Hubble Space Telescope Ultraviolet Imaging and Spectroscopy of the Bright Starburst in the Wolf-Rayet Galaxy NGC 4214
We have obtained an HST Faint Object Camera ultraviolet image and Faint Object Spectrograph ultraviolet spectra of the central starburst region in the nearby amorphous galaxy NGC 4214. The ultraviolet image reveals a bright, compact, starburst knot surrounded by more than 200 fainter pointlike sources. Spectral synthesis modeling of the ultraviolet spectrum of the central starburst knot suggests the stellar population has an age of 4-5 Myr. The extinction-corrected ultraviolet flux implies that hundreds of 0-type stars are contained within a diameter of at most 5 pc. The inferred number of Wolf-Rayet (W-R) stars implies that the star formation episode occurred in a short duration burst. The starburst knot contains roughly twice the number of O stars as in a similar size region centered on R136a in 30 Doradus, but it is slightly older. The knot in NGC 4214 is comparable in size and luminosity to the starburst knots identified in the core of the W-R galaxy He 2-10 and to the super star clusters found in other star-forming galaxies. The number of Lyman continuum photons inferred from Hα measurements is at least a factor of 4 smaller than that predicted from the observed numbers of hot stars; this starburst knot is thus "density bounded" to ionizing radiation. The fainter pointlike sources seen in the ultraviolet image are probably individual hot stars or small groups of stars. The ultraviolet luminosity function of these objects is similar to that found for stars within 30 Doradus.