Two accurate time-delay distances from strong lensing: Implications for cosmology

TL;DR

This work demonstrates that time-delay distances from strong gravitational lenses enable precise, independent cosmological inferences. By conducting a blind analysis of RXJ1131-1231 using COSMOGRAIL delays, deep HST imaging, a new Keck velocity-dispersion measurement, and line-of-sight analyses with Millennium simulations, the authors infer the time-delay distance $D_{\Delta t}$ with about 6% precision, accounting for all known systematics. The results are consistent with those from another lens, B1608+656, and when combined with WMAP7 data, yield competitive constraints on $H_0$, $\Omega_{de}$, and $w$ (and curvature) across multiple cosmologies. The study highlights the complementarity of time-delay lenses to other cosmological probes and their value as an independent cross-check of the standard cosmological model and general relativity. It also outlines a path toward tighter constraints with additional lenses from current and upcoming surveys.

Abstract

Strong gravitational lenses with measured time delays between the multiple images and models of the lens mass distribution allow a one-step determination of the time-delay distance, and thus a measure of cosmological parameters. We present a blind analysis of the gravitational lens RXJ1131-1231 incorporating (1) the newly measured time delays from COSMOGRAIL, (2) archival HST imaging of the lens system, (3) a new velocity-dispersion measurement of the lens galaxy of 323+/-20km/s based on Keck spectroscopy, and (4) a characterization of the line-of-sight structures via observations of the lens' environment and ray tracing through the Millennium Simulation. Our blind analysis is designed to prevent experimenter bias. The joint analysis of the data sets allows a time-delay distance measurement to 6% precision that takes into account all known systematic uncertainties. In combination with the WMAP7 data set in flat wCDM cosmology, our unblinded cosmological constraints for RXJ1131-1231 are: H_0=80.0+5.8/-5.7km/s/Mpc, OmegaDE=0.79+/-0.03 and w=-1.25+0.17/-0.21. We find the results to be statistically consistent with those from the analysis of the gravitational lens B1608+656. The joint constraints from the two lenses and WMAP7 are H_0=75.2+4.4/-4.2km/s/Mpc, OmegaDE=0.76+0.02/-0.03 and w=-1.14+0.17/-0.20 in flat wCDM, and H_0=73.1+2.4/-3.6km/s/Mpc, OmegaL=0.75+0.01/-0.02 and OmegaK=0.003+0.005/-0.006 in open LCDM. Time-delay lenses constrain especially tightly the Hubble constant (5.7% and 4.0% respectively in wCDM and open LCDM) and curvature of the universe. They complement well other cosmological probes, and provide an independent check of unknown systematics. Our measurement of the Hubble constant is completely independent of those based on the local distance ladder method, providing an important consistency check of the standard cosmological model and of general relativity.

Figures (12)

• Figure 1: HST ACS F814W image of the gravitational lens RXJ1131$-$1231. The lensed AGN images of the spiral source galaxy are marked by A, B, C and D, and the star forming regions of the spiral galaxy form the spectacular lensed structures. The primary lens galaxy and the satellite lens galaxy are indicated by G and S, respectively.
• Figure 2: Top panel: The LRIS spectrum of RXJ1131$-$1231 (black line) with a model generated from 9 INDO-US templates and a 5$^{\rm th}$ order continuum overplotted (red line, with green showing the continuum). The gray shaded areas were not included in the fit. Bottom panel: the residuals of the model fit. From the spectrum and model, we measure a central velocity dispersion of $\sigma = 323 \pm 20\, {\rm km\, s^{-1}}$, including systematic uncertainties.
• Figure 3: Posterior of the key lens model parameters for the lensing and time-delay data. Each color represents a particular source resolution that is the dominant systematic uncertainty in the modeling of the ACS data. The solid curves are a Gaussian fit to the PDF by weighting each source resolution chain equally. The contours/shades mark the 68.3%, 95.4%, 99.7% credible regions.
• Figure 4: ACS image reconstruction of the most probable model with a source grid of 64$\times$64 pixels. Top left: observed ACS F814W image. Top middle: predicted lensed image of the background AGN host galaxy. Top right: predicted light of the lensed AGNs and the lens galaxies. Bottom left: predicted image from all components, which is a sum of the top-middle and top-right panels. Bottom middle: image residual, normalized by the estimated 1$\sigma$ uncertainty of each pixel. Bottom right: the reconstructed host galaxy of the AGN in the source plane. Our lens model reproduces the global features of the data.
• Figure 5: $11.5'$$\times$$10.5'$ R-band image obtained from stacking 60 hours of the best-quality images in the COSMOGRAIL monitoring. The lens system is marked by the box near the center. Galaxies (stars) in the field are indicated by solid (dashed) circles. The radius of the solid circle is proportional to the flux of the galaxy. X-ray map from ChartasEtal09 are overlaid on the image within the dashed box. The concentrations of mass structures to the east of the lens are consistent with the modeled external shear and convergence gradient directions.