For a company beleaguered by a federal antitrust investigation, Sotheby's pulled off a impressive sale of Impressionist and modern art last night. Its York Avenue salesroom was overflowing with collectors, dealers and curiosity seekers. The regulars came to watch as millions of dollars' worth of art by masters like Monet and Matisse, Picasso and van Gogh changed hands. But some in the audience were drawn by the publicity Sotheby's has received since the resignations of A. Alfred Taubman, its chairman and major stockholder, and Diana D. Brooks, its president and chief executive, in February. Sotheby's faces the possibility of steep fines and dozens of lawsuits alleging that the company colluded with its archrival Christie's to fix the prices it charges buyers and sellers. None of this affected the strong prices last night. The sale totaled $140.3 million, in the middle of its estimate of $117 million to $163 million. Of the 50 lots, only 8 went unsold.