MNRAS 000, 1–15 (2016) Preprint 6 January 2017 Compiled using MNRAS LATEX style file v3.0

H0LiCOW I. H0 Lenses in COSMOGRAIL’s Wellspring:

Program Overview

S. H. Suyu,1,2,3⋆ V. Bonvin,4 F. Courbin,4 C. D. Fassnacht,5 C. E. Rusu,5 D. Sluse,6

T. Treu,7 K. C. Wong,8,2 M. W. Auger,9 X. Ding,7,10 S. Hilbert,11,12 P. J. Marshall,13

N. Rumbaugh,5 A. Sonnenfeld,14,7,15 M. Tewes,16 O. Tihhonova,4 A. Agnello,17

R. D. Blandford,13 G. C.-F. Chen,5,2 T. Collett,18 L. V. E. Koopmans,19 K. Liao,7

G. Meylan,4 C. Spiniello11Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany2Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box 23-141, Taipei 10617, Taiwan3Physik-Department, Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany4Laboratoire d’Astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, CH-1290 Versoix,Switzerland5Department of Physics, University of California, Davis, CA 95616, USA6STAR Institute, Quartier Agora - Allée du six Août, 19c B-4000 Liège, Belgium7Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA8National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan9Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK10Department of Astronomy, Beijing Normal University, Beijing 100875, China11Exzellenzcluster Universe, Boltzmannstr. 2, 85748 Garching, Germany12Ludwig-Maximilians-Universität, Universitäts-Sternwarte, Scheinerstr. 1, 81679 München, Germany13Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94035, USA14Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan15Physics Department, University of California, Santa Barbara, CA, 93106, USA16Argelander-Institut für Astronomie, Auf dem Hügel 71, D-53121 Bonn, Germany17ESO-European Southern Observatory, D-85748 Garching bei München, Germany18Institute of Cosmology and Gravitation, University of Portsmouth, Burnaby Rd, Portsmouth PO1 3FX, UK19Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700-AV Groningen, The Netherlands

Accepted XXX. Received YYY; in original form ZZZ

ABSTRACT

Strong gravitational lens systems with time delays between the multiple images allowmeasurements of time-delay distances, which are primarily sensitive to the Hubbleconstant that is key to probing dark energy, neutrino physics, and the spatial curva-ture of the Universe, as well as discovering new physics. We present H0LiCOW (H0Lenses in COSMOGRAIL’s Wellspring), a program that aims to measure H0 with <3.5% uncertainty from five lens systems (B1608+656, RXJ1131−1231, HE 0435−1223,WFI2033−4723 and HE 1104−1805). We have been acquiring (1) time delays throughCOSMOGRAIL and Very Large Array monitoring, (2) high-resolution Hubble SpaceTelescope imaging for the lens mass modeling, (3) wide-field imaging and spectroscopyto characterize the lens environment, and (4) moderate-resolution spectroscopy to ob-tain the stellar velocity dispersion of the lenses for mass modeling. In cosmologicalmodels with one-parameter extension to flat ΛCDM, we expect to measure H0 to< 3.5% in most models, spatial curvature Ωk to 0.004, w to 0.14, and the effectivenumber of neutrino species to 0.2 (1σ uncertainties) when combined with current CMBexperiments. These are, respectively, a factor of ∼ 15, ∼ 2, and ∼ 1.5 tighter thanCMB alone. Our data set will further enable us to study the stellar initial mass func-tion of the lens galaxies, and the co-evolution of supermassive black holes and theirhost galaxies. This program will provide a foundation for extracting cosmological dis-tances from the hundreds of time-delay lenses that are expected to be discovered incurrent and future surveys.

Key words: gravitational lensing: strong – cosmological parameters – distance scale– quasars: individual: B1608+656, RXJ1131−1231, HE0435−1223, WFI2033−4723,HE1104−1805 – galaxies: structure

c© 2016 The Authors

2 S. H. Suyu et al.

1 INTRODUCTION

In the past decade, the so-called “flat ΛCDM” cosmologi-cal model consisting of dark energy (with density charac-terized by a cosmological constant Λ) and cold dark matter(CDM) in a spatially flat Universe has emerged as the stan-dard cosmological model. This simple model has providedexcellent fit to various cosmological observations includingthe temperature anisotropies in the cosmic microwave back-ground (CMB) and galaxy density correlations in baryonacoustic oscillations (BAO). Recent CMB experiments,particularly the Wilkinson Microwave Anisotropy Probe(WMAP; Komatsu et al. 2011; Hinshaw et al. 2013) and thePlanck satellite (Planck Collaboration et al. 2013, 2015),and BAO surveys (e.g., Anderson et al. 2014; Ross et al.2015; Kazin et al. 2014), have yielded stringent constraintswith unprecedented precision on cosmological parameters inthe spatially-flat ΛCDM model.

