2003ApJ...586..112A

We present new high-resolution H I observations of the disk of the collisional infrared luminous (L_IR=2.2x10¹¹L_☉) galaxy NGC 1144, which reveal an apparent large-scale azimuthal and kinematic segregation of neutral hydrogen relative to the molecular gas distribution. Even among violently collisional galaxies, the CO/H I asymmetry in NGC 1144 is unusual, both in the inner regions and in the outer disk. We suggest that we are observing Arp 118 at a special moment, shortly after a high-speed collision between NGC 1144 and its elliptical companion NGC 1143. H I emission with an average molecular fraction f_mol<0.5 is observed on one side (northwest) of the rotating disk of NGC 1144, while the other side (southeast) is dominated by dense molecular complexes in which f_molis almost unity. The interface region between the warm- and cool-cloud dominated regions lies on a deep spiral-like dust lane that we identify as a shock wave responsible for the relative shift in the dominance of H I and H₂gas. A strong shock being fed by diffuse H I clouds with unusually large (>400 km/s) rotational velocities can explain (1) the CO/H I asymmetries, (2) a large velocity jump (185 km/s) across the arm as measured by H I absorption against a radio bright continuum source that straddles the arm, and (3) the asymmetric distribution of star formation and off-nuclear molecular gas resulting from likely streaming motions associated with the strong shock. The new results provide for the first time a coherent picture of Arp 118's many peculiarities and underline the potentially complex changes in the gas phase that can accompany large gravitational perturbations of gas-rich galaxies.