Blue Interacting Disk Galaxies: Massive Stellar Contents from IUE and Optical Observations
Session 75 -- Interacting Galaxies and Starbursts
Display presentation, Wednesday, 11, 1995, 9:20am - 6:30pm

## [75.03] Blue Interacting Disk Galaxies: Massive Stellar Contents from IUE and Optical Observations

A. Baleisis (UIUC), S. A. Lamb (UIUC), H. A. Bushouse (STSCI)

We are interested in the effects of collisions involving gaseous disk galaxies on the global production of massive stars during the ensuing dynamical readjustments of the system. The interaction timescale for systems of galaxies is of the order of $10^8$ years and we seek information on this and smaller timescales. Analysis of optical and UV spectra allows an investigation of the massive star formation over the relevant dynamical time scale. To aid in this investigation we have obtained UV and optical spectra of seven interacting disk galaxies. This enables us to put observational constraints on both the current massive star populations and those of the preceding $10^8$ years. The galaxies were chosen from the Bushouse (1986 AJ, 91, 255) optically selected sample of closely interacting disk galaxies to have optical indications of current star formation, such as strong optical HII emission-line features and blue continuum colors. The short wavelength IUE ($\lambda$$\lambda$ 1000 - 2000 $\AA$) UV spectra of these objects is typically dominated by velocity broadened C IV $\lambda$ 1550 $\AA$ and Si IV $\lambda$ 1400 $\AA$ doublet lines originating in the outflowing atmospheres of young, massive stars. The shape and strength of the composite C IV and Si IV line profiles can, when compared with synthesized UV spectra give information about the massive star formation history over the previous $10^6$ - $10^7$ years, provide indications of the upper mass cut-off of the initial mass function (IMF), constrain the slope of the IMF for the most massive stars, and distinguish between continuous and episodic star formation. The IUE results can be placed in the larger context of star formation on a time scale of $10^8$ years by comparison with the results obtained from interpretation of relevant optical spectra. Our optical observations were obtained at the Kitt Peak 36" and the Steward observatory 90" telescopes, using Reticon spectrographs, at various times from 1984 to 1990. Here the star formation history is also inferred from population synthesis studies which involve fitting library spectra (based on synthetic narrow-band continuum colors) to the composite galaxy spectra.