Galaxy Structures and External Perturbations in Gravitational Lenses

In modeling strong gravitational lens systems, one often adopts simplemodels, such as singular isothermal elliptical density plus lowest-orderexternal perturbation. However, such simple models may mislead us if the realmass distribution is more complicated than that in the assumed models. Inparticular, assumptions on mass models are crucial in studying flux ratioanomalies that have been suggested as evidence for a cold dark-mattersubstructure. We reinvestigated four quadruple lens systems using power-lawFourier models, which have advantages of clear physical meanings and theapplicability of a linear method, as well as a simple singular isothermalelliptical density model. We also investigated the effect of externalperturbations, including a singular isothermal sphere, lowest-order expansion,and next-order expansion. We have found that the $\cos3\theta$ terms of theprimary galaxy and/or of external perturbation significantly reduce the \chi^2in PG 1115+080 and B1422+231. In particular, we could reproduce the flux ratiosof B1422+231 with next-order external perturbation assuming 5% fluxuncertainties, suggesting that external perturbation cannot be described by asimple singular isothermal sphere approximation. On the other hand, we couldnot fit B0712+472 and B2045+265 very well even with our models, although the\chi^2 were reduced compared with the case of using simple models. Our resultsclearly demonstrate that both the primary lens galaxy and the externalperturbation are often more complicated than we usually assume.Comment: 9 pages, 1 figure, to appear in PASJ, Vol. 56, No.