Integral field spectroscopy of the quadruply lensed quasar 1RXS J1131-1231: New light on lens substructures

We have observed the quadruply lensed quasar 1RXS J1131-1231 with the integral field spectrograph mode of the Kyoto Tridimensional Spectrograph II mounted on the Subaru telescope. Its field of view has simultaneously covered the three brighter lensed images A, B, and C, which are known to exhibit anomalous flux ratios in their continuum emission. We have found that the [O III] line flux ratios among these lensed images are consistent with those predicted by smooth-lens models. The absence of both microlensing and millilensing effects on this [0 III] narrow-line region sets important limits on the mass of any substructures along the line of sight, which is expressed as ME < 105 M_⊙ for the mass inside an Einstein radius. In contrast, the Hβ line emission, which originates from the broad-line region, shows an anomaly in the flux ratio between images B and C, i.e., a factor 2 smaller C/B ratio than predicted by smooth-lens models. The ratio of A/B in the Hβ line is well reproduced. We show that the anomalous C/B ratio for the Hβ line is most likely caused by micro-/millilensing of image C. This is because other effects, such as the differential dust extinction and/or arrival time difference between images B and C, or the simultaneous lensing of another pair of images A and B, are all unlikely. In addition, we have found that the broad Hβ line of image A shows a slight asymmetry in its profile compared with those in the other images, which suggests the presence of a small microlensing effect on this line emitting region of image A.