Continuum removal in H\alpha\ extragalactic measurements
  We point out an important source of error in measurements of extragalactic
H-alpha emission and suggest ways to reduce it.
  The H-alpha line, used for estimating star formation rates, is commonly
measured by imaging in a narrow band and a wide band, both which include the
line. The image analysis relies on the accurate removal of the underlying
continuum. We discuss in detail the derivation of the emission line's
equivalent width and flux for extragalactic extended sources, and the required
photometric calibrations. We describe commonly used continuum-subtraction
procedures, and discuss the uncertainties that they introduce.
  Specifically, we analyse errors introduced by colour effects. We show that
the errors in the measured H-alpha equivalent width induced by colour effects
can lead to underestimates as large as 40% and overestimates as large as 10%,
depending on the underlying galaxy's stellar population and the
continuum-subtraction procedure used. We also show that these errors may lead
to biases in results of surveys, and to the underestimation of the cosmic star
formation rate at low redshifts (the low z points in the Madau plot). We
suggest a method to significantly reduce these errors using a single colour
measurement.
