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Session 24 - Disks, Bars & Halos.
Oral session, Wednesday, January 07
International Ballroom East,

[24.03] Galaxy Structure and Evolution at the End of the Spiral Sequence

L. D. Matthews (SUNY at Stony Brook)

I will present the results of a multiwavelength observational study of a sample of nearby, extreme late-type spiral galaxies. These are moderate-to-low surface brightness Sc-Sm spirals that fall at the low end of spirals in terms of properties such as HI mass, optical luminosity, and optical size. They include examples of some of the smallest rotationally-supported disk galaxies that still exhibit regular optical structures and centralized light concentrations. In terms of disk structures, the sample is diverse. It includes thick, Magellanic-type disks, BCD/spiral transition objects, and thin, extended disks that appear to be dynamically very cold.

CCD photometry and 21-cm spectroscopy show that in terms of their physical properties, extreme late-type spirals often appear as transition objects between giant spirals and irregular galaxies, having moderately blue colors, high gas fractions, and in some instances, radial reddening of the disk. Bars are common in extreme late-type spirals, and a number of the sample galaxies have unresolved, point-like nuclei. Several of the galaxies show distinctive surface brightness ``steps'' in their disks, demonstrating that even small, late-type spirals can have multi-component disks. Several of the sample objects fall below the Tully-Fisher relation defined by brighter galaxies, suggesting that extreme late-type spirals are often dark matter-dominated. Longslit spectroscopy reveals that many extreme late-type spirals have slowly rising rotation curves out to the last measured point in the optical galaxy. Taken together, my observations suggest that extreme late-type spiral galaxies are some of the least evolved nearby galaxies in a dynamical and in a star-formation sense.

Program listing for Wednesday