SF340 flight crew suffers hydraulic system anomalies immediately after takeoff. Returned to departure airport under emergency conditions and performed zero flap landing.

Narrative

Everything was normal until just after takeoff when I called for 'gear up; heading; indicated airspeed (IAS).' The Captain selected the gear up; heading and IAS modes. We both then noticed that the green down-lock lights had extinguished but the orange gear handle light was still illuminated with the hydraulic pump not running. Immediately thereafter; we received a master caution with a hydraulic look-down light. The Captain; as Pilot Not Flying; completed the memory items for the hydraulic look-down light and proceeded to the appropriate checklist in the Emergency/Non-Normal checklist. The Captain coordinated with Dispatch and ATC while I flew the plane and backed him up on the radios. Upon turning around to return to ZZZ; we agreed that in the interest of safety; we should declare an emergency in consideration of our limited amount of hydraulic pressure available. The Captain briefed the Flight Attendant on the situation and we received vectors back to ZZZ for a visual approach. We had previously computed the appropriate adjustments to our landing data and performance for a zero-flap landing; which we agreed that was the safest approach to make in our situation. A routine landing was made; after which we turned off the runway and received a tow back to the gate; per the checklist.