Physics
Scientific paper
Jul 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002hst..prop.9288f&link_type=abstract
HST Proposal ID #9288
Physics
Hst Proposal Id #9288
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 will test this hypothesis with WFC images deep enough to detect all main sequence stars that can become type II supernova {i.e. O and B stars}. This will directly constrain the disk heating rate from stellar populations. The target NGC2915 {DDO154 as a reserve orbit candidate} has one of the most spectacular HI disks; it extends over five times the optical {Holmberg} radius and displays HI spiral arms. Recently we discovered three faint HII regions slightly beyond the optical radius, indicating some outer disk star formation. Parallel HRC observations will image one of these HII regions to determine if it is ionized by a cluster or single star {only one star is needed to produce the ionizing flux}. Observations of an extended faint young outer stellar population, if found, will also [1] constrain IMF parameters {upper mass limit, slope}; [2] imply that the ISM is not primordial; [3] imply that the target is not as dark matter dominated as previously measured.
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