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Cosmic shear is a powerful probe of cosmological parameters, but itspotential can be fully utilised only if galaxy shapes are measured with greataccuracy. Two major effects have been identified which are likely to accountfor most of the bias for maximum likelihood methods in recent shear measurementchallenges. Model bias occurs when the true galaxy shape is not wellrepresented by the fitted model. Noise bias occurs due to the non-linearrelationship between image pixels and galaxy shape. In this paper weinvestigate the potential interplay between these two effects when an imperfectmodel is used in the presence of high noise. We present analytical expressionsfor this bias, which depends on the residual difference between the model andreal data. They can lead to biases not accounted for in previous calibrationschemes. By measuring the model bias, noise bias and their interaction, weprovide a complete statistical framework for measuring galaxy shapes with modelfitting methods from GRavitational lEnsing Accuracy Testing (GREAT) likeimages. We demonstrate the noise and model interaction bias using a simple toymodel, which indicates that this effect can potentially be significant. Usingreal galaxy images from the Cosmological Evolution Survey (COSMOS) we quantifythe strength of the model bias, noise bias and their interaction. We find thatthe interaction term is often a similar size to the model bias term, and issmaller than the requirements of the current and shortly upcoming galaxysurveys.

S\'{e}rsic galaxy models in weak lensing shape measurement: model bias, noise bias and their interaction