Assigned ID: 919 ----- Attempting Submission 1 (Sat Feb 26 01:53:02 GMT 2011) ----- Extragalactic giant HII regions (EGHRs) are sites of active, concentrated star formation, and thus provide excellent labs to analyze starburst phenomenon. Although they have been known for a long time, ground-based observations cannot resolve the physical structures and stellar content of EGHRs. The high resolution and sensitivity of Hubble Space Telescope (HST) are ideal for detailed studies of EGHRs. We have searched the Hubble Legacy Archives (HLA) and found 17 nearby galaxies, within ~15 Mpc, with H-alpha and continuum images. We propose to use these images to study the three top-ranking HII regions in each galaxy. While these 51 HII regions span a range in luminosity, most of them are bonafide EGHRs, with H-alpha luminosity greater than a few times 10^39 ergs/s. We will use the H-alpha image to study the distribution and physical structure of the gas in each HII region and determine its H-alpha luminosity and required ionizing flux. We will use the continuum images to determine whether concentrated clusters or distributed OB associations reside in these HII regions, and use photometric measurements to determine the mass and age spread of the resolved stellar population. These will then be compared with the interstellar structures. The results will help us understand the starburst phenomenon and answer questions such as: Are globular clusters formed in EGHRs? How does star formation proceed in an EGHR? How does the physical structure of an EGHR relate to the luminosity, physical location in the host galaxy, and the host galaxy type?

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