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A. Gil de Paz (Carnegie Observatories and Universidad Complutense de Madrid), B. F. Madore, S. Boissier, Y. H. Joe (Carnegie Observatories), D. Thilker, L. Bianchi (Johns Hopkins University), GALEX Science Team
With the objective of understanding the physical conditions governing the formation of stars in the extended UV (XUV) disks recently discovered by GALEX in the nearby galaxies M83 and NGC4625 we have completed the spectroscopic observations of two dozen star-forming regions found in the outermost parts of these galaxies. These regions were previously identified in deep Halpha images taken from the ground as faint, compact HII regions with emission-line luminosities compatible with being powered by single stars.
The multi-object spectroscopic observations were carried out using the 5m Hale telescope at Palomar Observatory and the 6.5m Magellan-I telescope at Las Campanas. The internal dust extinction (derived from the Balmer decrement) is found to vary betwen A(V)=0-1.5mag, with a median of A(V)=0.6mag. The line ratios measured suggest metallicities for the ionized gas of one tenth solar or less in the outermost parts of the extended UV disks gradually rising to reach the abundances of the optical disk in the inner regions of the XUV disks. These preliminary results suggest that the conditions under which the star formation takes place in the XUV disks, although not dramatically different from those found in the main body of these or other galaxies, might be similar to those present in spiral disks during an earlier stage of evolution.
GALEX (Galaxy Evolution Explorer) is a NASA Small Explorer, launched in April 2003. We gratefully acknowledge NASA's support for construction, operation, and science analysis for the GALEX mission.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.