Other
Scientific paper
Jul 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008hst..prop11750d&link_type=abstract
HST Proposal ID #11750. Cycle 17
Other
Scientific paper
The properties of dwarf galaxies fall into two broad classes. At one extreme are dwarf irregulars, which have blue colors and irregular morphologies. They are actively star forming, with high gas mass fractions and current star formation rates that are comparable to or higher than their past average. At the other extreme are dwarf spheroidal and dwarf elliptical galaxies, which have smooth morphologies and redder colors. They are devoid of gas, have no current star formation, and are preferentially found in denser environments. For decades, scientists have debated the origin of these two distinct classes, postulating that dwarf spheroidals and dwarf ellipticals are either former dwarf irregulars whose star formation was truncated by environmental processes {e.g. ram pressure or tidal stripping} or that they are a distinct class of galaxies with a unique evolutionary history. Given that the suppression of dwarf galaxies' star formation is central to reconciling the paucity of dwarfs with predictions of Lambda CDM, discriminating amonst these models transcends squabbles about the origin of dwarf galaxy morphologies. A recent paper has claimed a definitive answer to these questions, based in large part on their finding that, at the same luminosity, the old stars in dwarf irregulars have systematically lower metallicities than those in dwarf spheroidals. These metallicities were estimated from the colors of red giant branch {RGB} stars, which become redder with increasing metallicity for old stellar populations. However, these authors neglected that when younger stellar populations are present, the color of the RGB becomes bluer and is contaminated with evolving AGB and core helium-burning stars. The resulting bluer colors lead to the false impression of lower metallicities if a purely old age is incorrectly assumed. The observed offset of young dIrr galaxies to lower inferred metallicities may reflect exactly this mistake. We propose to reanalyze a large sample of dwarf galaxies with high quality multi-color ACS imaging. We will rederive the metallicity distribution of the old stellar population, after correcting for the presence of young stars on the RGB. We will then analyze the resulting trends in metallicity as a function of galaxy mass, luminosity, morphology, and environment. Because this analysis will be carried out as a function of position within each galaxy, we will be able to examine radial trends in metallicity as well.;
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