An interesting result from Planck is its predicted valueof the Hubble constant (H0), a key cosmological param-eter that sets the present-day expansion rate as well asthe age, size, and critical density of the Universe. Planckdoes not directly measure H0, but rather enables its in-direct inference through measurements of combinations ofcosmological parameters given assumptions of the back-ground cosmological model. Intriguingly, Planck’s value ofH0 = 67.8 ± 0.9 km s

−1 Mpc−1 (Planck Collaboration et al.2015), from Planck temperature data and Planck lensingunder the flat ΛCDM model, is lower than recent directmeasurements based on the distance ladder, of 73.24 ±1.74 km s−1 Mpc−1 from the SH0ES program (Riess et al.2016) and of 74.3±2.1 kms−1 Mpc−1 (Freedman et al. 2012)from the Carnegie-Chicago Hubble Program (Beaton et al.2016). On the other hand, Planck’s H0 value is similarto the results of some of the megamaser measurements(e.g., H0 = 68.9 ± 7.1 kms

−1 Mpc−1 from Reid et al. 2013,H0 = 73

+26−22 kms

−1 Mpc−1 from Kuo et al. 2015, and H0 =66.0±6.0 km s−1 Mpc−1 from Gao et al. 2016), although theuncertainties of these maser H0 measurements are still sub-stantial relative to that of Planck. A 1% direct measurementof the Hubble constant is highly needed: such 1% measure-ments of H0 would address the possible tension with theCMB value which, if significant, would point towards de-viations from the standard flat ΛCDM and new physics.In fact, when one relaxes, for example, the flatness or Λassumption in the CMB analysis, strong parameter degen-eracies between H0 and other cosmological parameters ap-pear, and the degenerate H0 values from the CMB becomecompatible with the local H0 measurements from the dis-tance ladder (Planck Collaboration et al. 2015; Riess et al.2016; Freedman et al. 2012). Thus, a 1% measurement ofH0 is crucial for understanding the nature of dark energy,neutrino physics, the spatial curvature of the Universe andthe validity of General Relativity (e.g., Hu 2005; Suyu et al.2012a; Weinberg et al. 2013). In particular, the dark energyfigure of merit of any survey that does not directly mea-sure H0 improves by ∼ 40% if H0 is known to 1%. Further-more, independent methods to measure H0 are necessary toovercome systematic effects, such as the known unknowns(e.g., the effects of crowding or metallicity dependence inthe cosmic distance ladder) and the unknown unknowns in

order to robustly verify or rule out the standard cosmologicalparadigm.

Strong gravitational lenses with measured time delaysbetween the multiple images provide a competitive approachto measuring the Hubble constant, completely independentof the local distance ladder: we have demonstrated thatwe can constrain H0 to ∼ 7 − 8% precision from a sin-gle time-delay lens system with ancillary data (Suyu et al.2010, 2014). The time-delay method was first proposed byRefsdal (1964) even before the discovery of the first stronggravitational lens system (Walsh et al. 1979), consisting of aforeground mass distribution that is located close along theline of sight to a background source (see Treu & Marshall2016, for a recent review). The light from the backgroundsource is deflected by the foreground “lens” mass distribu-tion; such light bending produces distorted and, in rare casesof “strong lensing”, multiple and often spectacular images ofthe background source (e.g., Figure 1).

When the background source is one that variesin its luminosity, such as an active galactic nucleus(AGN; e.g., Vanderriest et al. 1989; Schechter et al.1997; Fassnacht et al. 1999, 2002; Kochanek et al.2006; Courbin et al. 2011) or a supernova (SN; e.g.,Quimby et al. 2014; Kelly et al. 2015, 2016; Grillo et al.2016; Kawamata et al. 2016; Treu et al. 2016; Goobar et al.2016; More et al. 2016a), the variability is manifest in eachof the multiple images, but delayed in time relative to eachother due to the different light paths. This time delay (∆t)thus depends on the “time-delay distance” (D∆t) and thelens mass distribution. Specifically, ∆t = D∆t∆φ/c, where∆φ is the Fermat potential difference that is determined bythe lens mass distribution and c is the speed of light. There-fore, by measuring the time delay from photometric lightcurves of the quasar images and modeling the lens massdistribution, one can determine the time-delay distance tothe lens system and use the distance-redshift relation toconstrain cosmological models.

