Towards Understanding Galaxy Clusters and Their Constituents:Projection Effects on Velocity Dispersion, X-Ray Emission Mass Estimates, Gas Fraction and Substructure

We study the projection effects on various observables of clusters of galaxies at zero redshift, including cluster richness, velocity dispersion, temperature, X-ray luminosity, three total mass estimates (velocity based, temperature based and gravitational lensing derived), gas mass estimate and substructure, utilizing a simulation of a realistic cosmological model (a cold dark matter model with the following parameters: $H_0=65$km/s/Mpc, $\Omega_0=0.4$, $\Lambda_0=0.6$, $\sigma_8=0.79$). Some combinations of the various quantites are also examined in detail. It is shown that projection effects are important for some quantities but insignificant for others. In particular, it is shown that, on average, the gas to total mass ratio in clusters appears 30-40\% higher than the global ratio due to projection, and the observed broad distribution of gas to total mass ratio is adequately accounted for by projection effects, alleviating the need to invoke other physical processes. The lensing deduced mass is, on average, 20-30\% higher than the velocity or temperature based mass, while a significant high end tail of the ratio of the former to the latter ($\sim 2.0$) exists. It seems that projection effects can account for the existing observations of clusters with lensing, dynamically and temperature based masses. Projection inflates substructure measurements in galaxy maps, but affects X-ray maps somewhat less, if appropriate measures of substructure are chosen. Given that most clusters ($\ge 90\%$) in the simulated cosmological model do not contain significant intrinsic substructure without projection effects, whereas about half of clusters would have been ``observed" to have statistically significant substructure as measured by the Dressler- Shectman $\Delta$ statistic, a