A woman was in critical condition after being shot in South Boston early Sunday, police said. Officers responded to a call at 12:18 a.m. and found the woman inside 15 Dorchester St., said Officer Katherine Shea, a Boston police spokeswoman. The woman was taken to Boston Medical Center, Shea said, and remained in critical condition Sunday afternoon. Police arrested the man who fired the gun used in the shooting, a Quincy man in his 40s, but Shea said she could not identify him because the shooting was under investgation. Shea said the man did not attempt to flee. He was expected to be arraigned on firearms charges on Monday.