Optical Kinematics

Our Fabry-Perot observations provide velocity information across the entire outflow in M82. Two well-known features of this emission line gas should be emphasized a priori. First, the velocity signature of outflow is clearly observed along the minor axis in both H$\alpha$ and [NII] (see Fig. 3). The well-established disk inclination angle of 81.^o5 ([Lynds & Sandage 1963]) and the increased opacity toward the northern filaments clearly establishes the large-scale kinematics as arising from outflow (cf. [Elvius 1972]; [Solinger, Morrison, & Markert 1977]). Second, the optical emission lines split into two distinct velocity components in large regions along the minor axis of the galaxy. The velocity structure of the individual components, as well as the difference between them, provides convincing evidence for the presence of outflowing gas along the minor axis of M82 ([Axon & Taylor 1978]). Figure 9 illustrates the velocity structure of the H$\alpha$-emitting gas in M82, as observed in the frames of the Fabry-Perot data cube. Both minor-axis outflow lobes span velocity ranges exceeding $v_{proj}\sim300$ km s^-1, overlapping at the systemic velocity.

 

Figure: Mosaic of 16 monochromatic frames from the H$\alpha$ Fabry-Perot data cube. The field of view is 3.'5. The frames are separated by $\sim$45 km s^-1, where the frame marked with an asterisk corresponds roughly to the systemic velocity of the galaxy. The circular region in the southeast corner is a masked ghost from the star in the southwest corner.  

We now describe our attempts to understand the complex dynamics of the outflow in M82 with the aid of kinematic models. We have extracted one-dimensional velocity cuts and synthetic two-dimensional spectra from the Fabry-Perot data cubes. These cuts have been made relative to the outflow axis, at an estimated position angle of $\sim$150^o ($\sim$85^o relative to the major axis of the galaxy; [McKeith et al. 1995]). While some authors have argued for a spherical wind in M82 (e.g., [Seaquist & Odegard 1991]), most recent models find evidence for a bipolar outflow, usually in the shape of cones with opening angles much less than 90^o. This is borne out by our study: the line flux, splitting, and ratio maps all exhibit marked azimuthal variations indicative of an aspherical outflow morphology, strongly weighted toward the minor axis of the galaxy. Studies of the supernova distribution (e.g., [Kronberg, Biermann, & Schwab 1985]) and other disk observations (e.g., [Achtermann & Lacy 1995]) indicate that even the central injection zone is probably not spherical, but rather in the form of a flattened disk, with dimensions $\sim$600 pc wide and $\sim$200 pc thick.

The specific velocities predicted by [Chevalier & Clegg 1985] do not agree with our observations. An estimated supernova rate of 0.3 yr^-1 ([Rieke et al. 1980]) yields an outflow velocity from their model of 2000-3000 km s^-1 at the edge of the starburst injection zone. This value is several times larger than the deprojected velocities observed at optical wavelengths ($v\sim$525-655 km s^-1; see below) on much larger scales. However, the extensive radio continuum halo ([Seaquist, Bell, & Bignell 1985]; [Seaquist & Odegard 1991]) could arise from synchrotron radiation due to a population of relativistic electrons, as they are transported outward in a wind at velocities in the range considered by Chevalier & Clegg. The relationship between such a wind and the slower, denser minor-axis outflow seen at optical and x-ray wavelengths remains unclear. We suspect that much of the H$\alpha$ filamentation arises from large-scale shocks from a high-speed wind plowing into the gaseous halo and entrained disk gas. Only a small fraction of the total wind energy is encompassed by the radio halo ($\sim$2%; [Seaquist & Odegard 1991]).

 

Figure: The radial velocity profile from fits to the H$\alpha$ line components along the axis of the outflow (position angle of 150^o), from both the Fabry-Perot data and the longslit observations of [McKeith et al. 1995]. A systemic velocity of 200km s^-1 has been subtracted from all values. The Fabry-Perot points have been interpolated over a band 7'' wide through the 2.2 micron nucleus. Dual H$\alpha$ components are resolved in both the north and south outflow lobes. The shaded region represents a velocity cut along the axis of a two-cone Monte-Carlo simulation.  

In modeling the bipolar outflow in M82, we model the individual line components rather than flux-weighted velocity profiles. In Figure 10, we show the velocities of line fits to the dual H$\alpha$ components along the minor axis of the galaxy. While the northern outflow components show a relatively constant projected separation of $\sim$300 km s^-1, the southern region of split lines reveals an intriguing variation. Within 200 pc south of the nucleus, where the favorable inclination of the disk allows us to measure line profiles closer to the starburst, the individual components of H$\alpha$ are separated by a much smaller velocity, comparable to our resolution ($\sim$50 km s^-1). Between 200 and 500 pc from the nucleus, the components rapidly diverge, remaining at a constant separation of $\sim$300 km s^-1 beyond 500 pc. Maps of the line component splitting reveal a separation of this order throughout the spatial extent of both lobes. Also plotted in Figure 10 are fits to the H$\alpha$ profiles from the long-slit optical spectra of [McKeith et al. 1995], which clearly confirms our observed minor-axis trend in the line splitting. An identical radial trend is seen in the [NII] line components along the southern outflow axis, but is not shown in the figure.