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Spectropolarimetry of the superwind filaments of the starburst galaxy M82: kinematics of dust outflow

Spectropolarimetry results for the starburst galaxy M82 are presented. The optical emission lines of the filaments in the energetic outflow ("superwind") from the nuclear starburst region of M82 are substantially polarized. The H-alpha polarization degrees and angles measured by our study are consistent with previous narrowband imaging polarimetry data. The polarized emission lines are redshifted with respect to the emission lines in the total light and systemic motion of the galaxy. The emission line intensity ratios [NII]/H-alpha and [SII]/H-alpha in the polarized light are similar to those of the nuclear star-forming region. In addition, the electron density N_e derived from the[SII]6731/6717 line ratio of the polarized light is 600 - 1000 cm^-3 at a distance of more than 1 kpc from the nucleus, whereas the N_e derived from the total light are less than 300 cm^-3. These facts strongly suggest that the emission from the nuclear starburst of M82 is scattered by dust grains entrained and transported outward by the superwind. A simple hollow biconical outflow model shows that the velocity of the outflowing dust grains, v_d, ranges from 100 to 200 km/s near the nucleus, decreases monotonically with the distance from the nucleus, and reaches about 10 km/s at around 1 kpc. The motion of the dust is substantially slower than that of both ionized gas (~600 km/s) and molecular gas (~200 km/s) at the same distance from the nucleus of M82. This indicates that dust grains in the superwind are kinematically decoupled from both gas components at large radii. Since the dust velocity v_d is much less than the escape velocity of M82 (~170 km/s at 1.5 kpc from the nucleus), most of the dust entrained by the superwind cannot escape to intergalactic space, and may fall back into the galaxy disk without any additional acceleration mechanisms (such as radiation pressure).