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The Asymmetric Wind in M82

P. L. Shopbell[*]
Rice University, Department of Space Physics & Astronomy, MS 108, Houston, TX 77251;


J. Bland-Hawthorn
Anglo-Australian Observatory, P.O. Box 296, Epping, NSW 2121, AUSTRALIA;


We have obtained detailed imaging Fabry-Perot observations of the nearby galaxy M82 in order to understand the physical association between the high-velocity outflow and the starburst nucleus. The high spatial and kinematic resolution of our observations has allowed us to perform photometric analyses of H$\alpha$, [NII], and [OIII] spectral lines at roughly one hundred thousand positions across the extent of the galaxy.

The observed velocities of the emitting gas in M82 reveal a bipolar outflow of material, originating from the bright starburst regions in the galaxy's inner disk, but misaligned with respect to the galaxy spin axis. The deprojected outflow velocity indicated by the optical filaments increases with radius from 525 to 655 km s-1. All three spectral lines show double components in the centers of the outflowing lobes, with the H$\alpha$ line split by $\sim$300 km s-1 over a region almost a kiloparsec in size. The filamentary lobes lie along an axis tilted by 15o with respect to the spin axis, confirmed by the regions of line splitting and the ionization pattern over the outflow. The filaments are not simple surfaces of revolution, nor is the emission distributed evenly over the surfaces. We model these lobes as a composite of cylindrical and conical structures, collimated in the inner $\sim$500 pc but expanding at a larger opening angle of $\sim$25o beyond that radius. We compare our kinematic model with simulations of starburst-driven winds in which disk material surrounding the source is entrained by the wind. There is some evidence for rotation of the wind filaments about the outflow axis in support of entrainment, and we find strong similarities between the observed and predicted structures.

The data reveal a remarkably low [NII]/H$\alpha$ ratio in the region of the outflow, indicating that photoionization by the nuclear starburst may play a significant role in the excitation of the optical filament gas, particularly near the nucleus. An increase in the [OIII]/H$\alpha$ ratio along the outflow is observed. At larger radii, the line diagnostics and a strong spatial correlation between H$\alpha$ and soft x-ray filaments are consistent with shock ionization.

A smooth spherical halo around M82 is observed in emission lines, extending to at least 2 kpc. We propose that the dusty halo is the primary source of the linearly polarized optical emission. A diffuse ionized medium (DIM) with enhanced [NII]/H$\alpha$ emission pervades throughout the stellar disk. We discuss likely sources of ionization and heating.

Keywords: galaxies: individual (M82, NGC 3034); galaxies: starburst; ISM: jets and outflows; galaxies: kinematics and dynamics; techniques: interferometric

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Patrick Shopbell