DALLAS — A partnership between two Dallas-based development firms, Gault Co. and RTG Capital LLC, has purchased a 170,000-square-foot office building in North Dallas. Situated on four acres at 5757 Alpha Road across from the Valley View Mall, which is being redeveloped into a mixed-use destination, the office property was formerly known as Alpha Tower. Following a renovation project for its landscaping, lobby and common areas that is scheduled to be complete by year’s end, the building will be rebranded Northwood Tower. The property was built in 1983 and was 32 percent leased at the time of sale. The seller and sales price were not disclosed.