Spatially Resolved Spitzer-IRS Spectral Maps of the Superwind in M82
  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 $M_{warm}\sim5-17\times10^6$ M$_{\odot}$ and
$E_{K}\sim6-20\times10^{53}$ 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.
