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Session 81 - Dwarf Galaxies.
Display session, Friday, January 09
Exhibit Hall,
We present an analysis of HST images obtained of the Blue Compact Dwarf (BCD) galaxy VIIZw403 (=UGC6456) obtained as part of the WFPC1 GTO program. BCDs are tiny, high surface brightness irregular galaxies currently engulfed in star formation. It is generally believed that BCD's are undergoing an extreme and short-lived burst of star formation, in some cases perhaps the first real episode of star formation. In addition BCDs have been typically thought of as containing large massive star populations located in a small space, and it is sometimes argued that the stellar mass function and star formation efficiency might be unusual. Thus, these galaxies are interesting as extrema among small galaxies.
The HST images of VIIZw403, a nearby BCD with M_B=-14, reveal young, massive stars in a main sequence to early B stars and red supergiants. Analysis of two rich OB associations in the galaxy show their massive star contents to be similar in number and spatial density to typical OB associations in Local Group galaxies. That is, the star formation units are not unusual in this galaxy. VIIZw403 is the first BCD to be resolved into an extended sea of red giant stars. In addition a distinctive asymptotic giant branch is also measured. Thus, an older, lower mass star population exists in this galaxy, and the galaxy is not young. Furthermore, the red giants rather than red supergiants dominate the light in the I-band, with implications for global photometry of other BCDs. The tip of the red giant branch was used to determine the distance to the galaxy, placing it \sim25% more distant than had been previously assumed. We discuss the star formation history of this galaxy from modelling the color magnitude diagram and show that the star formation history defies the standard assumptions.
