Need help finding 1854 emigrants

My great great grandfather, Hendrik Peterson, a policeman and jail warden in Kristiansand, emigrated with his wife Mary and his children, including my great grandfather, August Peterson in 1854. They first settled in WI, and in 1858, moved to Freeborn county MN. I recently found both men among the Norwegian Americans who fought in the civil war and, thus, am this far. Now I need help finding the ship on which they sailed to America.