H0LiCOW I. $H_0$ Lenses in COSMOGRAIL's Wellspring: Program Overview

TL;DR

Time-delay cosmography uses observed delays between multiple images of a variable source to infer the time-delay distance $D_{\Delta t}$, defined by $\Delta t = D_{\Delta t} \Delta \phi / c$ with $D_{\Delta t} = (1+z_d) \frac{D_d D_s}{D_{ds}}$. The H0LiCOW program targets $H_0$ precision better than $3.5\%$ by combining accurate time delays, high-resolution imaging of Einstein rings, environment characterization, and lens-galaxy velocity dispersions under blind-analysis to mitigate systematics. The paper outlines the five-lens sample, the observational follow-up plan, and cosmographic forecasts that show strong complements to CMB measurements in constraining $\Omega_k$, $w$, and $N_{\rm eff}$, while enabling studies of IMF and SMBH–host co-evolution. It positions H0LiCOW as a scalable framework for exploiting the hundreds of time-delay lenses anticipated from current and future surveys, paving the way toward sub-percent $H_0$ in the longer term.

Abstract

Strong gravitational lens systems with time delays between the multiple images allow measurements of time-delay distances, which are primarily sensitive to the Hubble constant that is key to probing dark energy, neutrino physics, and the spatial curvature of the Universe, as well as discovering new physics. We present H0LiCOW ($H_0$ Lenses in COSMOGRAIL's Wellspring), a program that aims to measure $H_0$ with $<3.5\%$ uncertainty from five lens systems (B1608+656, RXJ1131-1231, HE0435-1223, WFI2033-4723 and HE1104-1805). We have been acquiring (1) time delays through COSMOGRAIL and Very Large Array monitoring, (2) high-resolution Hubble Space Telescope imaging for the lens mass modeling, (3) wide-field imaging and spectroscopy to characterize the lens environment, and (4) moderate-resolution spectroscopy to obtain the stellar velocity dispersion of the lenses for mass modeling. In cosmological models with one-parameter extension to flat $Λ$CDM, we expect to measure $H_0$ to $<3.5\%$ in most models, spatial curvature $Ω_{\rm k}$ to 0.004, $w$ to 0.14, and the effective number of neutrino species to 0.2 (1$σ$ uncertainties) when combined with current CMB experiments. These are, respectively, a factor of $\sim15$, $\sim2$, and $\sim1.5$ tighter than CMB alone. Our data set will further enable us to study the stellar initial mass function of the lens galaxies, and the co-evolution of supermassive black holes and their host galaxies. This program will provide a foundation for extracting cosmological distances from the hundreds of time-delay lenses that are expected to be discovered in current and future surveys.

Figures (3)

• Figure 1: H0LiCOW lens sample, consisting of four quadruply lensed quasar systems in various configurations and one doubly lensed quasar system. The lens name is indicated above each panel. The color images are composed using 2 (for B1608$+$656) or 3 (for other lenses) HST imaging bands in the optical and near-infrared. North is up and east is left.
• Figure 2: HST WFC3 F160W observation of HE 0435$-$1223, WFI2033$-$4723 and HE 1104$-$1805 from left to right in the top panels. In the bottom panels, the lens-galaxy light has been subtracted, revealing the Einstein ring of the AGN host galaxy that is needed for accurate and precise lens mass modeling. The full modeling of HE 0435$-$1223 is detailed in H0LiCOW Paper IV. The lens subtraction for WFI2033$-$4723 and HE 1104$-$1805 in the bottom-middle and bottom-right panels, respectively, is based on an initial PSF model without PSF reconstruction (which we defer to future work), hence the visible residuals. In each of the panels, north is up, and east is left.
• Figure 3: Forecasted cosmographic constraints from the H0LiCOW lens sample through measurements of $D_{\Delta t}$ and $D_{\rm d}$. Columns from left to right are, respectively, the constraints from the H0LiCOW lenses alone (with uniform prior on cosmological parameters), lenses in combination with WMAP 9-year results, lenses in combination with Planck 2015 results, and marginalized constraints on $H_0$ from the previous three columns. The H0LiCOW lenses primarily constrain $H_0$, which in turn break CMB parameter degenercies to elucidate the spatial curvature of universe ($\Omega_{\rm k}$, top row), dark energy equation of state ($w$, middle row) and effective number of relativistic species ($N_{\rm eff}$, bottom row). H0LiCOW lenses provide an independent, complementary and competitive probe of cosmology.