Fig 2: This plot shows the hydrostatic mass bias for galaxy clusters, i.e. the difference between the true mass and the estimated mass (with different models). Open symbols give the estimated mass without correction, the filled symbols the results when applying the new method. The three areas of the figure show simulated clusters in different dynamical stages, namely, the top 20% (rather relaxed), 50% (less disturbed) and 100% (all) clusters according how dynamically relaxed they are. The new method works well for all types of clusters and irrespective of the detailed procedures used in estimating the masses (indicated by different colors). © MPA
Different methods are used to determine the mass of a galaxy cluster depending on both the dynamical relaxation state of the cluster (relaxed/ disturbed, evident from its X-ray image) and the spatial resolution of the observed image (well-resolved or not). With our solution to the hydrostatic mass bias problem, we can improve the accuracy of mass estimation for clusters belonging to all these categories. We will also be able to take advantage of the spatial information in the observations for the well-resolved, dynamically disturbed clusters. © MPA
Booming observations of galaxy clusters provide great opportunities for exploring the nature of Dark Energy. At the same time, they post great challenges to scientists. The "hydrostatic mass bias" problem, which leads to a systematic error in estimating the mass of galaxy clusters, is one big limitation when doing precision cosmology with galaxy clusters. Now researchers at MPA have developed a method to correct for it.
Still, much more galaxy clusters will be detected without immediate spatially-resolved data. However the newly developed method is also useful for them. For example, the eROSITA survey will measure the X-ray emission of more than 50,000 galaxy clusters and their progenitors. Most of them will not be spatially well-resolved. Masses for these objects will be obtained by scaling relations between the mass and spatially-averaged observables, such as the mean X-ray luminosity, temperature, or their combination. Correcting the hydrostatic mass bias will lead to a more accurate calibration of the scaling relations, and thus allow researchers to better exploit the huge number of galaxy clusters to explore the nature of Dark Energy.