David Landry

Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)



Committee Chair

Massimiliano Bonamente

Committee Member

Marshall Joy

Committee Member

Martin Weisskopf

Committee Member

John Fix

Committee Member

James Miller


X-ray astronomy., Cosmology., Astrophysics.


Clusters of galaxies are the largest known bound systems in the Universe and are formed from the collapse of primordial density fluctuations. Since clusters are extremely large and massive, the cluster's baryons and dark matter originated from approximately the same comoving volume, and thus it is believed that their ratio should be representative of the Universe. Current studies indicate that the cluster baryon fraction is lower than the cosmic baryon fraction as measured by the Omegab / OmegaM parameter. To address this issue, the gas mass fraction, fgas, is measured out to large radii for a complete sample of the 35 most luminous clusters from the Brightest Cluster Sample and its extension at redshifts z = 0.15-0.30 using Chandra X-ray data. This sample includes relaxed and unrelaxed clusters, and the data were analyzed using two different models for the gas density and temperature of the cluster. In accord with earlier studies, the gas mass fraction is shown to increase with radius, and thus the value of fgas depends on the radius used. However, the background of the surface brightness and temperature profiles often limits the radius out to which masses can be measured accurately. Therefore, measurements for the entire sample were limited to r500, the radius within which the mass density is 500 times the critical density. The average gas mass fraction for this sample of clusters at r500 is measured to be fgas = 0.163 ± 0.032, which is in agreement with the cosmic baryon fraction (Omegab / OmegaM = 0.167 ± 0.006) at the 1σ level, after adding the stellar baryon fraction. In this analysis, it is shown that the most X-ray luminous clusters in the redshift range z=0.15-0.30 have a gas mass fraction that is consistent with the cosmic value at r500.



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