Delta and Northwest announced yesterday evening that the two companies had officially merged, hours after the U.S. Department of Justice said the deal to create the world's largest airline did not raise anti-trust concerns for the government.

The combined airline will be called Delta and headquartered in Atlanta. It will be the world's largest by several measures, including number of passengers flown, number of departures and number of destinations. Operations of the two carriers are expected to be combined over the next 14 to 16 months.

In a statement, Delta chief executive Richard Anderson said the merger gave the new company "increased flexibility to adapt to the economic challenges ahead." Anderson will retain the chief executive role at the combined company.

The outlook for U.S. airlines appears to be mixed over the next several months. Already, there are signs that the economic slowdown is leading to a pull back from air travel. Airlines, however, are benefiting from a dramatic drop in oil prices from peaks this summer that led to talk of airline bankruptcies and wider industry consolidation.

After a six-month investigation, the Justice Department's anti-trust division removed the last hurdle to the proposed merger yesterday, saying it "is likely to produce substantial and credible efficiencies that will benefit U.S. consumers and is not likely to substantially lessen competition."