Ultraviolet imaging of the irregular galaxy NGC 4449 with the ultraviolet imaging telescope: Photometry and recent star formation history

The bright Magellanic irregular galaxy NGC 4449 was observed during the Ast ro-2 Space Shuttle mission by the Ultraviolet Imaging Telescope (UIT), whic h obtained images of a 40' field centered on the galaxy in two broad far-ul traviolet (FUV) bands centered at 1520 and 1620 Angstrom, with 3 "-5 " spat ial resolution. Together with H alpha and H beta fluxes from ground-based F abry-Perot images, these data are analyzed in order to explore the recent s tar formation history of NGC 4449. Maps of the H alpha-to-FUV and FUV-to-bl ue continuum flux ratios are presented and interpreted using evolutionary s ynthesis models. Photometry is presented both for 22 apertures containing l arge OB complexes and for 57 small apertures containing compact FUV-emittin g knots. The OB complexes along the northern edge of the visible system hav e high H alpha-to-FUV ratios and thus appear to be more dominated by the cu rrent generation of stars than are other parts of the galaxy. However, youn g sources do exist elsewhere and are particularly conspicuous along the bar . The small-aperture analysis shows three candidate regions for sequential star formation. Surface brightness profiles are consistent with an exponent ial disk in both the FUV and the optical continuum.