Polish-born collector Barbara Piasecka Johnson, widow of the heir to the Johnson & Johnson pharmaceutical fortune, is parting with her monumental Rembrandt. “Portrait of a Man, Half-Length, With His Arms Akimbo,” from 1658, not seen by the public for nearly 40 years, is being sold at Christie’s in London on Dec. 8. The estimate is $29.7 million to $41.2 million, the highest estimate ever for an old master painting coming to auction.