Plumbing help?

[Router Help]

Any plumbing experts here?

We're remodeling our kitchen and one thing I need to do is move the kitchen sink about 2 feet to the left. Our sink only has one drain, but the drain pipe coming out of the wall is split in two. One half goes to the garbage disposal and then the sink drain. The other half is capped by this strange copper piece as picture below. The copper piece is open on the top so water could pour out of it if there was a sever enough backup. Conversely, random crap could fall into the drainage system.

My question is, what is this thing? I'm guessing the previous owners had a sink with two drains, changed it to a single drain, and capped it with this thing. Am I safe in cutting out the Y piece and replacing it with a straight piece to my single drain, or is this some kind of vent system?

[Router Help]

Figured it out! We starting ripping out the kitchen so long ago that I had completely forgotten that the dishwasher drain was connect here! DOH!