August 2015 • 2015MNRAS.451.2640B
Abstract • We have mapped the superwind/halo region of the nearby starburst galaxy M82 in the mid-infrared with Spitzer - IRS. The spectral regions covered include the H2 S(1)-S(3), [Ne II], [Ne III] emission lines and polycyclic aromatic hydrocarbon (PAH) features. We estimate the total warm H2 mass and the kinetic energy of the outflowing warm molecular gas to be between Mwarm ∼ 5 and 17 × 106 M⊙ and EK ∼ 6 and 20 × 1053 erg. Using the ratios of the 6.2, 7.7 and 11.3 μm PAH features in the IRS spectra, we are able to estimate the average size and ionization state of the small grains in the superwind. There are large variations in the PAH flux ratios throughout the outflow. The 11.3/7.7 and the 6.2/7.7 PAH ratios both vary by more than a factor of 5 across the wind region. The northern part of the wind has a significant population of PAH's with smaller 6.2/7.7 ratios than either the starburst disc or the southern wind, indicating that on average, PAH emitters are larger and more ionized. The warm molecular gas to PAH flux ratios (H2/PAH) are enhanced in the outflow by factors of 10-100 as compared to the starburst disc. This enhancement in the H2/PAH ratio does not seem to follow the ionization of the atomic gas (as measured with the [Ne III]/[Ne II] line flux ratio) in the outflow. This suggests that much of the warm H2 in the outflow is excited by shocks. The observed H2 line intensities can be reproduced with low-velocity shocks (v < 40 km s-1) driven into moderately dense molecular gas (102 < nH < 104 cm-3) entrained in the outflow.