Exploring the Authorship of Luke and Acts

The Gospel of Luke and the book of Acts were both allegedly written by a companion of Paul, Luke "the gentile physician." But the books never name their author.

So why Luke? Are are there any good grounds for naming him in particular? Or any grounds at all? Were the books probably written by a doctor? Was he probably a gentile? *Was* there a Luke? If picking his name was just guess-work -- would it affect how we interpret the books or understand their reliability?