X-ray analysis of the galaxy group UGC 03957 beyond R 200 with Suzaku

Abstract

Context. In the last few years, the outskirts of galaxy clusters have been studied in detail and the analyses have brought up interesting results such as indications of possible gas clumping and the breakdown of hydrostatic, thermal, and ionization equilibrium. These phenomena affect the entropy profiles of clusters, which often show deviations from the self-similar prediction around R200. However, significant uncertainties remain for groups of galaxies. In particular the question, of whether entropy profiles are similar to those of galaxy clusters. Aims. We investigated the gas properties of the galaxy group UGC 03957 up to 1.4 R200 ≈ 1.4 Mpc in four azimuthal directions with the Suzaku satellite. We checked for azimuthal symmetry and obtained temperature, entropy, density, and gas mass profiles. Previous studies point to deviations from equilibrium states at the outskirts of groups and clusters and so we studied the hydrodynamical status of the gas at these large radii. Methods. We performed a spectral analysis of five Suzaku observations of UGC 03957 with ~138 ks good exposure time in total and five Chandra snapshot observations for point source detection. We investigated systematic effects such as point spread function and uncertainties in the different background components, and performed a deprojection of the density and temperature profile. Results. We found a temperature drop of a factor of ~3 from the center to the outskirts that is consistent with previous results for galaxy clusters. The metal abundance profile shows a flat behavior towards large radii, which is a hint for galactic winds as the primary ICM enrichment process. The entropy profile is consistent with numerical simulations after applying a gas mass fraction correction. Feedback processes and AGN activity might be one explanation for entropy modification, imprinting out to larger radii in galaxy groups than in galaxy clusters. Previous analyses for clusters and groups often showed an entropy flattening or even a drop around ~ R200, which can be an indication of clumping or non-equilibrium states in the outskirts. Such entropy behavior is absent in UGC 03957. The gas mass fraction is well below the cosmic mean but rises above this value beyond R200, which could be a hint for deviations from hydrostatic equilibrium at these large radii. By measuring the abundance of the α-elements Si and S at intermediate radii we determined the relative number of different supernovae types and found that the abundance pattern can be described by a relative contribution of 80%-100% of core-collapse supernovae. This result is in agreement with previous measurements for galaxy groups.