Imaging spectrophotometry of ionized gas in NGC 1068. I. Kinematics of the narrow-line region
Creators: name: Cecil, G.
name: Tully, R.B.
name: Bland, J.
Publisher: name: Institute of Physics Publishing
Year: 1990

description: We have used an imaging Fabry-Perot interferometer and CCD at the University of Hawaii 2.2 m telescope to synthesize [N II] λλ6548, 6583 emission-line profiles with (&lt;1″, ∼140 km s-1) FWHM (spatial, velocity) resolution across the inner 1′ diameter of the nearby Seyfert galaxy NGC 1068. The stack of monochromatic images spatially resolved the high-velocity gas (∼6″ radius and ∼3000 km s-1 line widths) and has been extensively analyzed for kinematic and photometric content. Our profiles agree well with previous long-slit work, but their complete spatial coverage has now allowed us to constrain the gas volume distributions. We find that the narrow-line region (NLR) is distributed in a thick (FWHM ∼3″.3 = 230 pc) center-darkened, line-emitting cylinder that envelopes the collimated radio jet. The cylinder is composed of three distinct kinematic subsystems, which we discuss. 1. High-velocity gas emits 75% of the total dereddened narrow-line region (NLR) [N II] flux (corresponding to a mass of ∼38,000 [104.5 cm-3/ne] M⊙, with ne the mean electron density). A kinematic model consistent with the data is used to argue that this gas fills the cylinder with two families of nested cones of increasing opening angle in the NE and SW quadrants. The maximum opening angle may be as large as 150°. The axial inclination to the line of sight in the NE is 75°, so the cylinder axis is inclined ∼45° to the disk plane. The brightest high-velocity flux in the NE is concentrated in the conical shell with opening angle ∼82°. The shell is thick, with Gaussian dispersion ∼3″.3 = 230 pc FWHM perpendicular to the cone axis. The emitting filaments are optically thick, with Av ∼ 1.3 mag of internal extinction. Line-of-sight velocities are approximately proportional to r0.6 (with r the cylindrical radius from the jet axis), and so are roughly those of a constant-speed conical outflow. Intrinsic gas velocities deproject to ∼1500 km s-1 with respect to systemic velocity, and we infer an average mass loss of ∼0.15 [104.5 cm-3/ne] M⊙ yr-1. The large extranuclear line widths in this NLR apparently arise from geometrical projection and spatial averaging of an asymmetric, large-scale flow, with no evidence for intrinsic "turbulence" in the outflowing filaments. The filaments are too massive (≥10-2 M⊙) to be broad-line region (BLR) clouds blown to resolvable radii. We argue instead that molecular clouds with average density ∼102.5 cm-3 are forced by the stellar bar (prominent in near-IR images) from 10″ radius into the inner NLR, where they are exposed to a high-speed (0.1c) nuclear wind whose kinetic luminosity is similar to the ionizing luminosity of the nucleus. Fragments stripped by fluid instabilities are crushed to densities of ∼103-105 cm-3 during their wind acceleration, but they are stable to conductive thermal evaporation in the hot shocked wind for masses and velocities derived from our spectra. 2. Gas near systemic velocity is concentrated in a narrow kinematic feature brightened along the boundaries of the NE radio lobe. It accounts for 8% of the total NLR [N II] flux (corresponding to ∼1.2 ×10 5 [103 cm-3/ne] M⊙), but negligible kinetic energy. This component is also apparent in Ha images and so represents a mass enhancement rather than an excitation effect. Its deprojected velocity deviates by less than 175 km s-1 from the bar-forced flow field defined by gas in the large-scale disk. The only significant velocity deviation (80 km s-1 along the line of sight) is localized in an expanding "ripple" around the base of the NE lobe at ∼3″ radius, with no associated flux enhancement. 3. Nine percent of the total NLR [N II] flux (corresponding to ∼1000 [105 cm-3/ne] M⊙) is emitted by a component which dominates within 1″ radius and extends 2″ along ∼100° P.A. Its centroid is blueshifted by ∼270 km s-1 from systemic, and it has line width ∼600 km s-1 (i.e., typical of a Seyfert 2 galaxy). The brightest flux from this component apparently arises in spatially unresolved kinematic substructure that is associated with the nuclear triple radio-source (∼0″.7 extent). Our highest resolution images show a tight correlation between the brightest line emission and the radio source, in agreement with speckle line imaging.