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We report the discovery of giant ionization cones in the 140-kpc nebulaaround 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 theweak double-lobed radio source observed on smaller scale <30 kpc). They have anopening angle ~120deg +/- 10deg and subtend ~65-90% of 4pi steradians, wherethe large uncertainty takes into account possible projection effects. Thematerial in the outer ionization cones is matter-bounded, indicating that allionizing photons emitted through the cones escape from the system. The quasarionizing flux is ~2-3 times fainter outside of these cones, despite the largelysymmetric geometry of the nebula in [O III]. Overall, adding up thecontributions from both inside and outside the cones, we find that ~65-95% ofthe quasar ionizing radiation makes its way out of the system. These resultsemphasize the need for line ratio maps to quantify the escape fraction ofionizing radiation from quasars and the importance of quasar radiative feedbackon the intergalactic medium.

MMTF Discovery of Giant Ionization Cones in MR 2251-178: Implications for Quasar Radiative Feedback