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Session 63 - Galaxy Evolution II.
Oral session, Tuesday, January 16
Corte Real, Hilton
We present VLA H\,I and optical spectra of the peculiar galaxy Arp 10. Originally believed to be an example of a classical colliding ring galaxy with multiple rings, Arp 10 attracted our attention because it exhibited a threshold behavior in the star formation rate along its bright optical ring. The new observations show a more complicated picture with a large disturbed neutral hydrogen disk extending 2.7 times the radius of the bright optical ring as well as optical shells or ripples in the outer optical isophotes of the galaxy is reminiscent of the ripples seen in some early type systems. The H\,I disk comprises of two main parts: a very irregular outer structure, containing peculiar loops and filaments and a more regular inner disk associated with the main bright optical ring. In both cases, the H\,I structures do not exactly trace the optical morphology. This effect is most apparent in the outer parts, where the H\,I emission resembles the H\,I loops seen in some shell elliptical galaxies. Even the inner H\,I disk does not correspond well morphologically nor kinematically to the optical rings. These peculiarities lead us to believe that the potential in which the H\,I disk resides is time varying --- a situation which would inherently produce rings of star formation.
We suggest that Arp 10 is the result of the intermediate stage of a merger between a large H\,I rich disk and a gas-poor disk system. As such, it may represent an example of a class of mergers which lies intermediate between the ``ripple and shell'' accretion systems and the head-on collisional ring galaxies.
