Simulated galaxy clusters show complexity of identifying centre
Cosmological structure formation models assume lower mass systems to merge to form more massive structures, with galaxy clusters representing the final state of this process. These clusters are not only good cosmological probes but also valuable laboratories for testing models of gravitational structure formation, galaxy evolution, thermodynamics of the intergalactic medium and plasma physics. Observationally, galaxy clusters are usually identified through optical images, X-ray observations or gravitational lensing. A fundamental step in any of these procedures is the identification of the cluster centre though, and the preferred measures depend on the signal used. For instance, the minimum of the gravitational potential (which expects to define the centre if the cluster is in dynamical equilibrium) is usually adopted when using strong and weak lensing, whereas luminosity peaks are common measures when using optical and X-ray images. In a new publication by CAASTRO members Dr Weiguang Cui and Prof Chris Power (both at ICRAR-UWA) and colleagues, the researchers employed simulations to determine the difference in the estimation of the centre of a galaxy cluster when assuming different mass distribution and physical processes.
They created a statistical sample of clusters drawn from a suite of cosmological simulations in which they explored a range of galaxy formation models: Dark Matter only, CSF (hydrodynamical models that include gas cooling, star formation and supernova feedback) and AGN (feedback from supermassive Black Holes included in addition to CSF). The team investigated how the location of the galaxy cluster centre was affected by the choice of these observables. What they found was that the “brightest cluster galaxy position” from the optical images correlated more strongly with the minimum of the gravitational potential than the X-ray defined centres. The feedback from supermassive Black Holes, on the other hand, significantly enhanced the offset between the peak X-ray luminosity and the minimum gravitational potential. These results highlight the importance of identifying the cluster centre when interpreting cluster observations, in particular when comparing theoretical predictions and observational data.
Weiguang Cui, Chris Power et al. in MNRAS (2015): “How does our choice of observable influence our estimation of the centre of a galaxy cluster? Insights from cosmological simulations.”