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 H _ { 2 } S ( 1 ) - S ( 3 ) , [ NeII ] , [ NeIII ] emission lines and PAH features .
We estimate the total warm H _ { 2 } mass and the kinetic energy of the outflowing warm molecular gas to be between M _ { warm } \sim 5 - 17 \times 10 ^ { 6 } M _ { \odot } and E _ { K } \sim 6 - 20 \times 10 ^ { 53 } erg .
Using the ratios of the 6.2 , 7.7 and 11.3 micron 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 five 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 disk or the Southern wind , indicating that on average , PAH emitters are larger and more ionized .
The warm molecular gas to PAH flux ratios ( H _ { 2 } / PAH ) are enhanced in the outflow by factors of 10-100 as compared to the starburst disk .
This enhancement in the H _ { 2 } / PAH ratio does not seem to follow the ionization of the atomic gas ( as measured with the [ NeIII ] / [ NeII ] line flux ratio ) in the outflow .
This suggests that much of the warm H _ { 2 } in the outflow is excited by shocks .
The observed H _ { 2 } line intensities can be reproduced with low velocity shocks ( v < 40 km s ^ { -1 } ) driven into moderately dense molecular gas ( 10 ^ { 2 } < n _ { H } < 10 ^ { 4 } cm ^ { -3 } ) entrained in the outflow .