where can I find the logs for shutdown by a root user?

Question

Distributor ID: Ubuntu
Description:    Ubuntu 14.04.3 LTS
Release:        14.04
Codename:       trusty

The issue I am facing is that, when I type in the command "shutdown -h now" as a root user, it is not logged anywhere in syslog, auth.log or kern.log.

But when a user with sudo permission does the same, it is logged in auth.log.

So, where can I find the logs for the first case?.