More precisely, the time-delay distance is

D∆t ≡ (1 + zd)DdDsDds

(1)

(Refsdal 1964; Suyu et al. 2010), where zd is the redshiftof the foreground deflector (also referred to as the stronglens), Dd is the angular diameter distance to the deflector,Ds is the angular diameter distance to the source, and Ddsis the angular diameter distance between the deflector andthe source. This time-delay distance is for a single stronglens plane, with other line-of-sight mass distributions onlyweakly perturbing the strong lens system and characterizedvia external shear and convergence. For cases where there aremassive line-of-sight mass distributions at a different redshiftfrom the strong lens galaxy yet close in projection to it suchthat these massive structures cannot be well approximatedby an external shear/convergence, it is necessary to use themulti-plane lensing formalism (e.g., Blandford & Narayan1986; Schneider et al. 1992). In general, multi-lens plane raytracing does not yield a single time-delay distance but ratherseveral combinations of distances. Nonetheless, even in someof these cases, we can derive an effective time-delay distance.

As a result of the unique combination of these threeangular diameter distances, the time-delay distance D∆tis primarily sensitive to the Hubble constant, in con-

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H0LiCOW program overview 3

trast to other non-local distance probes such as super-nova (SN) that probe relative luminosity distances (e.g.,Riess et al. 1998; Perlmutter et al. 1999; Conley et al. 2011;Suzuki et al. 2012; Betoule et al. 2014) and BAO (e.g.,Eisenstein et al. 2005; Percival et al. 2010; Blake et al. 2011;Anderson et al. 2014) that yield absolute angular diame-ter distances. We note though that BAO, together withthe CMB, can be used to calibrate the absolute magni-tude of SN; assuming that the absolute magnitude of SNdoes not evolve with redshift, this combination of BAOand SN provides an “inverse-distance ladder” for the Hub-ble constant that is insensitive to assumptions on dark en-ergy properties and spatial curvature (e.g., Heavens et al.2014; Aubourg et al. 2015). While BAO and the time-delaymethod both provide angular diameter distance measure-ments, the distinction is that BAO gives angular diameterdistances at specific redshifts whereas the time-delay methodyields time-delay distances (D∆t) which are each a combi-nation of three angular diameter distances. One could infact determine the angular diameter distance to the lensDd in addition to D∆t for time-delay lenses that have stel-lar velocity dispersion measurements of the foreground lensgalaxy (Paraficz & Hjorth 2009; Jee et al. 2015). Withouttime delays, lenses with stellar velocity dispersion measure-ments can still offer a way to determine the cosomolgi-cal matter and dark-energy density parameters via a ra-tio of angular diameter distances (e.g., Futamase & Hamana1999; Futamase & Yoshida 2001; Grillo et al. 2008). Re-cently Jee et al. (2016) have shown that measurements ofD∆t and Dd from a modest sample of time-delay lenses withlens velocity dispersion measurements yield competitive con-straints on cosmological models. In practice, both distancesappear as intermediate quantities between the sought aftercosmological parameters and the observed quantities.

In order to measure distances precisely and accuratelyfrom time-delay lenses, we need four key ingredients in addi-tion to the spectroscopic redshifts of the lens and the source:(1) time delays, (2) high-resolution and high signal-to-noiseratio images of the lens systems, (3) characterization of thelens environment, and (4) stellar velocity dispersion of thelens galaxy. These can be obtained via imaging and spec-troscopy from Hubble Space Telescope (HST ) and ground-based observatories. In Section 2, we detail each of theserequirements.

We initiated the H0LiCOW (H0 Lenses in COSMO-GRAIL’s Wellspring) program with the aim of measuringthe Hubble constant with better than 3.5% precision and ac-curacy (in most background cosmological models), througha sample of five time-delay lenses. We obtain the key ingre-dients to each of the lenses through observational follow-upsand novel analysis techniques. In particular, we have high-quality lensed quasar light curves, primarily obtained viaoptical monitoring by the COSMOGRAIL (COSmologicalMOnitoring of GRAvItational Lenses; e.g., Courbin et al.2005; Vuissoz et al. 2008; Courbin et al. 2011; Tewes et al.2013b) and Kochanek et al. (2006) teams but also via radio-wavelength monitoring (Fassnacht et al. 2002). COSMO-GRAIL has been monitoring more than 20 lensed quasarsfor more than a decade. The unprecendented quality ofthe light curves combined with new curve shifting algo-rithms (Tewes et al. 2013a) lead to time delays with typ-ically ∼3% accuracy (Fassnacht et al. 2002; Courbin et al.

