Abstract details

I will report the results of two deep HST/ACS imaging studies of the extremely metal-poor star-forming dwarf galaxies I Zw 18 and SBS 1415+437. Deep broad-band imaging yields high-quality CMDs of the resolved stars that can be used to infer the star-formation history over a large fraction of a Hubble time, as well as the distance through the TRGB-method, average AGB star brightness, or Cepheid variability. Chemically-unevolved systems like I Zw 18 or SBS 1415+437 represent the closest analog to primordial galaxies in the early universe and, as such, offer the best place where to study star formation and stellar evolution in a pristine environment. More interestingly, it has been argued that some of these objects (e.g., I Zw 18) may be genuine 'young' galaxies in the local universe. However, by revealing the existence of an evolved red stellar population in both I Zw 18 and SBS 1415+437, our new HST/ACS observations do not support such an interpretation. I will discuss these findings in the context of results that we are obtaining with similar set-up for other enigmatic dwarf starburst galaxies such as NGC 4449 and NGC 1569, and in the context of popular theories for the formation and evolution of galaxies. This work is based on HST projects GO 9361, 10585, 10586, and 10885 (PI Aloisi, with various collaborators) with grant support from NASA/STScI.