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P. M. Knezek, K. R. Sembach (JHU), J. S. Gallagher (UWisc)
Dwarf galaxies continue to confound astronomers. These are presumably the simplest galaxies formed, yet despite considerable effort recently, the formation and evolution of both rotationally supported (dIrrs) and pressure supported (dE/dSphs) dwarf systems remain poorly understood. In particular, a possible evolutionary link between the two different morphological classes of dwarfs continues to be debated. Do dIrrs evolve into dE/dSphs? Even in the Local Group the situation is unclear. It is now known that many of the Local Group dwarfs have complex star formation histories and kinematic peculiarities, and at least 5 of the dwarfs possess optical morphologies that place them in a ``transition'' category between dIrr and dE/dSph (e.g. Mateo 1998, ARAA, 36, 435). Detailed studies of such ``transition'' objects, both within the Local Group and in other environments, may provide insight into the possible evolutionary link between the two classifications.
A prior broadband (B and R) and H\alpha study of three potential dwarf ``transition'' galaxies identified by Sandage & Hoffman (1991, ApJ, 379, 45), NGC~3377A, NGC~4286, and IC~3475 (Knezek, Sembach, & Gallagher 1998, ApJ, in press), indicated that the evolutionary history of even these `simple' systems can be quite complex. We found that none of these systems is clearly a ``transition'' object. The two galaxies with ongoing star formation, NGC~3377A and NGC~4286, can continue to form stars at their current rate for more than a Hubble time. The galaxy without detected H\alpha emission, IC~3475, is too blue to be a canonical dE, as originally proposed, and has an optical bar. In an effort to further disentangle the evolutionary status of these three systems we have obtained spectroscopy of some of the HII regions of the two dwarf galaxies with detected H\alpha emission.We have also obtained B, R, and H\alpha data on a larger sample of ``transition'' objects to try and place these galaxies in the context of ``transition'' dwarfs as a whole. We will present a detailed analysis of the star forming regions of two ``transition'' galaxies, and use that knowledge with the star formation properties of a larger sample to search for a coherent picture of the evolutionary history of these systems.
The author(s) of this abstract have provided an email address for comments about the abstract: pmk@pha.jhu.edu