not RPS tails but slingshot tails and discuss other potential candidates.
Highlights
Galaxy clusters grow through the sequential merging and accretion of galaxies, groups, and subclusters (Kravtsov & Borgani 2012)
When we see the plane of the merger almost edge on, the arc-shaped slingshot tail may not point sideways; in this scenario the homogeneous brightness and sharp edge of the tail can be used to distinguish it from a ram pressure stripped tail
In this paper we have visually inspected a suite of idealized binary cluster merger simulations to show that in addition to ram pressure stripped tails, there is a second class of gas tails, named slingshot tails
Summary
Galaxy clusters grow through the sequential merging and accretion of galaxies, groups, and subclusters (Kravtsov & Borgani 2012). As one of the latter begins the merging process, it must traverse the intracluster medium (ICM) of its host cluster This motion through the ICM acts as a headwind on a galaxy or subcluster, producing a ram pressure that progressively strips its gaseous atmosphere (Gunn & Gott 1972; Nulsen 1982). We describe two different forms of slingshot tails, highlighting the need for caution in drawing conclusions regarding both the subhalo’s direction of motion based on the tail direction and the flow patterns surrounding slingshot tails To this end, we analyze slingshot tails in binary cluster merger simulations, focusing on distinguishing slingshot tails from classic ram pressure stripped tails. For clarity, we term the more massive merger partner (e.g., a cluster) the primary and the less massive merger partner (e.g., subcluster or earlytype galaxy), the secondary
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