The Stellar Populations in Blue Compact Dwarf Galaxies

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Session 39 -- Spiral, Dwarf and Irregular Galaxies
Display presentation, Wednesday, June 14, 1995, 9:20am - 6:30pm

[39.04] The Stellar Populations in Blue Compact Dwarf Galaxies

D. L. Sudarsky and J. J. Salzer (Wesleyan University)

\par The stellar populations in Blue Compact Dwarf (BCD) galaxies are investigated using optical CCD imaging data in combination with theoretical models. The very blue colors characteristic of BCDs are due to their intense star-forming region(s). The light from the starbursts can dominate the optical and ultraviolet appearance of BCDs, sometimes to the point of masking any previously-existing underlying stellar population. Using a combination of broad-band UBV and narrow-band H$\alpha$ images, we have isolated the starburst and host galaxy components in a sample of ten BCDs in order to study the relative contributions to the overall galaxian mass and light from the two distinct populations. In order to produce color indices that are characteristic of the stellar components of BCDs, the UBV broad-band images are corrected for the presence of nebular line and continuum emission with empirical and semi-empirical methods. The analysis of our data yields colors and luminosities for both the young starburst population and the older stars of the host galaxy, allowing for the first time the comparison of the properties of the BCD hosts with those of quiescent dwarf galaxies.

\par The separated starburst and host galaxy color indices differ by 0.2--0.6 magnitudes, clearly indicating the presence of an underlying older stellar population. These, as well as the color indices of the composite galaxy, are used in combination with theoretical models to provide a quantitative understanding of the stellar populations. By modeling the starbursts, our data allow us to set limits on the total mass present in the older stellar population. Typically, the older stars of the host galaxy contribute 30-80 times the mass of the starburst and roughly equal amounts of stellar luminosity. The B-band luminosity enhancement (brightening) of the host galaxy due to the starburst component is typically 0.6--1.0 magnitude. All BCDs studied show clear evidence for an older, underlying population of stars from an apparently normal dwarf galaxy, except for I Zw 18, which appears to be forming stars for the first time.

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