Large-scale bipolar wind in M82

Abstract

In 1963 it was reported¹ that an enormous explosion (∼10⁵⁵ erg) had occurred about one million years ago in the nuclear region of the nearby irregular galaxy M82. Deep photographic plates showed a spectacular filamentary system extending to 200 arc s (∼3 kpc) along the minor axis of the galaxy and Hα spectroscopy revealed a steep velocity gradient in this direction, suggesting expulsion of the ionized gas from the core of the galaxy^1,2. Thereafter, broadband polarimetry showed the filaments to be linearly polarized (15–30%) at optical wavelengths, with electric vectors generally perpendicular to the radius vector from the centre^3,4, indicating that the light has been scattered by particles into our line of sight⁵. The explosion hypothesis⁶ was questionable in that the observed blueshifts and redshifts cannot arise from explusion or infall, as the particles would scatter only redshifted or blueshifted light respectively on both sides of the galaxy. Although the nature of M82 remains a mystery, two significant observations have been largely overlooked. From imaging polarimetry, Schmidt et al⁷. demonstrated that most of the visual polarized flux arises from a smooth component which is symmetric over the main body of the galaxy. Axon and Taylor found well organized Hα line-splitting over ∼60 arc s at two long-slit positions south of the galaxy and parallel to the major axis⁸. Here we present new imaging Fabry–Perot observations of M82 for the Hα and [N II] λ6,583 spectral lines⁹. Our observations show for the first time that the line and continuum emission above and below the galactic disk arise from two morphologically and kinematically distinct components. The first component consists of filamentary, narrow-line emission (full width at half maximum, FWHM <200 km s⁻¹) organized into a bipolar outflow over the surfaces of two elongated bubbles. We present direct evidence for a second component of broad-line (at FWHM 300 km s⁻¹) and continuum emission arising from a faint exponential halo that rotates slowly with the disk.