Consider a cluster of galaxies that, besides the galaxies, contains hot
dilute gas visible in the X-ray band. These clusters are still forming,
as new materials is constantly falling into their gravitational well. 
Estimate the time scale on which a massive cluster of 1e15 solar masses and radius 2 Mpc would form.
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Also discuss whether or not you expect shocks to form where the infalling
material collides with the existing intracluster gas.