Other
Scientific paper
Jan 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21733527a&link_type=abstract
American Astronomical Society, AAS Meeting #217, #335.27; Bulletin of the American Astronomical Society, Vol. 43, 2011
Other
Scientific paper
Observations of the Taffy Galaxies with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope have revealed powerful rotational lines of molecular hydrogen from both the galaxy disks and the bridge region between them. The emission shows strong similarities with the molecular hydrogen emission from the giant inter-group shock structures in Stephan's Quintet. We use excitation diagrams to characterize the warm molecular gas, finding a high average surface mass of
5 Msolar/pc2 and typical excitation temperatures of 150-170 K. This is likely to be a lower limit to the surface density because of limitations in the spectral coverage over most of the bridge, allowing only one excitation temperature component to be modeled. Emission is also seen in the galaxy disks, with the larger ringlike galaxy, UGC 12914, exhibiting exceedingly strong H2 lines which dominate other mid-IR line features. We investigate several possible warming mechanisms for the gas, including cosmic rays, magnetic reconnection, and shocks, with the latter seeming to be the most likely. The very powerful H2 luminosity in the Taffy bridge gas appears to be another example (like Stephan's Quintet) of how large H2 line luminosities, which dominate the mid-IR cooling, can be generated by high-speed encounters. In this case two counter-rotating interpenetrating disks may have helped in increase the kinetic energy available to heat the molecular gas in a post-shocked "Taffy” layer as the galaxies pull apart. Since the cooling time is so short in the warm H2 gas, shocks must be permeating the bridge region in order to continue to heat the molecular hydrogen. Collisions between gas-rich systems may be more common at high-redshift, and so the observations have implications for future IR missions which might be sensitive to rotational cooling lines of molecular hydrogen shifted into the far-IR.
Appleton Philip N.
Boulanger Francois
Cluver Michelle
Guillard Pierre
Helou George
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