Exploring a Potential Bias in Dark Matter Investigations Using Strongly Lensed Quasars

Simulations based on ΛCDM cosmology predict thousands of substructures under galactic scale have not been detected in the local universe. One hypothesis proposes that most of these substructures are dark for various astrophysical reasons. Gravitational lensing provides a powerful alternative way to probe dark substructures in distant galaxies by detecting their gravitational perturbations and therefore provides insights into the nature of dark matter. Lensed quasars with certain image configurations are especially promising for probing substructure abundance in lens galaxy halos. When the observed flux ratios of the lensed quasar images deviate from the smooth mass model predictions, these “flux-ratio anomalies” are considered to be the evidence of gravitational perturbations. While the standard analysis of flux-ratio anomalies assumes that substructures are the only cause of anomalies, we found that in two edge-on disk lenses, B1555+375 and B0712+472, their flux anomalies can be explained by including disk components into their mass models. Our results bring up a concern with a potential bias in the previous analyses of flux-ratio anomalies. To further investigate the baryonic effects in flux-ratio anomalies, we create mock quasar lenses by selecting disk and elliptical galaxies in the Illustris simulation. Our analysis shows that baryon-induced flux anomalies can be found in all morphological types of lens galaxies. The baryonic effects increase the probability of finding lenses with strong anomalies by 8% in ellipticals and 10~20% in disk lenses, showing that the baryonic effects are unneglectable in the analysis. As future large-scale surveys are expected to bring numerous lensed quasar samples, further investigations on baryonic effects should be done in order to achieve precise constraints on dark matter in the future