24. A computer program product, encoded on a non-transitory computer-readable medium, operable to cause data processing apparatus to perform operations comprising: receiving a search query; determining that the search query matches a topic among a plurality of topics, each topic of the plurality of topics i) describing information included by a document of multiple documents and ii) distinguishing the document from the remaining documents of the multiple documents; identifying multiple identities associated with the topic based on multiple documents in which the topic is referred, each identity of the multiple identities corresponding to an expert of one or more experts; for each document of the multiple documents, obtaining a composite score of each identity of the document based on an aggregation of aggregate scores of the identity for the document, the aggregate score of each identity based on a topic score associated with the document and an identity score of each identity associated with the document, the identity score for each identity indicating a degree of relevance between the identity and the document, and the topic score for the topic indicating a degree of relevance between the topic and the document; ordering the multiple identities based on a respective composite score associated with each document of the multiple documents; and presenting the multiple identities based on the ordering.
24. A computer program product, encoded on a non-transitory computer-readable medium, operable to cause data processing apparatus to perform operations comprising: receiving a search query; determining that the search query matches a topic among a plurality of topics, each topic of the plurality of topics i) describing information included by a document of multiple documents and ii) distinguishing the document from the remaining documents of the multiple documents; identifying multiple identities associated with the topic based on multiple documents in which the topic is referred, each identity of the multiple identities corresponding to an expert of one or more experts; for each document of the multiple documents, obtaining a composite score of each identity of the document based on an aggregation of aggregate scores of the identity for the document, the aggregate score of each identity based on a topic score associated with the document and an identity score of each identity associated with the document, the identity score for each identity indicating a degree of relevance between the identity and the document, and the topic score for the topic indicating a degree of relevance between the topic and the document; ordering the multiple identities based on a respective composite score associated with each document of the multiple documents; and presenting the multiple identities based on the ordering. 26. The program product of claim 24 , further operable to cause data processing apparatus to perform operations comprising: identifying the plurality of topics from a corpus of documents.