top command shows 2 process of
console-kit-dae each using 22% of memory.
I would like to know why and if I can kill them?
There have been reports of some versions ConsoleKit consuming, reserving or appearing to consume more memory than expected. If you are running an older version of ubuntu, you may want to look at upgrading.
ConsoleKit does a few things, it effectively lets ordinary users do certain system level tasks. These would include mounting usb drives, and rebooting the machine.
So killing it will probably not hurt any normal operations - email, surfing, documents etc. However it will probably get respawned after being killed.