Authentication is a service that is obviously used by other microservices, otherwise it would have no use.
But with microservices coupling is as loose as possible.
Your authentication microservice provides an interface (REST or whatever) independent on the underlying implementation and it does not matter how it implemented. As long has the interface 'contract' is fulfilled you can re-implement it from Java to Rails, or move it from here to there, if you wish and that is completely transparent to all other microservices.
But with microservices coupling is as loose as possible.
Your authentication microservice provides an interface (REST or whatever) independent on the underlying implementation and it does not matter how it implemented. As long has the interface 'contract' is fulfilled you can re-implement it from Java to Rails, or move it from here to there, if you wish and that is completely transparent to all other microservices.