Bipolar Ionization Cones in the Extended Narrow-line Region of Nearby QSO2s

Abstract

We have used narrowband [O iii ] λλ 4959, 5007 and H α +[N ii ] λλ 6548, 84 Hubble Space Telescope ( HST ) images of nine luminous ( L [O iii ] > 10 42 erg s −1 ) type 2 QSOs with redshifts 0.1 <  z  < 0.5 in order to constrain the geometry of their extended narrow-line regions (ENLRs), as recent ground-based studies suggest that these regions become more spherical at high luminosities due to destruction of the torus. We instead find elongated ENLRs reaching 4–19 kpc from the nucleus and bipolar ionization cones in [O iii ]/(H α +[N ii ]) excitation maps indicating that the torus survives these luminosities, allowing the escape of ≈10 times higher ionizing photon rates along the ionization axis than perpendicular to it. The exceptional HST angular resolution was key to our success in arriving at these conclusions. Combining our measurements with previous ones based on similar HST data, we have revisited the relation between the ENLR radius R maj and L [O iii ] over the range 39 < log( L [O iii ]) < 43.5 ( L in erg s −1 ): log( R maj ) = (0.51 ± 0.03) log( L [O iii ])−18.12 ± 0.98. The radius of the ENLR keeps increasing with L [O iii ] in our data, implying that the ENLR can extend to distances beyond the limit of the galaxy if gas is present there—e.g., from active galactic nucleus (AGN) outflows or interactions, seen in six objects of our sample. We attribute the flattening previously seen in this relation to the fact that the ENLR is matter-bounded, meaning that ionizing photons usually escape to the intergalactic medium in luminous AGNs. Estimated ionized gas masses of the ENLRs range from 0.3 to 2 × 10 8 M ⊙ , and estimated powers for associated outflows range from <0.1% to a few percent of the QSO luminosity.