Arrests need warrants. And if you're thinking about the police, that's also part of the executive, mostly. Judges can't do anything unless some other branch of the government asks them to.
Arrests need probable cause. They can either be done on a warrant or without a warrant (in the latter case, in the federal system, a complaint must be filed and the arrested person must be brought before a magistrate for a hearing on probable cause within 72 hours after arrest.)
