  We report the discovery of giant ionization cones in the 140-kpc nebula
around quasar MR 2251-178 based on deep [O III] 5007/H-beta and [N II]
6583/H-alpha flux ratio maps obtained with the Maryland-Magellan Tunable Filter
(MMTF) on the Baade-Magellan 6.5m telescope. These cones are aligned with the
weak double-lobed radio source observed on smaller scale <30 kpc). They have an
opening angle ~120deg +/- 10deg and subtend ~65-90% of 4pi steradians, where
the large uncertainty takes into account possible projection effects. The
material in the outer ionization cones is matter-bounded, indicating that all
ionizing photons emitted through the cones escape from the system. The quasar
ionizing flux is ~2-3 times fainter outside of these cones, despite the largely
symmetric geometry of the nebula in [O III]. Overall, adding up the
contributions from both inside and outside the cones, we find that ~65-95% of
the quasar ionizing radiation makes its way out of the system. These results
emphasize the need for line ratio maps to quantify the escape fraction of
ionizing radiation from quasars and the importance of quasar radiative feedback
on the intergalactic medium.
