Bipolar ionization cones in the Extended Narrow-Line Region of nearby
  QSO2s
  We have used narrow-band [OIII]$\lambda\lambda$4959,5007 and
H$\alpha$+[NII]$\lambda\lambda6548,84$ Hubble Space Telescope (HST) images of 9
luminous (L[OIII]$>10^{42}$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
(ENLR), as recent ground-based studies suggest these regions become more
spherical at high luminosities due to destruction of the torus. We find instead
elongated ENLRs reaching 4 to 19 kpc from the nucleus and bipolar ionization
cones in [OIII]/(H$\alpha$+[NII]) excitation maps indicating that the torus
survives these luminosities, allowing the escape of $\approx$10 times higher
ionizing photon rates along the ionization axis than perpendicularly to it. The
exceptional HST angular resolution was key to our success in arriving at these
conclusions. Combining our measurements with previous ones based on similar HST
data, we have revisited the relation between the ENLR radius R$_{maj}$ (in pc)
and L[OIII] (in erg s$^{-1}$) over the range $39<$log(L[OIII])$<43.5$:
log(R$_{maj}) = (0.51\pm0.03)$ log(L[OIII])$-18.12\pm0.98$. The radius of the
ENLR keeps increasing with L[OIII] 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 AGN outflows or interactions, seen in 6 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 AGN. Estimated ionized gas masses of the ENLRs
range from 0.3 to $2\times10^8$ M$_{\odot}$, and estimated powers for
associated outflows range from $<0.1\%$ to a few percent of the QSO luminosity.
