Observations are presented of the distribution of 12CO J = 3-2 emission in the starburst galaxy M82 covering a region 3'' × 3'' (2.8 × 2.8 kpc). This area includes the halo region involved in the superwind outflow. More limited coverage is presented for 13CO J = 3-2 and C18O J = 3-2. The mass of molecular gas in the halo is about 5 × 108 M☉, with a dynamical timescale of the order of 107 yr. The results show the region of the outflow at higher CO excitation than previous published observations. Comparison with recently made observations of 12CO J = 2-1 shows that the CO gas becomes progressively de-excited at larger distances from the starburst disk, and the isotopic ratio 13CO/12CO J = 3-2 also becomes smaller outside the starburst disk. These effects are interpreted as differences in excitation and optical depth between the starburst region and the outflow and outer disk. A comparison between the 12CO J = 3-2 emission with a published 850 μm continuum map shows that CO makes a significant contribution to the continuum in this band and that the fractional contribution is greatest near ±30'' from the nucleus approximately along the major axis. The progressively slower rotation of the halo gas with distance above and below the disk, coupled with consideration of the conservation of angular momentum, is analyzed to reveal the pattern of the outflow. The flow appears to diverge more strongly below the disk, with a cone angle of about 90°, which compares to about 40° above the disk. The mass and energetics of the halo molecular gas suggest the possibility that the molecular material and dust in the halo will not escape from M82 but are instead being recycled through the halo after injection as supershells by one or more transient starburst events.
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