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Session 81 - Dwarf Galaxies.
Display session, Friday, January 09
Exhibit Hall,

[81.05] Ultraviolet Imaging Telescope and Optical Observations of Holmberg II: Insights on Star Formation in Dwarf Galaxies

S. G. Stewart (USNO), M. N. Fanelli, J. K. Hill (HSTX), K. -P. Cheng (CSU,Fullerton), R. W. O'Connell (UVa), R. C. Bohlin (STScI), G. G. Byrd (Alabama), S. G. Neff (NASA/GSFC), M. S. Roberts (NRAO), A. M. Smith, T. P. Stecher (NASA/GSFC)

We present the first study using spatially-resolved ultraviolet imagery to characterize star formation in a dwarf galaxy. Far-ultraviolet (FUV, 1520 Åimages of dwarf galaxy Holmberg II were taken with the Ultraviolet Imaging Telescope (UIT) during the Astro-2 mission in March 1995 providing a direct image of the recently-formed, massive stars in the galaxy. Using the FUV data and corresponding optical observations in the U, B, and R bands, total magnitudes and colors for the galaxy are derived. We determine surface brightness profiles in the plane of the galaxy and find that the optical surface brightness profiles are normal in comparison to other dwarf galaxies showing a central light depression at radii close to the dynamical center. The FUV surface brightness profile exhibits a central depression and has two peaks in the surface brightness which are at higher levels than the central region. The FUV flux and (FUV-B) color of individual star forming regions are used to determine the approximate age of each region. An estimate for the number of equivalent early-O stars in each region is made and compared to estimates using other methods. A comparison between the relative morphologies of the FUV and H\alpha images shows FUV knots embedded in H\alpha shells. We also find H II regions which have no corresponding FUV emission surrounding FUV knots suggestive of sequential star formation. Holmberg II has an interesting H I morphology (Puche et al. 1992) consisting of numerous holes in the neutral medium which in theory are related to the star formation process. A comparison between the FUV and H I morphologies shows that none of the bright FUV knots lie within the H I holes and that they are more likely to be found immediately outside of a hole boundary. In addition, we find that all of the FUV emission lies within the solid body regime of the rotation curve of the neutral gas disk.

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