Star Formation in the Field and Clusters of NGC 5253

Abstract

We investigate the star formation history of both the bright star clusters and the diffuse ``field star'' population in the dwarf starburst galaxy NGC 5253 using long-slit ultraviolet spectroscopy obtained with the Space Telescope Imaging Spectrograph. The slit covers a physical area of 370 pc×1.6 pc and includes eight apparent clusters and several intercluster regions of diffuse light that we take to be the field. The diffuse light spectrum lacks the strong O-star wind features that are clearly visible in spectra of the brightest clusters. This discrepancy provides compelling evidence that the diffuse light is not reflected light from nearby clusters but originates in a UV-bright field star population, and it raises the issue of whether the star formation process may be operating differently in the field than in the clusters. We compare our spectra to STARBURST99 evolutionary synthesis models that incorporate a new low-metallicity (~1/4 Z_solar) atlas of O-star spectra. The clusters are well fitted by instantaneous burst models with a Salpeter initial mass function (IMF) extending up to 100 M_solar, and we derive ages for them ranging from 1 to 8 Myr. Reasonable fits to the field spectrum are obtained by continuous star formation models with either an upper mass cutoff of ~30 M_solar or an IMF slope steeper than Salpeter (α~3.5). We favor a scenario that accounts for the paucity of O stars in the field without requiring the field to have a different IMF than the clusters: stellar clusters form continuously and then dissolve on ~10 Myr timescales and disperse their remaining stars into the field. We consider the probable contribution of an O-star-deficient field population to the spatially unresolved spectra of high-redshift galaxies. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.