2015MNRAS.451.1606F

Photometric observations of galaxies at submillimetre to millimetre wavelengths (50-1000 GHz) are susceptible to spatial variations in both the background cosmic microwave background temperature and cosmic infrared background emission that can be comparable to the flux from the target galaxy. We quantify the residual uncertainty when background emission inside a circular aperture is estimated by the mean flux in a surrounding annular region, assumed to have no contribution from the source of interest. We present simple formulae to calculate this uncertainty as a function of wavelength and aperture size. Drawing on examples from the literature, we illustrate the use of our formalism in practice and highlight cases in which uncertainty in the background subtraction needs to be considered in the error analysis. We make the code used to calculate the uncertainties publicly available on the web.