Observations of CO J=3-2 in the outflow of the starburst galaxy M82

Observations are presented of the distribution of (CO)-C-12 J = 3-2 emission in the starburst galaxy M82 covering a region 3" x 3" (2.8 x 2.8 kpc). This area includes the halo region involved in the superwind outflow. More limited coverage is presented for (CO)-C-13 J = 3-2 and (CO)-O-18 J = 3-2. The mass of molecular gas in the halo is about 5 x 10(8) Mo., with a dynamical timescale of the order of 10(7) yr. The results show the region of the outflow at higher CO excitation than previous published observations. Comparison with recently made observations of (CO)-C-12 J = 2-1 shows that the CO gas becomes progressively de-excited at larger distances from the starburst disk, and the isotopic ratio (CO)-C-13/(CO)-C-12 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 (CO)-C-12 J = 3-2 emission with a published 850 mum 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 degrees, which compares to about 40 degrees 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.