Recycling of ghost galaxies : The origin of giant HI ring around NGC 1533

We propose that the giant HI ring recently discovered by HIPASS for S0 galaxy NGC 1533 is formed by unequal-mass merging between gas-rich LSB (low surface brightness: “ghost”) galaxies and HSB disks. The NGC 1533 progenitor HSB spiral is transformed into a barred S0 during merging and the outer HI gas disk of the LSB is transformed into the giant HI ring. We also discuss two different possibilities for the origin of isolated star-forming regions (“ELdot” objects) in the giant gas ring. 1. Ring formation from tidal disruption of LSBs Recent observational results of NGC 1533 HI rings and numerical models for this objects are described in detail by Ryan-Weber et al. (2002) and by Bekki et al. (2003, in preparation), respectively. We accordingly summarize briefly the important three results derived in this numerical studies (See Figure 1 for the model details). Firstly, the strong tidal filed of NGC 1533’s progenitor HSB strips the outer diffuse HI gas disk of the LSB and completely destroy the stellar disk of the LSB. The stripped HI gas forms an incomplete gas ring and the LSB is swallowed by the HSB finally. This outer HI ring formation is different from the merger scenario of polar ring formation proposed by Bekki (1998). Secondly, the HSB forms a bar and loses its spiral arms owing to tidal heating of the interaction, and therefore can be classified as a barred S0 after merging. Thirdly, star formation can proceed in the giant gas ring either by the cloud-cloud collisions within the ring or by strong external gaseous pressure of the hot gas surrounding the HI ring. The proposed merger models thus explain (1) the observed large total mass of the HI ring, (2) structure and kinematics of the HI ring, and (3) the morphology of the host S0 galaxy (NGC 1533).