Suzaku X-ray Observations of the Nearest Non-Cool Core Cluster, Antlia: Dynamically Young but with Remarkably Relaxed Outskirts

Ka-Wah Wong, Minnesota State University Mankato
Jimmy A. Irwin, University of Alabama - Tuscaloosa
Daniel R. Wik, Johns Hopkins University
Ming Sun, University of Alabama, Huntsville
Craig L. Sarazin, University of Virginia
Yutaka Fujita, Osaka University
Thomas H. Reiprich, Universitat Bonn


We present results of seven Suzaku mosaic observations (>200 ks in total) of the nearest non-cool core cluster, the Antlia Cluster, beyond its degree-scale virial radius (R200≈887 kpc = 76′) in its relaxed direction to the east. The temperature drops by a factorof three from ∼2 keV near the center out to R200, consistent with the scaled temperature profiles of many other clusters. Its pressure follows the universal profile. The density slope in its outskirts is significantly steeper than that of Virgo, which is the nearest cool-core cluster with a similar temperature as Antlia, but shallower than those of the massive clusters. The entropy increases all the way out to R200, consistent with the baseline model predicted by a gravity heating-only mechanism in the outskirts. However, the entropy inside ∼R500 is significantly higher than the baseline model, similar to many other nearby low mass clusters or groups. The enclosed gas mass fraction does not exceed the cosmic value out to 1.3 R200. Thus, there is no evidence of significant gas clumping, electron-ion non-equipartition, or departure from the hydrostatic equilibrium (HSE) approximation that are suggested to explain the entropy and gas fraction anomalies found in outskirts of some massive clusters. Compared to the Virgo Cluster and two fossil groups with measurements out to R200, which are all dynamically older than Antlia, the east direction is remarkably relaxed, in contrast to our expectations. We observe a diversity of gas properties among these low mass groups and we address the different gas properties found in group and cluster outskirts. We also present scaling relations for the gas fraction (fgas,200), entropy (K200), and temperature (T500) using 22 groups and clusters with published data in the literature. We find that the fgas,200–T500 relation has a power-law slope of 0.328±0.166 for the sample with HSE mass measurements, which drops to 0.168±0.221 when including three massive clusters with weak lensing mass measurements. The enclosed baryon fraction at R200 is consistent with the cosmic value. The power-law slope of the K200–T500 relation is 0.638±0.205. The entropy deficit at R200 cannot be fully accounted by the bias or deviation in gas fraction.