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Astron. Astrophys. 326, 537-553 (1997)

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5. Conclusion

Using interferometric HI and CO observations, we found an extreme spatial segregation between the atomic and molecular gas in Arp 105 (NGC 3561A/B). These gas distributions are consistent with the idea that, as result of a collision, one fraction of the atomic gas of NGC 3561A has been tidally pulled out into the intergalactic medium, supplying, for one part, tidal dwarf galaxies, and for the other, the active nucleus of NGC 3561B, whereas another fraction has sunken towards its central region, where it was transformed into molecular gas. In that respect, Arp 105 exhibits all the phenomena previously observed in typical mergers, although it is a far less evolved interacting system.

Our multi-wavelength study reveals a complex dynamics. In the central region of the spiral, the molecular and ionized gas show flat rotation curves. In the southern part of the system, the HI clouds associated with NGC 3561B and A105S have discrepant velocities, which suggests that either they have different origins, or that they used to belong to one structure, part of which has collapsed and has formed the tidal dwarf A105S. A similar phenomenon might have occurred in the northern HI clouds, where we observe, within the HI tail, a kinematically decoupled component, associated with the star forming dwarf A105N. Both tidal objects show possible signs of rotation.

The observations reported here provide for the first time indications that some tidal dwarfs might already be gravitationally bound and are therefore likely to become independent galaxies.

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© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997