Physical Conditions in the X-ray Emission-line Gas in NGC 1068
  We present a detailed, photoionization modeling analysis of
XMM-Newton/Reflection Grating Spectrometer observations of the Seyfert 2 galaxy
NGC 1068. The spectrum, previously analyzed by Kinkhabwala et al. (2002),
reveals a myriad of soft-Xray emission lines, including those from H- and
He-like carbon, nitrogen, oxygen, and neon, and M- and L-shell iron. As noted
in the earlier analysis, based on the narrowness of the radiative recombination
continua, the electron temperatures in the emission-line gas are consistent
with photoionization, rather than collisional ionization. The strengths of the
carbon and nitrogen emission lines, relative to those of oxygen, suggest
unusual elemental abundances, which we attribute to star-formation history of
the host galaxy. Overall, the emission-lines are blue-shifted with respect to
systemic, with radial velocities ~ 160 km/s, similar to that of [O III] 5007,
and thus consistent with the kinematics and orientation of the optical
emission-line gas and, hence, likely part of an AGN-driven outflow. We were
able to achieve an acceptable fit to most of the strong emission-lines with a
two-component photoionization model, generated with Cloudy. The two components
have ionization parameters and column densities of logU = -0.05 and 1.22, and
logN(H) = 20.85 and 21.2, and covering factors of 0.35 and 0.84, respectively.
The total mass of the X-ray gas is roughly of an order of magnitude greater
than the mass of ionized gas determined from optical and near-IR spectroscopy,
which indicates that it may be the dominant component of the narrow line
region. Furthermore, we suggest that the medium which produces the
scattered/polarized optical emission in NGC~1068 possesses similar physical
characteristics to those of the more highly-ionized of the X-ray model
components.
