The Blackstone Group has received the go-ahead from antitrust authorities to acquire TRW Inc.’s automotive parts unit for almost $4.73 billion U.S. The acquisition by Blackstone, a New York buyout firm, had been known for several months, but was delayed until Northrop Grumman Corp.’s purchase of TRW this month. Northrop bought TRW for its defense business and said it would sell or spin off TRW’s auto parts operations. The antitrust division of the U.S. Justice Department ended its investigation of the deal without taking any action, antitrust officials said. Blackstone said last month that the group plans to retain the current management at the auto parts unit and would likely hold a public stock offering in the coming years. Terms of the transaction call for Blackstone to pay $4.725 billion U.S. for the automotive unit, including about $4 billion in cash and $600 million in debt securities. Northrop, the No. 2 U.S. defense contractor, is expected to retain an approximately 20% equity interest in the operation after the deal closes.