Gas segregation in the interacting system Arp 105

Arp 105 is an example of a collision between an infrared luminous spiral (NGC 3561A) and an elliptical (NGC 3561B) galaxy in a cluster of galaxies. Duc & Mirabel (1994) reported in this system the genesis of star{forming tidal dwarf galaxies at the ends of the tidal tails emanating from the spiral. In this paper, the gas distribution in Arp 105 is analyzed based on HI and 12CO(1{0) emission, mapped with the Very Large Array and the Plateau de Bure interferometers. The observations reveal an extreme segregation between the atomic and molecular gas distributions. HI clouds as massive as 6 109 M are found along the tidal tails and are associated with the dwarf galaxies at their tip; the spiral disk itself contains less than 5 107 M of HI. About 1010 M of molecular gas is concentrated in the central 3 kpc region of the spiral. Towards the elliptical, HI is seen in absorption. The shape of the absorption line suggests inflow and capture of gas by the giant galaxy. Arp 105 thus shows direct evidence of mass transfer between a spiral and an elliptical. All these phenomena are interpretated as a direct consequence of the interaction between NGC 3561A and B. A study of the dynamics of the system reveal that the HI in the northern tail is composed of two individual components, one along the optical tail, and a second one, kinematically decoupled, which shows signs of rotation. Indications of rotation are also seen in H in the tidal object at the tip of the southern tail. These observations show that these tidal dwarf galaxies might have already acquired dynamical independence.