THE KINEMATICS OF A VIOLENT OUTFLOW FROM THE NUCLEUS OF THE SC GALAXY NGC-3079

We describe an extensive set of long-slit CCD spectra of the peculiar, nearly edge-on Sc galaxy NGC 3079. The disk is cluttered with H II regions which define a rotation curve of normal appearance. In addition, however, clouds having the optical emission-line characteristics of shock-heated gas are visible near the nucleus and roughly along the minor axis. This gas exhibits very complicated kinematics; we see expanding shells of gas similar to those in planetary nebulae, as well as extremely steep linear velocity gradients and spectacular velocity reversals. Radial velocities of up to approximately 1200 km s-1 are observed at a projected distance of approximately 700 pc from the nucleus. Despite the complex kinematics, the dominant motions in NGC 3079 appear to be expansion away from the nuclear region along a hollow conelike surface. Deprojected speeds exceeding 2000 km s-1 are derived if the opening angle of the cone is 90-degrees, as observed in a narrowband H-alpha + [N II] image of the galaxy previously published by Ford et al. [ApJ, 311, L7 (1986)]. A majority of the kinematically distinct gas is coincident with the "loop" of H-alpha + [N II] emission discovered by Ford et al., which is itself aligned with the bridges and loops of radio emission studied by Duric et al. [ApJ, 273, L11 (1983)] and by Duric & Seaquist [ApJ, 326, 574 (1988)]. In agreement with Heckman et al. [ApJS, 74, 833 (1990)], we conclude that NGC 3079 hosts an energetic, bipolar outflow-a "galactic superwind" produced by violent activity in the nuclear region of NGC 3079. Such winds are often seen in other galaxies having very large far-infrared luminosities. However, the anomalously high observed gas velocities, together with evidence from radio observations, suggest that the superwind in NGC 3079 is largely driven (or at least initiated) by accretion onto a compact object, rather than by a vigorous starburst. Most of the kinematically distinct, optically emitting gas along the minor axis of NGC 3079 is probably interstellar gas entrained and shock heated by the wind, perhaps with a smaller contribution of material that cooled directly from plasma in the hot wind. Curiously, the radial velocity of this gas generally increases with increasing projected distance from the nucleus. It is possible that most of the currently observed motion was caused by a relatively isolated explosive event, instead of the more continuous, long-term outflow. A more probable alternative, however, is that the velocity gradient is produced by the gradual acceleration of gas entrained by the wind. On the western side of the galaxy, the outflow is aligned with the direction to NGC 3073 (Mrk 131), a dwarf S0 companion galaxy which has a redshifted H I tail pointing away from the nucleus of NGC 3079. This morphology suggests that the superwind is in its blowout phase, and it strongly supports the hypothesis of Irwin et al. [ApJ, 313, L91 (1987)] that the ram pressure of a wind from NGC 3079 is stripping neutral hydrogen away from NGC 3073. The wind-galaxy interaction may have also induced one or more bursts of star formation in NGC 3073, whose optical spectrum reveals the presence of OB stars as well as A-type stars. NGC 3079 demonstrates that galactic superwinds may have a profound effect on their environment.