Biases in hydrostatic mass profiles introduced by hot gas substructures:Chandra study of four galaxy clusters

By analyzing the azimuthal variations of total gravitating mass profiles in the central 300 h^-1 71 kpc regions of four galaxy clusters with Chandra data, we find that the azimuthally-averaged mass profiles may have been systematically underestimated by 16^＋9 -8% at lσ significance in the 50-100 h^-1 71 kpc regions, probably due to the prevailing existence of 2-D hot gas substructures in 100-300h^-1 71 kpc. The mass biases become negligible（-7＋11 _9＋ %） at 〉 150 h^-1 71 kpc. We confirm the results that the gas temperature maps can be used to probe the departure from hydrostatic equilibrium and help quantify the systematic biases in X-ray mass measurements in the central regions of clusters.

Central gas entropy excess as direct evidence for AGN feedback in galaxy groups and clusters

By analyzing Chandra X-ray data of a sample of 21 galaxy groups and 19 galaxy clusters, we find that in 31 sample systems there exists a significant central （R ≤ 10 h^-171 kpc） gas entropy excess （AK0）, which corresponds to = 0.1 - 0.5 keV per gas particle, beyond the power-law model that best fits the radial entropy profile of the outer regions. We also find a distinct correlation between the central entropy excess △K0 and K-band luminosity LK of the central dominating galaxies （CDGs）, which is scaled as △K0 ∝ L K 1.6±04, where LK is tightly associated with the mass of the supermassive black hole hosted in the CDG. In fact, if an effective mass-to-energy conversionefficiency of 0.02 is assumed for the accretion process, the cumulative AGN feedback E AGN feedack=ηMBHc2 yields an extra heating of = 0.5 - 17.0keV per particle, which feedback is sufficient to explain the central entropy excess. In most cases, the AGN contribution can compensate the radiative loss of the X-ray gas within the cooling radius （= 0.002 - 2.2 keV per particle）, and apparently exceeds the energy required to cause the scaling relations to deviate from the self-similar predictions （=0.2 - 1.0 keV per particle）. In contrast to the AGN feedback, the extra heating provided by supernova explosions accounts for = 0.01 - 0.08 keV per particle in groups and is almost negligible in clusters. Therefore, the observed correlation between △K0 and Lx can be considered as direct evidence for AGN feedback in galaxy groups and clusters.

X-ray spectroscopy of clusters of galaxies

Clusters of galaxies are the most massive objects in the Universe and precise knowledge of their mass structure is important to understand the history of structure formation and constrain still unknown types of dark contents of the Universe. X-ray spectroscopy of galaxy clusters provides rich information about the physical state of hot intracluster gas and the underlying potential structure. In this paper, starting from the basic description of clusters under equilibrium conditions, we review properties of clusters revealed primarily through X-ray observations considering their thermal and dynamical evolutions. The future prospects of cluster studies using upcoming X-ray missions are also mentioned.

Group-scale asymmetric abundance structures in the NGC 533 group

By performing a two dimensional spectral analysis on the galaxy group NGC 533 with high-quality Chandra data, we find that the metal abundance distribution in this group is anisotropic. In the area around 2.5′ from the center, we find two concentrations of high abundance structures, in which the abundances are significantly higher than their surrounding regions at the 90% confidence level. We find that the total iron mass in these two regions agrees with the iron mass synthesized in the central dominant galaxy in n 19＋0.08 Gyr. The double-sided configuration of the high abun- 0.03 dance structure, together with the point-like radio emission in the center suggests that the abundance structures could have been transported from the center to their present positions by active galactic nucleus （AGN） activity. We further calculate the energy required for transport, and find that it could have been supplied during an AGN period. However, considering that this group is reported to have experienced a recent merger, the possibility that this merger is responsible for the abundance structures still cannot be excluded.