The Star Formation History of the Blue Compact Dwarf Galaxy UGCA 290

Abstract

We present the star formation history of UGCA 290, a galaxy with properties intermediate between Blue Compact Dwarfs and Dwarf Irregulars. This galaxy is particularly interesting because its young stellar population is extremely similar to that of the well-studied type iE Blue Compact Dwarf VII Zw 403, despite its different spatial morphology and old stellar content. Our Hubble Space Telescope/Wide Field Planetary Camera 2 single-star photometry for UGCA 290 extends over nine magnitudes in I, and allows a detailed study of its star formation history. Using synthetic color-magnitude diagrams, we show that the recent "burst" which gives this galaxy its BCD status is a moderate enhancement in star formation which lasted for approximately 20 Myr, at a rate about ten times above its previous rate. The star formation history for most of the previous billion years is consistent with a constant rate, although enhancements as large as the current one are possible at times ealier than 400 Myr ago. We estimate that the total mass converted into stars in UGCA 290 more than one billion years ago is about three times the astrated mass since that time. The initial mass function is consistent with a Salpeter slope, and the stellar metallicity is bracketed by Z_sun/50 and Z_sun/5, with evidence for metallicity evolution. Similar results for the star formation history over the past 600 Myr apply to VII Zw 403. Our main result is that despite the traditional picture of BCDs, the current bursts in these two galaxies are neither remarkably intense nor short-lived, and that most of their star formation occured more than a billion years ago.