Extended Halo

Early polarization observations detected a strong linear polarization throughout the halo of M82, with the position angles oriented perpendicular to a radial vector from the nucleus ([Elvius 1969]). This polarization was interpreted as evidence for a scattering component in the galaxy (e.g., [Solinger 1969]), probably consisting of electrons illuminated by the nucleus. The more difficult issue has been the polarization of the optical filaments themselves. The first observations in this regard ([Visvanathan & Sandage 1972]) determined that the minor axis filaments and the halo were equally polarized at optical wavelengths, suggesting that perhaps there was no ``explosion'' in M82 and that the off-axis filaments were merely density enhancements in a dusty cloud through which the galaxy was moving ([Solinger, Morrison, & Markert 1977]).

Although the discovery of split emission lines ([Axon & Taylor 1978]) and the minor-axis X-ray halo ([Watson, Stanger, & Griffiths 1984]) have virtually eliminated this alternate interpretation for the optical filaments (although see [Rohan, Morrison, & Sadun 1987]), the polarization measurements remain poorly understood. Recent observations (e.g., [Scarrott, Eaton, & Axon 1991]) indicate that the optical filaments may indeed contain a scattered component, but even then there is uncertainty as to the source of the polarization, i.e. the nucleus ([Visvanathan 1974]) or the entire disk ([Solinger & Markert 1975]). Unfortunately, with few exceptions (e.g., [Schmidt, Angel, & Cromwell 1976]; [Bland & Tully 1988]; [Doane 1993]; [Suchkov et al. 1994]), the presence of a halo component in M82 has been largely ignored.

Our detailed analyses in H$\alpha$ and [NII] clearly confirm the existence of the smooth exponential halo noted by [Bland & Tully 1988]. Figure 7 illustrates the flux along a narrow ($\sim$9'') band parallel to and approximately 45'' southeast of the major axis of the galaxy. The flux due to the outflow filaments can be seen superimposed upon an exponentially decreasing background. The halo has be detected across our entire region of fit lines, approaching a flux level of 10^-15 ergs cm^-2 sec^-1 arcsec^-2 at a radius of 1 kpc.

 

Figure: A profile of the peak H$\alpha$ emission along a band parallel to and approximately 725 pc south of the major axis of M82, illustrating the bright filaments and underlying diffuse halo component. The dashed line represents a cut through an exponential halo model. Tick marks on the spatial axis are spaced at 1' intervals.  

We have estimated the radial profile of the halo flux at H$\alpha$ with an exponential function:

$$I(\hbox{\hbox{H$\alpha$}}) = I_0(\hbox{\hbox{H$\alpha$}}) e^{-r/r_e},$$ (1)

where $I_0(\hbox{\hbox{H$\alpha$}}) \sim 1.6\times10^{-15}$ ergs cm^-2 sec^-1 arcsec^-2 Å^-1 and $r_e \sim 315$ pc. This profile has been overlayed on the cut in Figure 7. For the observed halo line width of $\sim$350 km s^-1, the integrated H$\alpha$ flux is $\sim 2.5\times10^{-11}$ ergs cm^-2 sec^-1. This compares with an observed flux from the filaments and unsaturated nucleus of $\sim 9.9\times10^{-11}$ ergs cm^-2 sec^-1. For a distance of 3.25 Mpc this implies a total H$\alpha$ luminosity from the halo of $3.2\times10^{40}$ ergs sec^-1.

The azimuthally symmetric polarization pattern of the halo in broadband light ([Schmidt, Angel, & Cromwell 1976]) suggests a scattering origin. The halo is known to comprise cold neutral atoms ([Cottrell 1977]), relativistic electrons ([Seaquist & Odegard 1991]), dust ([Visvanathan & Sandage 1972]) and warm ions ([Bland & Tully 1988]). If we associate the line-emitting halo with the polarized component, the line width ($\sim$350 km s^-1 FWHM) reflects either the motion of scattering mirrors embedded in a warm medium or the ``beam-averaged'' projected kinematics of the nuclear and large-scale disk gas. If the observed line dispersion arises from thermal motions of electrons, the kinetic temperature must be less than 1000 K, at which point the flux from recombination would overwhelm the scattered flux for any reasonable halo density ($n_H \approx 1$ cm^-3; [Cottrell 1977]). Dust scattering is expected to be much more efficient in any case. The ratio of the scattering optical depths can be written

$$R_{de} = {{\sigma_d n_d}\over{\sigma_e n_e}},$$ (2)

where n_d and n_e are the dust and electron densities; $\sigma_d$and $\sigma_e$ are the dust and electron scattering cross sections. For $\sigma_d$, we form the more conservative weighted mean of the MRN ([Mathis, Rumpl, & Nordsieck 1977]) grain distribution for which $\sigma_d \approx 0.01\mu$m. In the local ISM, the dust-to-gas number density ratio is $n_d / n_H \approx 10^{-12}$ ([Ostriker & Silk 1973]) although this depends strictly on the grain composition. With the conservative assumption of a totally ionized halo, we deduce a scattering ratio $R_{de} \sim 50$, verifying the relative importance of dust scattering. [Schmidt, Angel, & Cromwell 1976] demonstrate that a reasonable optical depth to dust scattering is $\tau_d \sim 0.1$ from a comparison of the halo broadband flux and the estimated disk flux.

