In the galaxy, you are always visible, and your contacts can always see when you get online.
There should be an option to Hide from the galaxy; And to go Offline.
Here is a picture:
Appear Offline: You will appear as offline, your contacts will receive the message that you are offline and this option will remember it’s selection. If you reconnect, you will receive the message
" You are appearing as offline."
Hidden: This will hide you from the galaxy, if someone in your contacts tries to locate you, it will say that this user has hidden their location.