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Session 89 - Starburst Galaxies.
Oral session, Wednesday, January 15
Piers 4/5,
The Cartwheel is the archetype ring galaxy due to its large angular size, its 30 kpc diameter starburst outer ring, and its rich morphology. Because of this, it has been the subject of numerous observational and theoretical studies. One notable area of disagreement between models and observations has been the global distribution of star formation. Recent simulations have predicted robust star formation in inner ring and ``spokes'', despite the failure of spectroscopy and H-alpha imaging to find ionized gas apart from the outer ring. This restriction has been attributed to low gas densities throughout the ring galaxy's interior. We present new data showing that ionized gas is not in fact confined to the Cartwheel's outer ring, but exists throughout the inner ring, disk, and ``spokes'', though at low levels. Long-slit optical spectroscopy is used to constrain the nature of these faint line emitting regions. We consider the possibility that they are the result of (i) ongoing massive star formation, (ii) the infall of aging outer ring HII complexes, and (iii) shock ionization of gas raining down upon the disk from the tidal stream joining the Cartwheel and the fleeing Intruder galaxy.
