Kinematics of spatially extended high-velocity outflow from the nucleus of M51

Abstract

Kinematics of high-velocity (~1500 km s 1 extent) N + were mapped across the Seyfert-like narrow-line region (9" radius) at the center of the active spiral galaxy M51 (NGC 5194). A CCD and imaging Fabry-Perot interferometer were used in subarcsecond seeing at 65 km s _1 resolution. Two Ha velocity channels were also recorded. Complementary long-slit CCD spectroscopy provides S + densities and N + profiles with high signal-to-noise ratios at subarcsecond increments. Line intensity, line width, gas excitation and the radio continuum peak in an extranuclear cloud (XNC) ~4" south of the nucleus. Within the XNC, I have spatially resolved the high-velocity ionized gas into sharply defined features. In particular, line profiles and channel maps show N + emission enhanced in a shell of unresolved thickness (<40 pc). The Ha distribution is more uniform, indicating that the N + enhancement is driven by temperature/ionization variations rather than by mass. The [N n]/ Ha ratio has a distinct velocity-dependent component and peaks in a region occupied by relativistic charged particles, suggesting that cosmic-ray electrons are an important heat source. Gas of highest velocity (1500 km s' 1 range) is concentrated at the bright, arcuate southeastern boundary of the N + shell. The nuclear isopleths are elliptical in line emission and the radio continuum, with the major axis aimed at this surface. The kine-matics of this surface are deprojected to show that two superposed cloud systems, comprising ~4000 M 0 of ionized gas with velocity dispersion ~140 km s-1 , are expanding at ~500 km s _1 in the disk of the galaxy. Thus in this NLR, line broadening and the asymmetries of the emission line profiles arise from geometry, not selective obscuration. The emission morphology, excitation distribution, and positional variation of the N + kinematics in the XNC resemble those of galactic Herbig-Haro objects, and plausibly arise from thermaliza-tion of the kinetic energy of a directed nuclear outflow along well-defined shock fronts in the ambient medium. Models of high-velocity, steady flow, planar shocks can fully account for the observed Ha luminosity only if the average preshock density is greater than 50 cm-3. Thus gas in the XNC must be exposed to an additional, centrally concentrated but nonstellar ionizing source if this density is confirmed to be ~ 1 cm " 3 , as implied by the atomic hydrogen column. Subject headings: galaxies: individual (M51)-galaxies: nuclei-shock waves I. INTRODUCTION There are several unambiguous indicators of nonstellar activity in galactic nuclei. The strengths and spatial distributions of the X-ray and nonthermal-radio continua may not be those expected for a population of stellar remnants. The nuclear emission lines can be broadened to velocities in excess of the virial and thermal widths of 10 4 K gas rotating in a stellar gravitational field. These lines can also show evidence in their flux ratios for nonstellar excitation and for a broader range of ionization than found in galactic H + regions. Despite the ubiquity of these phenomena in active nuclei of all lumin-osities, their dynamical interconnection remains elusive. To first order, their relative prominence is likely to reflect the balance between spatially dispersed and centrally located energy sources. But it has not been possible to assess the dynamical significance of these phenomena because the associated energy flows can only be traced by the ionized gas, and most spectroscopic observations have been spatially unre-solved.