Discovery of high-latitude CO in an H I supershell in NGC 5775

We report the discovery of very high latitude molecular gas in the edge-on spiral galaxy NGC 5775. Emission from both the J = 1-0 and 2-1 lines of 12CO is detected up to 4.8 kpc away from the midplane of the galaxy. NGC 5775 is known to host a number of H I supershells. The association of the molecular gas (MH2,F2 = 3.1 × 107 M ̇) reported here with one of the H I supershells (labeled F2) is clear, which suggests that molecular gas may have survived the process that originally formed the supersheil. Alternatively, part of the gas could have been formed in situ at high latitude from shock compression of preexisting H I gas. The CO J = 2-1/J = 1-0 line ratio of 0.34 ± 40% is significantly lower than unity, which suggests that the gas is excited subthermally, with gas density a few times 102 cm-3. The molecular gas is likely in the form of cloudlets that are confined by magnetic and cosmicray pressure. The potential energy of the gas at high latitude is found to be 2 × 1056 ergs, and the total (H I+H2) kinetic energy is 9 × 1053 ergs. Based on the energetics of the supershell, we suggest that most of the energy in the supershell is in the form of potential energy and that the supershell is on the verge of falling and returning the gas to the disk of the galaxy.