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J. F. Helmboldt, R. A. M. Walterbos (New Mexico State University)
We use a spectroscopic sample 37,474 early-type galaxies selected by morphology from the second data release of the Sloan Digital Sky Survey to search for actively star-forming early-type galaxies that may be the progenitors of post-star-burst, early-type galaxies referred to as E+A galaxies. Within our sample, we find that about 3% of the galaxies show clear evidence for current star formation. Visual inspection of the most nearby galaxies estimates that about 1/3 of these star forming galaxies are bulge-dominated spiral galaxies; the rest are elliptical or lenticular galaxies. Visual inspection also revealed that a significant fraction of E+A galaxies within the sample are also bulge-dominated spirals. The star forming objects were found on average to be slightly higher surface brightness and to have lower velocity dispersions than the entire sample. The star forming galaxies also have lower velocity dispersions than the E+A galaxies in the sample, but are lower surface brightness. We find that the abundances of E+A and star forming galaxies within our sample both depend on environment in nearly the same way; higher numbers of both types of objects are found in lower density environments. We use population synthesis models to explore the possible distributions for the star formation time scales in these objects. The best fitting scenario requires that the star formation time scale be inversely proportional to the initial SFR and estimates the median time scale to be about 50 Myr with a standard deviation of about 1 dex. The fraction of galaxies with time scales greater than 1 Gyr for this distribution is about 10%. This fraction is significantly lower than what is required to explain the level of alpha-enhancement observed for the entire sample as quantified by the distribution of values for [Mg/Fe] without invoking any change in the initial mass function.
The author(s) of this abstract have provided an email address for comments about the abstract: helmbold@nmsu.edu
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.