Surrogate key is a unique identification key, it is like an artificial or  alternative key to production key, bz the production key may be alphanumeric or composite key but the surrogate key is always single numeric key. 

assume the production key is an alphanumeric field. if u create an index for this fields it will occupy more space, so it is not advicable to join/index, bz generally all the  datawarehousing fact table are having historical data.   these factable are linked with somany dimension table. if it's a numerical fields the performance is high