Fabry-Perot Maps

 The emission-line spectra were fit with Gaussian profiles across the field to generate spatial maps of line characteristics such as flux, velocity, and dispersion. Single-component fits were made to the H$\alpha$ and [NII] lines across the majority of the field, well into the halo of the galaxy. There are regions north of the galaxy's disk where double components were fit to the H$\alpha$ line profile, as well as regions south of the disk where double components were fit to both the H$\alpha$ and [NII] lines. The [OIII] line was fit with a single Gaussian component throughout the field, although visual inspection revealed signs of line splitting in specific regions.

The region of acceptable fits is identical for the H$\alpha$ and [NII] maps, although the [NII] fits in the northern-most regions of the maps have significantly higher errors, due to clipping of the 6583Å profile by the limited spectral coverage. The region with sufficient [OIII] flux for profile fitting is much smaller than that of H$\alpha$ or [NII]. The central regions of the galaxy are saturated in both the H$\alpha$ and [NII] observations.

We now describe the important features of each Fabry-Perot map. Each spatial map is presented at approximately the same scale, with tick marks separated by one arcminute ($\sim$950 pc). The maps from the H$\alpha$+[NII] data set contain regions in which the spectral lines are split; these maps show both a flux-weighted total for the galaxy as well as values for the individual components separately. These components are referred to as the high-velocity component (HVC) and the low-velocity component (LVC). North is up and east is to the left in all images. The position angle of the major axis of the galaxy is approximately 65^o.

 

Figure: Flux maps of M82 in the emission lines of (a.) H$\alpha$ 6563Å and (b.) [NII] 6583Å, in units of ergs cm^-2 sec^-1, log-scaled between -15.5 and -13.0. The upper maps represent the total flux in the line at each pixel, while the lower panels represent the flux in the high- (HVC) and low- (LVC) velocity components of the line, in the regions where the line is split (designated by an outline in the upper panel). Splitting of the [NII] emission line is only observed south of the disk.  

Figure 1 illustrates the flux distribution in the light of H$\alpha$ 6563Å and [NII] 6583Å. We measure a total unsaturated H$\alpha$ flux of $9.9\times 10^{-11}$ ergs cm^-2 sec^-1 and a total [NII] flux of $4.1\times 10^{-11}$ ergs cm^-2 sec^-1. A rough comparison with the H$\alpha$ imagery to be presented in the next section indicates that the saturated nuclear regions contribute an additional $\sim2.1\times 10^{-11}$ ergs cm^-2 sec^-1 in the H$\alpha$ line, or $\sim$17% of the total. The flux is concentrated in the nucleus and along the minor axis, with very little emission originating in the extended disk of the galaxy. The nuclear line emission has saturated the detector in two concentrations (``the eyes''), as well as a number of weaker ancillary regions. In contrast, the minor axis emission is spatially extensive and filamentary. Numerous long radial filaments can be seen extending more than a kiloparsec from the nucleus, as well as complex small-scale structures. Note, for example, the bright bow shock-like arc approximately 500 pc SSE of the nucleus.

The morphology of the extraplanar gas differs between the two sides of the galaxy, appearing more chaotic on the north side and showing signs of collimation in the south. The southern H$\alpha$ emission exhibits a sharp edge on the eastern side. Furthermore, the emission to the south is considerably brighter and more extensive, although there appears to be an abrupt reduction in flux at a distance of approximately 500 pc from the nucleus. Examination of the H$\alpha$ and [NII] emission maps reveals that, beyond 500 pc, the flux in the HVC drops rapidly, while the flux in the LVC remains more uniform. We also find pervasive diffuse emission at a low level throughout the halo of the galaxy.

 

Figure: Flux map of M82 in the emission line of [OIII] 5007Å, in units of ergs cm^-2 sec^-1, log-scaled between -16.0 and -14.2. The [OIII] emission line profiles were fit with single components throughout the field. A contour map of the H$\alpha$ flux has been superimposed.  

Figure 2 is a map of the flux from M82 at [OIII] 5007Å. We measure a total [OIII] flux of $2.6\times 10^{-12}$ ergs cm^-2 sec^-1, more than an order of magnitude below that seen at H$\alpha$ and [NII]. There is essentially no emission from the disk at this wavelength; the flux originates almost entirely within the nucleus and along the southern minor axis of the disk. Although the radial extent of the minor-axis emission is much smaller than that seen in [NII] and H$\alpha$, the filamentary morphology is similar. Indeed, excellent correlation is seen with the H$\alpha$ flux map, which has been contoured on Figure 2. A striking feature of the [OIII] flux map is the presence of two distinct streams of emitting gas, each extending along the southern minor axis from one of the bright nuclear emission regions. These streams can also be identified in the H$\alpha$ flux map (Figure 1), although the much more pervasive nature of the H$\alpha$ emission makes the structure more difficult to discern at small radii.

