I know I am kind of late to this discussion but I am just going to say what I want to say anyway...lol
I think it has to be a carrier.
Obviously this ship has gone WAY WAY outside maped space, it is most likely (as someone has probably stated and I mised) and expeditionary force. Its main mission is probably just to find new worlds and stuff (like the Star Ship Interprise from star Trek).
I think they came preparred for whatever they may meet, but they havent met any "aliens" yet (since this is the first conntact). So I think we are looking at a BIG carrier, it has to support all those troops, and vehicles comfortably and have a lot of food, water and the like.
Ait support probably wasnt a big issue to them, since they had no idea what they were going to find, and plus I doubt the rebel forces they thought they might encounter, (assuming there are rebal forces at this time) wouldnt be to much of a problem and thus they didnt have a lot of fighter ships.