There's no depth image directly available through Tango API. In fact, the raw data in Tango's consideration is point clouds. In order to get a depth image, you would have to project those points onto camera's image plane, and perform certain amount of upsampling to get a good result.
Tango Support Library does have serveral upsample methods. There's also a rgb-depth-sync example code from Tango shows how to perform the depth points projection.
