Other
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998mcdg.proc...77s&link_type=abstract
The Magellanic Clouds and Other Dwarf Galaxies, Proceedings of the Bonn/Bochum-Graduiertenkolleg Workshop, held at the Physikzen
Other
Irregular Dwarf Galaxies, Star Formation History, Stellar Populations
Scientific paper
Using Hubble Space Telescope WFPC2 photometry of individual stars in four nearby dwarf irregular galaxies, we have constructed color-magnitude diagrams in V and I to a limit of 26 in V. We use the blue, core helium burning stars as a tracer of the star formation rate over the past 600-900 Myr in these galaxies. We find that generally star formation can be found throughout the galaxies, with localized sites of enhanced star formation which last for roughly 100 Myr and vary in location around the optical extent of the galaxies. Comparing the star formation histories of all four galaxies, we find no evidence for starbursts. Given a total time baseline of roughly 2 Gyr, this implies that starbursts in dwarf irregulars are relatively rare. We also compare ISM abundances obtained from H II region observations. It is particularly interesting to compare Sextans A with Pegasus. While Sextans A shows a relatively high star formation rate recently, Pegasus has been relatively quiescent. We find that the ISM in Pegasus shows a relatively high N/O ratio, while Sextans A shows a relatively low N/O ratio. These results are consistent with the hypothesis that oxygen production is dominated by high mass stars, while nitrogen production is dominated by intermediate to low mass stars. Thus, the star formation history as recorded in the stars is consistent with the star formation history inferred from the ISM abundances. It appears that a good lower limit on the ability of oxygen to mix back into the cold/warm ISM is about 10 Myr and a good upper limit may be about 100 to 200 Myr. A good upper limit on the ability of nitrogen to mix back into the cold/warm ISM may be about 400 Myr.
Dohm-Palmer Robbie C.
Kobulnicky Henry A.
Skillman Evan D.
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