Evaluation of Statistical Tests for Substructure in Clusters of Galaxies
Abstract
We use Nbody simulations of galaxy cluster mergers to evaluate new and published statistical tests of substructure. These tests include 22 onedimensional (normality) tests, four twodimensional (spatial) tests, and five threedimensional (velocityspatial) tests. These tests are statistical in that they provide a significance level for the presence of substructure. All the tests are applied to the same data files so that their relative sensitivity can be compared. The data files contain positions and velocities of dark matter particles drawn randomly from the simulations. Three noncosmological simulations are run in which the subclusters begin as King spheres: a single isothermal cluster, a merger of a 1/6 mass subcluster, and a merger of a 1/3 mass subcluster. In this way, we examine the dependence of the tests on the subcluster's relative mass. We examine also the dependence on the total sample size, the epoch of merger (pre and postcorecrossing), and the projection angle of the merger axis. The results allow a quantitative comparison of the effectiveness of each estimator under different observational scenarios. In general, the higher the dimensionality of the test, the more sensitive it is to substructure. The sensitivity of individual diagnostics depends on the line of sight relative to the merger axis. The threedimensional tests are least sensitive to lines of sight perpendicular to the merger axis and are most sensitive to lines of sight 45%deg^60^deg^ from perpendicular. The twodimensional tests are most sensitive to lines of sight perpendicular to the merger axis. The onedimensional tests are the most sensitive to lines of sight parallel to the merger axis. No single substructure test is the most sensitive in all situations. Therefore, we recommend that a battery of tests be applied to each cluster. We provide a score for each test reflecting its relative sensitivity. We find that clusters with no merging components but a velocity dispersion gradient have an increased likelihood for a "falsepositive" response from some threedimensional tests. We examine also the signatures of merger and the detectability of mergers in redshift surveys. We find that cluster masses are overestimated by up to a factor of 2 for clusters undergoing mergers. We plot the dependence of mass estimators on projection angle and epoch of merger. We find that the detectability of postmerger states is hampered by small search radii (e.g., <2.0h_75_^1^ Mpc) in redshift surveys.
 Publication:

The Astrophysical Journal Supplement Series
 Pub Date:
 May 1996
 DOI:
 10.1086/192290
 Bibcode:
 1996ApJS..104....1P
 Keywords:

 GALAXIES: CLUSTERS: GENERAL;
 GALAXIES: DISTANCES AND REDSHIFTS;
 GALAXIES: INTERACTIONS;
 METHODS: NUMERICAL