Spectroscopic Gravitational Lensing and Limits on the Dark Matter Substructure

Spatially resolved spectroscopic data from the CIRPASS integral field unit (IFU) on Gemini are used to measure the gravitational lensing of the four-image quasar Q2237+0305 on different size scales. A method for measuring the substructure present in the lens using observations at multiple wavelengths is demonstrated to be very effective and independent of many of the degeneracies inherent in previous methods. The magnification ratios of the QSO's narrow-line region (NLR) and broad-line region (BLR) are measured and found to disagree with each other and with the published radio and mid-infrared magnification ratios. The disagreement between the BLR ratios and the radio/mid-infrared ratios is interpreted as the result of microlensing by stars in the lens galaxy of the BLR. This implies that the mid-infrared emission region is larger than the BLR and that the BLR is 0.1 pc in size. We find a small difference between the shape of the Hβ line in image A and that in the other images. We consider this difference too small and symmetric to be strong evidence for rotation or large-scale infall in the Hβ emission region. The disagreement between the radio/mid-infrared ratios and the NLR ratios is interpreted as a signature of substructure on a larger scale, possibly the missing small-scale structure predicted by the standard cold dark matter (CDM) model. Extensive lensing simulations are performed to obtain a lower limit on the amount of substructure that is required to cause this discrepancy as a function of its mass and the radial profile of the host lens. The substructure surface density is degenerate with the radial profile of the host lens, but if the expectations of the CDM model are taken into account, certain radial profiles and substructure surface densities can be ruled out. A substructure mass scale as large as 108 M☉ is strongly disfavored, while 104 M☉ is too small if the radio and mid-infrared emission regions have the expected sizes of ~10 pc. The standard elliptical isothermal lens mass profile is not compatible with a substructure surface density of Σsub < 280 M☉ pc-2, at the 95% confidence level. This is 4%-7% of the galaxy's surface density (depending on which image position is used to evaluate this). The required substructure surface density at the required mass scale is high in comparison with the present expectations within the CDM model. Lens mass profiles that are flatter than isothermal—where the surface density in dark matter is higher at the image positions—are compatible with smaller quantities of substructure.