Mathematics – Logic
Scientific paper
Jul 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008hst..prop11578a&link_type=abstract
HST Proposal ID #11578. Cycle 17
Mathematics
Logic
Scientific paper
A long standing question in astrophysics is the existence of young galaxies, in which stars are now forming for the first time, in the nearby {i.e., present-day} universe. Such galaxies would be the local analogs of primordial galaxies observed at high redshift. The most promising candidates have long been the most metal-poor systems, including dwarf irregulars {dIrrs} and blue compact dwarfs {BCDs}. However, in many dIrrs and BCDs studied with HST an old {1 Gyr} underlying stellar population, as traced by red giant branch {RGB} stars, has been unambiguously detected. Even in I Zw 18, which is the most-metal poor prototype of the class and long the most controversial case, our group has recently succeeded in detecting an RGB. Nonetheless, there remains the possibility that the star formation histories of BCDs/dIrrs vary from galaxy to galaxy, and that truly young galaxies do exist in the local universe. A new test of these issues has only recently become possible with the identification of DDO 68 as an extremely metal-poor galaxy with an oxygen abundance equal to that of I Zw 18 {12+{O/H}=7.21}. This galaxy is about a factor of 2-3 closer than I Zw 18, which yields the opportunity to avoid the many ambiguities that have plagued studies of I Zw 18. Also, DDO 68 resides in a void, making it more likely that star formation has been suppressed for a very long time. We will observe DDO 68 with ACS/WFC in F606W and F814W, plus F658N {Halpha} to correct the broad F606W for gas contamination. We will use WFC3 in parallel with the same filters to study radial population gradients. Deep color-magnitude diagrams {CMDs} reaching the depth of one magnitude below the predicted RGB tip will be constructed and interpreted based on synthetic CMD fitting. These data will determine unambiguously whether DDO 68 has an underlying old {RGB} stellar population or is forming stars for the first time. Finding just a single nearby ``young'' galaxy would have profound cosmological implications.;
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