Azimuthal and Kinematic Segregation of Neutral and Molecular Gas in Arp 118: The Yin‐Yang Galaxy NGC 1144

We present new high-resolution HI observations of the disk of the collisionalinfrared luminous (L$_{\rm IR}=2.2\times10^{11}$ L$_{\sun}$) galaxy NGC 1144,which reveal an apparent large-scale azimuthal and kinematic segregation ofneutral hydrogen relative to the molecular gas distribution. Even amongviolently collisional galaxies, the CO/HI asymmetry in NGC 1144 is unusual,both in the inner regions, and in the outer disk. We suggest that we areobserving Arp 118 at a special moment, shortly after a high-speed collisionbetween NGC 1144 and its elliptical companion NGC 1143. HI emission with anaverage molecular fraction f$_{mol}$ $<$ 0.5 is observed on one side (NW) ofthe rotating disk of NGC 1144, while the other side (SE) is dominated by densemolecular complexes in which f$_{mol}$ is almost unity. The interface regionbetween the warm-- and cool--cloud dominated regions, lies on a deepspiral-like dust-lane which we identify as a shock-wave responsible for therelative shift in the dominance of HI and H$_2$ gas. A strong shock being fedby diffuse HI clouds with unusually large ($>$ 400 km s$^{-1}$) rotationalvelocities can explain: 1) the CO/HI asymmetries, 2) a large velocity jump (185km s$^{-1}$) across the arm as measured by HI absorption against a radio brightcontinuum source which straddles the arm, and 3) the asymmetric distribution ofstar formation and off-nuclear molecular gas resulting from likely streamingmotions associated with the strong-shock. The new results provide for the firsttime a coherent picture of Arp 118's many peculiarities, and underlines thepotentially complex changes in the gas-phase that can accompany largegravitational perturbations of gas-rich galaxies.Comment: 20 pages, two tables, 9 Figure