The massive stellar content of the H II region NGC 604 and its evolutionary state

This paper analyses the integrated ultraviolet spectra taken with the International Ultraviolet Explorer (IUE) and optical ground-based spectra taken with the William Herschel Telescope (WHT) of the giant H II region NGC 604. These data are complemented with ultraviolet (WFPC2 through F170W) and Hα (WFPC2 through F656N) images retrieved from the Hubble Space Telescope (HST) archive. The ultraviolet resonance wind stellar lines, the nebular optical emission lines and the higher order terms of the Balmer series, and He I absorption lines detected in the spectra of NGC 604 are interpreted using evolutionary models optimized for young star-forming regions. The evolutionary state and the massive stellar content of the region are derived in a self-consistent way. The three techniques applied suggest that the central ionizing cluster in NGC 604 is very young, ≃3 Myr old, and that the stars in the cluster were formed in an instantaneous burst following a Salpeter (α = 2.35) or flatter (α = 1.5) initial mass function (IMF), having stars more massive that 80 M⊙. The stellar cluster is able to provide most of the ionizing photons needed to photoionize the whole nebula, and the wind power to form the central shell structure where the cluster core is located. The stellar cluster is affected by an extinction similar to the average extinction that affects the ionized gas. The estimated number of massive stars in the cluster is also in agreement with that derived from previous studies based on the detection of individual stars. The results that we present here support the use of these techniques for the interpretation of the integrated light of more distant star-forming regions.