Spitzer IRS Spectral Mapping of the Toomre Sequence: Spatial Variations
  of PAH, Gas, and Dust Properties in Nearby Major Mergers
  We have mapped the key mid-IR diagnostics in eight major merger systems of
the Toomre Sequence (NGC4676, NGC7592, NGC6621, NGC2623, NGC6240, NGC520,
NGC3921, and NGC7252) using the Spitzer Infrared Spectrograph (IRS). With these
maps, we explore the variation of the ionized-gas, PAH, and warm-gas (H_2)
properties across the sequence and within the galaxies. While the global PAH
interband strength and ionized gas flux ratios ([Ne III]/[Ne II]) are similar
to those of normal star forming galaxies, the distribution of the spatially
resolved PAH and fine structure line flux ratios is significant different from
one system to the other. Rather than a constant H_2/PAH flux ratio, we find
that the relation between the H_2 and PAH fluxes is characterized by a power
law with a roughly constant exponent (0.61+/-0.05) over all merger components
and spatial scales. While following the same power law on local scales, three
galaxies have a factor of ten larger integrated (i.e. global) H_2/PAH flux
ratio than the rest of the sample, even larger than what it is in most nearby
AGNs. These findings suggest a common dominant excitation mechanism for H_2
emission over a large range of global H_2/PAH flux ratios in major mergers.
Early merger systems show a different distribution between the cold (CO J=1-0)
and warm (H_2) molecular gas component, which is likely due to the merger
interaction. Strong evidence for buried star formation in the overlap region of
the merging galaxies is found in two merger systems (NGC6621 and NGC7592) as
seen in the PAH, [Ne II], [Ne III], and warm gas line emission, but with no
apparent corresponding CO (J=1-0) emission. Our findings also demonstrate that
the variations of the physical conditions within a merger are much larger than
any systematic trends along the Toomre Sequence.
