Heath Ledger, the 28-year-old Australian actor known alternatively for raw prestige work in movies like "Brokeback Mountain" and "Monster's Ball" and as a box-office pin-up in others like "A Knight's Tale," was found dead in a Manhattan apartment yesterday afternoon. The actor, increasingly respected in critical as well as commercial circles, had been the subject of intense speculation for his forthcoming stint as the Joker in the "Batman" sequel "The Dark Knight." Though the details of his death remain unclear - Web conjecture produced several rumors discounted by the star's publicist - many noted last night that the rabid media response to his death recalled the instant pop mythology that followed the death of other rising young actors like River Phoenix.