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.
We study the CO and the radiocontinuum emission in an active galaxy to analyze the interplay between the central activity and the molecular gas. We present new high-resolution observations of the CO(1-0) and CO(2-1) emission lines, and 3.5 cm and 20
cm radio continuum emission in the central region of the LINER/starburst galaxy NGC 6764. The galaxy has an outflow morphology in radio continuum, spatially coincident with the CO and H$alpha$ emission, and centered slightly off the radio continuum peak at the LINER nucleus. The total molecular gas mass in the center is about 7x10^8 msun, using a CO luminosity to total molecular gas conversion factor that is three times lower than the standard one. CO(1-0) emission is found near the boundaries of the radio continuum emission cone. The outflow has a projected expansion velocity of 25 km/s relative to the systemic velocity of NGC6764. About 4x 10^6 msun of molecular gas is detected in the outflow. The approximate location (~1 kpc) of the dynamical inner Lindblad resonance has been derived from the rotation curve. The peak of the CO emission is slightly (< 200 pc) offset from the peak of the radio continuum. The molecular gas has most likely been ejected by the stellar winds from the recent starburst, but the CO line ratios show indication of an interaction with the AGN. The energy released by the nuclear starburst is sufficient to explain the observed outflow, even if the data cannot exclude the AGN from being the major energy source. Comparison of the outflow with hydrodynamical simulations suggests that the nuclear starburst is 3--7 Myr old and the bubble-like outflow is still confined and not freely expanding.
