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The elliptical power-law (EPL) model of the mass in a galaxy is widely usedin strong gravitational lensing analyses. However, the distribution of mass inreal galaxies is more complex. We quantify the biases due to this modelmismatch by simulating and then analysing mock {\it Hubble Space Telescope}imaging of lenses with mass distributions inferred from SDSS-MaNGA stellardynamics data. We find accurate recovery of source galaxy morphology, exceptfor a slight tendency to infer sources to be more compact than their true size.The Einstein radius of the lens is also robustly recovered with 0.1% accuracy,as is the global density slope, with 2.5% relative systematic error, comparedto the 3.4% intrinsic dispersion. However, asymmetry in real lenses also leadsto a spurious fitted `external shear' with typical strength, $\gamma_{\rmext}=0.015$. Furthermore, time delays inferred from lens modelling withoutmeasurements of stellar dynamics are typically underestimated by $\sim$5%.Using such measurements from a sub-sample of 37 lenses would bias measurementsof the Hubble constant $H_0$ by $\sim$9%. Although this work is based on aparticular set of MaNGA galaxies, and the specific value of the detected biasesmay change for another set of strong lenses, our results strongly suggest thenext generation cosmography needs to use more complex lens mass models.

Systematic errors induced by the elliptical power-law model in galaxy-galaxy strong lens modeling