Substructure and Gas Clumping in the Outskirts of Abell 133
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Galaxy clusters are powerful tools for studying various astrophysical principles. Gas accreting onto the cluster is heated to 10^7-10^8 K through adiabatic compression and shocks, making clusters highly luminous in X-ray imaging. Measurements of the gas density and temperature profiles can be used to calculate the gas mass fraction f_gas, which is expected to closely match the cosmic baryon fraction Ω_b/Ω_m. Recent observations have found entropy profiles in cluster outskirts that are shallower than predicted and values of f_gas that are higher than the Universal baryon fraction inferred from the Cosmic Microwave Background (CMB). Abell 133 was an ideal candidate for studying this phenomenon, since it had recently been observed in a wide (R≈30') Chandra mosaic with an exposure time of ∼2 Ms. The X-ray imaging was combined with existing optical imaging from the Canada-France-Hawaii Telescope (CFHT) and spectroscopy obtained from the Magellan telescope, to search for any possible gas clumps and to study their properties. The photometric analysis yielded over 3200 red sequence galaxies to a depth of r'=22.5, which were used to create a Gaussian smoothed intensity map and a significance map of the cluster (compared to CFHT Legacy Survey data). About 6 significant overdensities were detected in the significance map, although these did not fully correspond to contours obtained from the X-ray image. Spectroscopy obtained on the cluster yielded ∼700 secure redshifts, of which about 180 were cluster members. This included data from the NOAO Fundamental Plane Survey (NFPS) and the 6 Degree Field Galaxy Survey (6dFGS). We found a cluster redshift of z=0.0561±0.0002 and a velocity dispersion of σ=743±43 km/s. The dynamical analysis gave a virial radius of r_v=1.44±0.03 Mpc and a virial mass of M_v=(5.9±0.8)×10^14 M_sun. We also found values of R_500=1.21±0.07 Mpc and M_500=(5.3±0.9)×10^14 M_sun for γ=1/3 and R_500=0.99±0.05 Mpc and M_500=(2.9±0.5)×10^14 M_sun for γ=1/2, where γ is a parameter related to the assumed density profile and the velocity anisotropy. About 30 overdensities with a radius R_c≥30" were detected as gas clumps on the X-ray image. The galaxy distribution in these clumps was analyzed, both for the stacked signal as well as the individual clumps, in ten parallel colour-magnitude bands to find any significant red sequences associated with them. Most of these clumps appeared to be background systems, some consisting of 1-2 galaxies, others being small groups or clusters. Only 2-3 clumps appeared to be associated with the cluster itself. This suggests that the cluster density profile is actually quite smooth, which may not agree with recent numerical simulations. Further studies are required to determine if the cluster density distribution is consistent with what is predicted and the nature of the background systems.
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Gandhali Joshi (2013). Substructure and Gas Clumping in the Outskirts of Abell 133. UWSpace. http://hdl.handle.net/10012/7837