The UV spectrum of a giant extragalactic HII region is compared here with the synthetic spectrum generated by a fitting procedure which uses a combination of stellar spectra to match the observed spectrum.
The ultraviolet observations of giant extragalactic HII regions provide very valuable information on the stellar population embedded in these complexes.
The UV spectra of HII regions in the IUE short-wavelength range, SWP (1152–1950Å), show a stellar continuum which rises steeply towards the shorter wavelengths. Superposed over this continuum, emission lines and P-Cygni profiles are often observed, as an indication of the presence of hot luminous early-type stars with mass loss.
NGC 604, the brightest HII complex in the nearby galaxy M33, is, because of its size and distance, a good candidate for a detailed study of a giant extragalactic HII region in the UV based on IUE data.
NGC 604 has been observed with IUE on different occasions in low resolution (6 Å mm−1) with the large aperture slit (10 × 20 arcsec). A total of 16 SWP and 8 LWR, or LWP (1950–3200 Å), has been obtained. The spectra correspond to observations of different zones within the region.
The general characteristics of the IUE spectra of NGC 604 in the short wavelength range are similar to the ones mentioned above. In the near UV, the observed continuum is practically constant, no emission lines are detected, there is no 2200-Å absorption feature, and only interstellar absorption lines are present.