  The observed ultraviolet continuum (UVC) slope is potentially a powerful
diagnostic of dust obscuration in star forming galaxies. However, the intrinsic
slope is also sensitive to the form of the stellar initial mass function (IMF)
and to the recent star formation and metal enrichment histories of a galaxy.
Using the galform semi-analytical model of galaxy formation, we investigate the
intrinsic distribution of UVC slopes. For star-forming galaxies, we find that
the intrinsic distribution of UVC slopes at z=0, parameterised by the power law
index beta, has a standard deviation of sigma_beta=0.30. This suggests an
uncertainty on the inferred UV attenuation of A_fuv=0.7$ (assuming a Calzetti
attenuation curve) for an individual object, even with perfect photometry.
Furthermore, we find that the intrinsic UVC slope correlates with star
formation rate, intrinsic UV luminosity, stellar mass and redshift. These
correlations have implications for the interpretation of trends in the observed
UVC slope with these quantities irrespective of the sample size or quality of
the photometry. Our results suggest that in some cases the attenuation by dust
has been incorrectly estimated.
