What is this metal tube behind intake manifold

 

That is a coolant pipe. All modern vehicles have engine coolant passing around a jacket in the throttle body, this is to prevent ice build up around the throttle plate during winter months. Many Z owners remove or bypass this on their throttle bodies since it does nothing in summer months. Also, performance can be marginally impacted (warm air = less dense vs. cold air).

Cheers!
-Icer

 

I just got the car. No hose hooked up there. I was worry about it. I will just leave like that since I am not going to drive during winter. Thanks for your knowledge.