Reddening and Star Formation in Starburst Galaxies

Abstract

The reddening properties and the star formation history of a sample of 19 starburst galaxies are investigated using multiwavelength spectroscopy and infrared broad band photometry. New photometric data in the J, H, and K bands of the central starburst regions are supplemented with previously published spectra, covering the wavelength range 0.12-2.2 mu m. In the infrared, the reddening value derived for the stellar continuum is in agreement with that of the ionized gas, but the two values diverge at shorter wavelengths; in the UV, the mean optical depth of the dust in front of the stars is smaller, being only 60%, than the optical depth of the dust in front of the nebular gas. This difference can be better understood if the UV-bright stellar population and the ionized gas are not co-spatial. A model of foreground clumpy dust, with different covering factors for the gas and the stars, is proposed to account for the difference in reddening. A ``template starburst spectrum'', derived by combining the reddening-corrected UV, optical, and infrared data of all the galaxies in the sample, is used to investigate the star formation history of the galaxies. Spectral synthesis models indicate that the observed UV emission can be attributed to a stellar population which is undergoing active star formation at a constant rate since ~ 2x10(7) yr, in agreement with the supernova rates derived from the [FeII] emission line in the infrared. At least two, and probably more, intermediate age populations (age<2x10(9) yr) contribute to the optical and infrared emission, while populations older than ~ 2x10(9) yr do not contribute significantly to the template. The stellar composition of the template spectrum suggests episodic star formation over the last 10(9) yr, with star formation rates as large as or larger than the present rates. The synthetic stellar populations are generated according to an Initial Mass Function (IMF) with Salpeter slope (alpha =2.35) in the mass range 0.1-100 Msun, and reproduce a number of observational constraints, such as the spectral energy distribution of the template spectrum, the equivalent width of the atomic hydrogen emission lines, and the mass-to-light ratios; the data, then, do not provide indication for a high-mass-star truncated or a low-mass-star deficient IMF in starburst galaxies.