Alaska Air is one step closer to acquiring Hawaiian Airlines after the U.S. Department of Justice chose not to challenge the $1.9 billion deal that the carriers say will create a company better able to serve travelers. Alaska Air announced in December that it would pay $18 in cash for each share of Hawaiian. The brands of both airlines would be preserved after the merger, which is unique in an industry where decades of acquisitions have left only four big carriers dominating the U.S. market. Alaska and Hawaiian say they have few overlapping routes and the intent of a tie-up is to allow the new airline to better compete with American, Delta, Southwest and United Airlines.