Você não pode usar su
para se tornar usuário root no Ubuntu (a menos que você o habilite, mas isso não é recomendado). Você deve usar sudo
:
By default, the Root account password is locked in Ubuntu. This means that you cannot login as Root directly or use the su command to become the Root user. However, since the Root account physically exists it is still possible to run programs with root-level privileges. This is where sudo comes in - it allows authorized users (normally "Administrative" users; for further information please refer to AddUsersHowto) to run certain programs as Root without having to know the root password.
This means that in the terminal you should use sudo for commands that require root privileges; simply prepend sudo to all the commands you would normally run as Root.
Referência: link
Para responder à sua última pergunta, os usuários do IAM não têm nada a ver com usuários do sistema operacional.