The halo dust could reside in an extended neutral or warm ionized medium, some fraction of which could be supplied by the energetic wind ([Burbidge, Burbidge, & Rubin 1964]). We now compare the timescale for dust destruction by sputtering with the estimated age of the starburst wind, $\tau_{wind} \sim 3 \times 10^6$ years (e.g., [Lynds & Sandage 1963]; [Bland & Tully 1988]). From [Ostriker & Silk 1973], the timescale for grain sputtering is  

$$\tau_{sput} \sim 3 \times 10^5\ n_e \left({{10^6}\over{T_e}}... ...{Y}}\right) \left({{r_d}\over{0.1}}\right) \quad \hbox{years,}$$ (3)

where Y is the sputtering ``yield,'' i.e., the number of atoms released per impact. Y varies from $\sim 5 \times 10^{-4}$ at the threshold sputtering temperature of $3 \times 10^5$ K to $\sim 0.01$for temperatures of 10⁷-10⁸ K.

At the low temperatures expected in a galactic halo, $T\lesssim10^5$ K, the lifetime of dust is greater than 10⁷ yrs, comparable to or longer than the lifetime of the outflow. However, any dust located directly in the hot ($T\sim10^8$ K) X-ray-emitting wind survives for no more than 10⁵ years, and has therefore been destroyed. Such an effect is supported by low-resolution radio maps of the M81/M82 region, which indicate an anti-correlation between the wind lobes and HI column density ([Cottrell 1977]). We note that more recent studies of gas-grain sputtering and grain-grain collisions ([Tielens et al. 1994]; [Jones, Tielens, & Hollenbach 1996]) suggest that grains may be able to survive much longer than previously thought. However, most of these studies are specific to the three-phase ISM in the Galaxy, and it remains unclear precisely how the results should be extended to galactic wind systems, which characteristically involve higher temperatures (10⁸ K vs. 10⁶ K) and larger velocities (600 km s^-1 vs. 200 km s^-1) than standard ISM models.

Given the sputtering timescales, if the dust has been delivered into the halo by the wind, it must be as a component of cooler material entrained by the hot wind itself. However, this implies that the halo dust and optical emission line filaments may then have the same origin and morphology, yet the polarization observations do not seem to indicate a minor-axis concentration in the dust distribution ([Solinger & Markert 1975]; [Schmidt, Angel, & Cromwell 1976]; [Scarrott, Eaton, & Axon 1991]). We also do not observe substantial redshifted emission south of the galaxy, as would be expected from mirrors moving with the outflow.

Alternatively, dust may have been forced into the halo at an early stage of the outflow, at a time when it was dominated much more by radiation from massive stars in the central burst than by supernovae. Such a mechanism has been hypothesized by [Ferrara 1997] to explain the appearance of high-z dust in the Galaxy and other edge-on spirals. It has been demonstrated that radiation pressure is sufficient to evacuate a large fraction of the dust near an active star-forming region into the halo, creating a dust distribution which varies slowly with height above the disk. Given the current importance of radiation effects in the inner portion of the M82 outflow (see Section 4.3.6 below) and the extensive nature of the observed dusty halo, such a scenario seems a reasonable model.

Regardless of any mechanism of relocating disk dust into the halo, clearly the hypothesized encounter between M81 and M82 $\sim10^8$ years ago ([Cottrell 1977]; [Yun, Ho, & Lo 1994]) has played an important role in the evolution of the halo in M82. (This encounter is also thought to have initiated the central starburst in M82, e.g., [Mihos & Hernquist 1994].) Radio observations have shown massive clouds of HI surrounding both galaxies, with large arcs and bridges joining them and the nearby galaxy NGC 3077 (e.g., [Davies 1974]; [Yun, Ho, & Lo 1994]). The large-scale velocity structure of this gas blends with the global H$\alpha$ velocity trends in M82, matching the systemic velocity and even the minor-axis velocity gradient ([Cottrell 1977]). The HI cloud is clearly extensive enough to replenish dust that has been destroyed by sputtering. Moreover, this massive reservoir of atomic gas should help to maintain the tenuous halo gas itself in M82, a component that is required by hydrodynamical models in order for the outflowing wind to produce observable structures ([Suchkov et al. 1994]).