Massachusetts Attorney General Thomas Reilly (D) yesterday threw his weight behind opponents of a plan to build a wind farm off the state’s coast. In a friend-of-the-court brief filed in federal court, Reilly argued that the seabed of Nantucket Sound belongs to the federal government and therefore cannot be casually granted to a private company. The proposal by Cape Wind Associates, a private developer, to erect 130 wind turbines in the sound has divided the environmental community. Fans say the wind farm, which would be the first of its kind in the U.S., would produce no greenhouse gases and provide enough power for nearly three-quarters of the residents of Cape Cod and nearby island. Opponents say the farm would obstruct scenic views and set a dangerous precedent — giving away federal property for free. Reilly said a decision by the U.S. Army Corps of Engineers to permit the project as if the feds didn’t own the area leaves the seabed “wide open to promoters and developers of all stripes and schemes.”