Ultraviolet line spectra of metal-poor star-forming galaxies

We present synthetic ultraviolet spectra of metal-poor star-forming galaxie s that were calculated with the Starburst99 package. A new spectral library was generated from Hubble Space Telescope observations of O stars in the L arge and Small Magellanic Clouds. The corresponding mean metallicity of the synthetic spectra is approximately 1/4 Z.. The spectra have a resolution o f 1 Angstrom and cover the spectral range 1200-1600 Angstrom. A set of mode l spectra was calculated for a standard initial mass function and star AZ. formation history and is compared to synthetic spectra at solar metallicity . We find that the spectral lines are generally weaker at lower metallicity , as expected from the lower elemental abundances. Stellar wind lines, howe ver, show a more complex behavior : the metallicity dependence of the ioniz ation balance can be important in trace ions, like N4+ and Si3+. Therefore, the strength of N V lambda 1240 and Si IV lambda 1400 does not scale monot onically with metallicity. We compare our new models to ultraviolet spectra of NGC 5253 and MS 1512-cB58, two star-forming galaxies with one-fourth so lar metallicity at low and high redshifts, respectively. The new library pr ovides significantly better fits to the observations than earlier models us ing the Z. library. We discuss the potential of utilizing stellar photosphe ric and wind lines to estimate the chemical composition of star-forming gal axies. The new metal-poor synthetic spectra are available via the Starburst 99 Web site.