Far‐Ultraviolet Spectra of Starburst Galaxies: Stellar Population and the Kinematics of the Interstellar Medium

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Abstract
The far-ultraviolet spectra of the four starburst galaxies NGC 6090, Mrk 66, Mrk 1267, and IRAS 0833+6517 were observed with the Hopkins Ultraviolet Telescope during the Astro-2 mission. Additional data were obtained for IRAS 0833+6517 with the Goddard High-Resolution Spectrograph (GHRS) on the Hubble Space Telescope. We analyze the observations in terms of the stellar content and the kinematics of the interstellar medium, and we discuss the implications of these results for the interpretation of the ultraviolet spectra of high-redshift galaxies. Evolutionary synthesis models are used to constrain the star formation history from the absolute ultraviolet flux and from the stellar components of the absorption lines of Si IV and C IV, and from the far-ultraviolet lines O VI + Lyβ + C II. The spectral energy distributions from these models are used as inputs for the photoionization code CLOUDY to predict [O III]/Hβ and column densities of the cool component of the ionized gas (Si II and C II) associated with the H II region. The results indicate continuous star formation during about 9 Myr, or, alternatively, bursts with ages between 3 and 6 Myr, and a mass consistent with that estimated from the Hα flux. Evidence for significant dilution 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. Other evidence for large-scale motions of the interstellar gas comes from blueshifts of several hundred km s-1 with respect to the systemic velocity in the interstellar lines of NGC 6090, Mrk 66, and IRAS 0833+6517. These outflows are most likely driven by the starburst. Lyα was detected in emission in three of the galaxies (NGC 6090, Mrk 66, and IRAS 0833+6517). The dereddened Lyα/Hβ ratio in IRAS 0833+6517 is close to the recombination value, indicating that extinction is more important than multiple resonant scattering effects. However, the GHRS spectrum of IRAS 0833+6517 clearly shows that the emission profile of Lyα is asymmetric, the blue wing being absorbed by neutral gas. This indicates that the velocity structure of the neutral gas and the scattering by H I atoms can also play an important role in the escape of the Lyα photons.