17. The method of claim 16 , wherein ranking the related entities further comprises: computing, for each of the related entities, a respective co-occurrence score based on how frequently a recognized reference to the related entity co-occurs with a recognized reference to the first entity in resources indexed by the search engine, wherein computing, for each of the related entities, the respective ranking score comprises: computing the ranking score for the related entity from the subsequent query score for the related entity and the co-occurrence score for the related entity.
17. The method of claim 16 , wherein ranking the related entities further comprises: computing, for each of the related entities, a respective co-occurrence score based on how frequently a recognized reference to the related entity co-occurs with a recognized reference to the first entity in resources indexed by the search engine, wherein computing, for each of the related entities, the respective ranking score comprises: computing the ranking score for the related entity from the subsequent query score for the related entity and the co-occurrence score for the related entity. 18. The method of claim 17 , further comprising: determining, for each related entity, that the respective co-occurrence score for the related entity exceeds a third threshold value; determining, for each related entity, that the respective subsequent query score for the related entity exceeds a fourth threshold value; and generating the mapping in the index in response to determining that the co-occurrence scores exceed the third threshold value and the subsequent query scores exceed the fourth threshold value.