The accuracy of the UV continuum as an indicator of the star formation
  rate in galaxies
  The rest-frame intrinsic UV luminosity is often used as an indicator of the
instantaneous star formation rate (SFR) in a galaxy. While it is in general a
robust indicator of the ongoing star formation activity, the precise value of
the calibration relating the UV luminosity to the SFR ($B_{\nu}$), is sensitive
to various physical properties, such as the recent star formation and metal
enrichment histories, along with the choice of stellar initial mass function.
The distribution of these properties for the star-forming galaxy population
then suggests that the adoption of a single calibration is not appropriate
unless properly qualified with the uncertainties on the calibration. We
investigate, with the aid of the {\sc galform} semi-analytic model of galaxy
formation, the distribution of UV-SFR calibrations obtained using realistic
star formation and metal enrichment histories. At $z=0$, we find that when the
initial mass function is fixed (to the Kennicutt IMF), the median calibration
is $B_{\rm fuv}=0.9$ where ${\rm SFR}/[{\rm M_{\odot}\,yr^{-1}}]=B_{\nu}\times
10^{-28}\times L_{\nu}/[{\rm ergs\,s^{-1}\,Hz^{-1}}]$. However, the width of
the distribution $B_{\rm fuv}$ suggests that for a single object there is
around a 20% {\em intrinsic} uncertainty (at $z=0$, rising to $\simeq 30%$ at
$z=6$) on the star formation rate inferred from the FUV luminosity without
additional constraints on the star formation history or metallicity. We also
find that the median value of the calibration $B_{\rm fuv}$ is correlated with
the star formation rate and redshift (at $z>3$) raising implications for the
correct determination of the star formation rate from the UV.