 

Figure: Velocity map of M82 in the emission line of H$\alpha$ 6563Å, in units of km s^-1. The upper map represents the total velocity of the H$\alpha$-emitting gas at each pixel. In regions of multiple velocity components, the component velocities have been combined with flux weighting to yield an ``average'' velocity. An ellipse centered on the 2.2 micron nucleus ([Waller, Gurwell, & Tamura 1992]) has been superimposed on the image. The lower panels represent the velocities in the high- (HVC) and low- (LVC) velocity components of the line, in the regions where the line is split (designated by an outline in the upper panel). Note that the velocity range is much greater for these component panels than for the total velocity map.  

Figure 3 is a radial velocity map for the H$\alpha$-emitting gas in M82. Two prominent trends are seen: first, there exists a strong velocity gradient along the major axis of the galaxy, consistent with normal disk rotation, with the eastern end moving away from the observer. Second, a velocity transition is evident along the minor axis of the galaxy: strong blue-shifting of the H$\alpha$ emission is seen south of the disk; strong red-shifting to the north. The major-axis rotation signature merges into this minor axis motion in a gradual fashion, although this may be due in part to the flux-weighting procedure.

The HVC consists of highly redshifted emission in the north and highly blueshifted emission in the south ($v_r\sim300$ km s^-1). The LVC consists of emission at roughly the systemic velocity of the galaxy ($v_{sys}\approx203$ km s^-1; [de Vaucouleurs et al. 1991]), in both the north and south. The northern and southern component pairs mirror each other almost exactly in terms of relative velocity and kinematic structure. Perhaps surprisingly, the velocities within each individual component exhibit little radial or azimuthal variation. The exception to this is the inner portion of the HVC, where the radial velocities approach the systemic velocity.

A map of the line-of-sight velocities of the [NII]-emitting gas was also produced, but is not presented here, as it provides little additional information. The trends along both the major and minor axes are identical to those seen at H$\alpha$, but at a lower signal-to-noise ratio due to the decreased flux of [NII] line emission. A velocity map of the [OIII]-emitting gas was not produced, for the reasons outlined above.

 

Figure: Flux ratio map of M82 for the ratio of the flux in the emission line of [NII] 6583Å to the flux in the emission line of H$\alpha$ 6563Å, log-scaled between -0.8 and +0.3. A contour map of the H$\alpha$ flux has been superimposed.  

Figure 4 shows the logarithm of the ratio of the [NII] 6583Å flux to the H$\alpha$ 6563Å flux from M82. The most remarkable feature in this map is the presence of a distinct region of low line ratio (0.0-1.0) along the minor axis, south of the disk. This region extends at least a kiloparsec in radius from the nucleus and is narrower than the regions of minor axis filaments visible in the H$\alpha$ and [NII] flux maps (Fig. 1). The region of low ratios can be separated into two distinct structures, originating with the bright central eyes of M82, similar to the structures seen at smaller radii in the [OIII] 5007Å flux map (Fig. 2).

Elsewhere in the galaxy, higher line ratios ($\gtrsim1.0$) prevail, particularly at large radii within the inclined disk. Discrete HII regions can be seen in the disk as small concentrations with the expected [NII]/H$\alpha$ ratio of $\sim$0.5 ([Osterbrock 1989]). The regions of extremely low line ratio to the north are a result of the incompletely sampled [NII] line, and should be ignored.

 

Figure: Flux ratio map of M82 for the ratio of the flux in the emission line of [OIII] 5007Å to the flux in the emission line of H$\alpha$ 6563Å, log-scaled between -2.2 and -0.5.  

Figure 5 is a map of the logarithm of the ratio of the [OIII] 5007Å flux to the H$\alpha$ 6563Å flux from M82. The ratio has been calculated over the entire spatial extent of the [OIII] emission, as seen in the flux map (Fig. 2). The observed ratios are low ($\sim$0.05) throughout the nucleus and regions south of the disk, with the only apparent trend being a gradual increase in the ratio with distance from the nucleus.