Without more information regarding your domain it isn't possible to make a suggestion regarding aggregate boundaries. The relationships among entities isn't as relevant as the behaviors associated with entities. A relationship between entities does not automatically translate to an object reference; relationships can also be implemented as repositories.
An aggregate is a consistency boundary. In other words, it aggregates state which must remain consistent upon execution of associated behaviors. In your case, it may very well be that each of those entities is an aggregate. Take a look at effective aggregate design to get a better feel for factoring a domain into aggregates.