Other
Scientific paper
Jul 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004hst..prop10287f&link_type=abstract
HST Proposal ID #10287
Other
Hst Proposal Id #10287
Scientific paper
Half of gas rich galaxies have HI disks extending well beyond their optical extents. This gas is a significant potential fuel repository for future star formation. How can galaxies can store such quantities of ISM without any apparent star formation? These extended gas disks have well maintained velocity dispersions and dynamical structure. What is keeping them warm? Perhaps there is star formation, but at such low levels it is not readily apparent. We have begun investigating this problem by looking for low-level star formation in extended HI disk galaxies. Our first target was the spectacularly dark matter dominated galaxy NGC2915. In this case our hypothesis was proven to be correct. We found indications for low level star formation in two ways. {1} Our ACS WFC images show a noticeable blue plume of stars in the g vs g-V as well as the V vs V-I color magnitude diagrams. The blues stars extend over the full area of the WFC and are highly clumped. {2} Previous deep Halpha images shows a few very faint HII regions near the Holmberg radius. The weakness of the HII regions and the large numbers of blue {B} stars suggests that the IMF is truncated at the upper end compared to typical IMF parameterizations. This could imply that H-alpha surveys underestimate the true star formation rate. Another surprising result of the ACS observations is that the old population also extends well passed the Holmberg radius and includes globular clusters. We propose to observe a second galaxy for this program - NGC4789A {DDO154}. Like NGC2915, it also has HI extending more than five times the Holmberg radius in a fairly well ordered disk. In this case the old stellar population is comparetively weak, and the foreground reddening is much less. This second set of observations will give us a check on the generality of the NGC2915 results. In particular an IMF that does not extend up to O star formation could have major implications for metal production, kinetic energy release and ionization of the intergalactic medium.
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