Linear Polarization of the Hα Emission Line in the Halo of M82 and the Radiation Mechanism of the Filaments
New polarization measurements have been made of five faint outer regions in the M82 halo to test if the electric vectors are perpendicular to the filaments, or to the radius vector from the center. The measurements agree to within 2 with a scattering model, but differ by 4 to 9 from the prediction for the synchrotron mechanism. Polarization of the Ha emission line in patch RD was measured to be 27 percent *3 percent at 0 = 54 , and was found to be the same in magnitude and in angle as continuum radiation from the same patch, suggesting that scattering is the dominant process in the halo. To the accuracy of our data (I 3 percent) there is no wavelength dependence of the continuum polarization over the spectral range of XX0.36-0.8 . This requires either that the scattering particles be electrons or that they be grains with much larger dimensions than the wavelength of the measured light. The absolute energy distributions of two filaments have been measured from .36 to 0.9 and are similar to the distribution of a central region of M82 that was measured by O'Connell and by Peimbert and Spinrad, as de-reddened by 2.8 mag. The equivalent width of Ha is also similar in the three regions. If more accurate measurements support these preliminary results, the central region M82 A may then be the source of the scattered light in the halo. Comparison of the filamentary structures on photographs taken in the continuum and in Ha light supports the scattering hypothesis. If the scattering is by electrons and if M82 A is the source of scattered light, then the electron density in the bright patch RD is 3400 . The filamentary surface brightness decreases with distance from the center as 2 65, which requires that the density of scattering particles declines at a rate between D- and Th1.65, depending on the geometry. The total electron mass in the halo would be 2 X 10 if we adopt a volume filling factor of (35)2; otherwise it is 101* o, which is large. But in this case the electron halo would be optically thick, which is unacceptable. But we are unconvinced, for the following reasons, that a simple scattering model explains the system. (I) The emission lines in the halo are narrower than AX 6 A, and this requires that the internal motions of the scattering particles be less than 300 km 1 The limit on the corresponding electron temperature is 7? < 6000 K. A blast-wave origin for the electrons would seem to be excluded. (2) At this temperature, why is there no recombination radiation? (3) Why is the observed polarization so much smaller than tOO percent? What is the depolarizing mechanism other than ad hoc geometry? (4) The observed variation of color of the filaments with distance from the center is not understood. Is it real? (S) The complex velocity field which led to the original explosion hypothesis is not understood in the presence of scattering (Sanders and Balamore). The galaxy remains a mystery to us.