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\begin{document}

\title{REDDENING AND STAR FORMATION IN STARBURST GALAXIES}

\author{Daniela Calzetti\/\thanks{Visiting Astronomer, Kitt Peak National
Observatory, operated by the Association of Universities for Research in
Astronomy, Inc. under contract with the National Science Foundation.}\\
\\
Space Telescope Science Institute\\
\\
3700 San Martin Drive\\
\\
Baltimore, MD 21218}

\tobe{January 1997}{The Astronomical Journal}
\recacc{22 July 1996}{14 October 1996}

\maketitle

\begin{abstract}
\medskip

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
$\sim2\times10^7$~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 $<2\times10^9$~yr) contribute to the optical and infrared
emission, while populations older than $\sim2\times10^9$~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~M$_{\odot}$, 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.
\end{abstract}