The Plymouth County district attorney’s office and the state Department of Children and Families are investigating the death of an infant found in a Marshfield apartment Friday afternoon. Results of an autopsy are pending, said Bridget Norton Middleton, spokeswoman for the district attorney. Emergency personnel responded to a call for assistance for a baby “approximately 6 months old,’’ at about 12:20 p.m. Friday, Middleton said. She declined to give the baby’s name, but authorities said the baby was found at Castle Green Condominiums, off School Street. Other children in the home have been placed in the care of DCF.