FAR-ULTRAVIOLET SPECTRA OF STARBURST GALAXIES - STELLAR POPULATION AND THE KINEMATICS OF THE INTERSTELLAR-MEDIUM

The far-ultraviolet spectra of the four starburst galaxies NGC 6090, M rk 66, Mrk 1267, and IRAS 0833+6517 were observed with the Hopkins Ult raviolet Telescope during the Astro-2 mission. Additional data were ob tained for IRAS 0833+6517 with the Goddard High-Resolution Spectrograp h (GHRS) on the Hubble Space Telescope. We analyze the observations in terms of the stellar content and the kinematics of the interstellar m edium, and we discuss the implications of these results for the interp retation of the ultraviolet spectra of high-redshift galaxies. Evoluti onary synthesis models are used to constrain the star formation histor y from the absolute ultraviolet flux and from the stellar components o f the absorption lines of Si IV and C IV, and from the far-ultraviolet lines O VI + Ly beta + C II. The spectral energy distributions from t hese models are used as inputs for the photoionization code CLOUDY to predict [O III]/H beta and column densities of the cool component of t he ionized gas (Si II and C II) associated with the H II region. The r esults indicate continuous star formation during about 9 Myr, or, alte rnatively, bursts with ages between 3 and 6 Myr, and a mass consistent with that estimated from the H alpha flux. Evidence for significant d ilution by a field star population is found in all galaxies except NGC 6090. Most of the interstellar absorption lines are saturated. Their equivalent widths indicate a large velocity dispersion in the gas. Oth er evidence for large-scale motions of the interstellar gas comes from blueshifts of several hundred km s(-1) with respect to the systemic v elocity in the interstellar lines of NGC 6090, Mrk 66, and IRAS 0833 6517. These outflows are most likely driven by the starburst. Ly alph a was detected in emission in three of the galaxies (NGC 6090, Mrk 66, and IRAS 0833 + 6517). The dereddened Ly alpha/H beta ratio in IRAS 0 833+6517 is close to the recombination value, indicating that extincti on is more important than multiple resonant scattering effects. Howeve r, the GHRS spectrum of IRAS 0833+6517 clearly shows that the emission profile of Ly alpha is asymmetric, the blue wing being absorbed by ne utral gas. This indicates that the velocity structure of the neutral g as and the scattering by H I atoms can also play an important role in the escape of the Ly alpha photons.