Simulations of single- and two-component galaxy decompositions for spectroscopically selected galaxies from the Sloan Digital Sky Survey

We present the results of fitting simulations of an unbiased sample of Sloan Digital Sky Survey (SDSS) galaxies utilizing the fitting routine galfit and analysis pipeline pymorph. These simulations are used to test the two-dimensional decompositions of SDSS galaxies. The simulations show that single Sersic models of SDSS data are recovered with Sigma(mag) approximate to 0.025 mag and Sigma(radius) approximate to 5 per cent. The global values (half-light radius and magnitude) are equally well constrained when a two-component model is used. Sub-components of two-component models present more scatter. SDSS resolution is the primary source of error in the recovery of models. We use a simple statistical correction of the biases in fitted parameters, providing an example using the Sersic index. Fitting a two-component Sersic + Exponential model to a single Sersic galaxy results in a noisier, but unbiased, recovery of the input parameters (Sigma(totalmag) approximate to 0.075 mag and Sigma(radius) approximate to 10 per cent); fitting a single Sersic profile to a two-component system results in biases of total magnitude and half-light radius of approximate to 0.05-0.10 mag and 5-10 per cent in radius. Using an F-test to select the best-fitting model from among the single- and two-component models is sufficient to remove this bias. We recommend fitting a two-component model to all galaxies when attempting to measure global parameters such as total magnitude and half-light radius.