telescope image, to determine the posterior probability distribution of model parameters given the observed data. The software uses the ensemble sampler [@emcee2013], which improves sampling efficiency compared to the [@patil_2010] version that was used in @Mechtley2014.

For each image of each source, we attempted two different models—one with both a point source and an underlying profile, and one with only a point source. We compared the results of the two models both visually and using the Bayesian Information Criterion as a model selection heuristic. *In all cases*, there was no evidence that the data required the additional profile—the seven additional free parameters were primarily fitting noise peaks rather than residual flux from the hosts.

For all further analysis, we model the quasar as a pure point source. We also model any surrounding galaxies within $\approx3''$ of the quasar with a profile[^3]. It should be stressed, however, that if the galaxies are associated with the quasar and undergoing a merger, their rest-frame UV emission need not be distributed in anything like a Sérsic profile. Rather, this approach is simply used to model their flux to avoid over-subtraction. We assume uniform priors over a reasonable range, for all of the model parameters. 