2011; Tewes et al. 2013b). In addition, we obtain HST imag-ing that reveal the“Einstein ring”of the lens systems in highresolution, and develop state-of-the-art lens modeling tech-niques (Suyu et al. 2009; Suyu & Halkola 2010; Suyu et al.2012b, Suyu et al. in preparation) and kinematic model-ing methods (Auger et al. 2010; Sonnenfeld et al. 2012) toobtain the lens mass distribution with a few percent un-certainty (e.g., Suyu et al. 2013, 2014). We further obtainwide-field imaging and spectroscopy to characterize the en-vironment of the field, as well as the spectroscopy of the lensgalaxy to obtain the stellar velocity dispersion. The exquisitefollow-up data set that we have acquired allow us not only toconstrain cosmology but also to study lens galaxy and sourceproperties for understanding galaxy evolution, including thedark matter distribution in galaxies, the stellar initial massfunction of galaxies and the co-evolution between supermas-sive black holes and their host galaxies.

A crucial aspect of our program is the use of blind anal-ysis (e.g., Conley et al. 2006; Suzuki et al. 2012; Suyu et al.2013; von der Linden et al. 2014) to test for residual sys-tematics and avoid subconscious experimenter bias. In par-ticular, we have developed core analysis techniques for thefirst lens whose dissection was not blinded (B1608+656;Suyu et al. 2010); we subsequently build upon these tech-niques and perform blind analysis on the other lenses in thesample. In the blind analysis, the idea is not to blind all themodel parameters being inferred, but rather just the cos-mological parameters that we aim to measure (as well asany derived parameters or summary statistics from whichwe could infer the cosmological parameters). We thereforeblind the time-delay distance and all cosmological param-eters in our analysis. Specifically, throughout the analysis,we only ever plot these blinded parameters offset by theirposterior median value. We can then still use the parametercorrelations and the uncertainties to cross check our anal-ysis, since the temptation to stop investigating systematicerrors when the “right answer” has been obtained has beenremoved. Only when the collaboration deems the analysis tobe final and complete do we “open the box”to reveal the me-dian values of the parameters, and then publish these resultswithout modifications.

This paper (hereafter, H0LiCOW Paper I) is the firstof the series, and gives an overview of the program. Thereare four more papers that detail the data sets and analysisof the H0LiCOW lens system HE0435−1223. In particular,Sluse et al. (2017, hereafter H0LiCOW Paper II) present thespectroscopic follow-up of the strong lens field to measureredshifts of massive and nearby objects close in projectionto the strong lens system and identify galaxy groups alongthe line of sight. Rusu et al. (2017, hereafter H0LiCOW Pa-per III) use our multi-band wide-field imaging to charac-terize the lens environment in combination with ray trac-ing with numerical simulations. Wong et al. (2017, hereafterH0LiCOW Paper IV) perform the lens mass modeling ofthe strong lens system incorporating the time delays, high-resolution imaging and lens stellar kinematics data sets toinfer the distance to the lens via blind analysis. Bonvin et al.(2017, hereafter H0LiCOW Paper V) present the time-delaymeasurements from COSMOGRAIL lens monitoring andthe cosmological inference based on the previous three pa-pers.

The outline of this paper is as follows. We describe the

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4 S. H. Suyu et al.

key ingredients for time-delay cosmography in Section 2,present the five H0LiCOW lens systems in Section 3, anddescribe our observational campaign in Section 4 . The keycomponents of the four analysis papers introduced aboveare summarized in Section 5. We show the forecasted cos-mographic constraints from the H0LiCOW sample in Sec-tion 6. We summarize in Section 7 with an outlook for theprogram.

2 OBSERVATIONAL REQUIREMENTS OF

THE TIME-DELAY METHOD

In this section, we describe the observational requirementsof the four ingredients for accurate and precise distance mea-surements from time-delay lenses.

(i) Time delays. Monitoring campaigns to map out thevariability of the multiple lensed images over time have beencarried out both in the radio and optical wavelengths (e.g.,Vanderriest et al. 1989; Schechter et al. 1997; Burud et al.2002; Hjorth et al. 2002; Fassnacht et al. 2002; Vuissoz et al.2007; Kochanek et al. 2006; Rumbaugh et al. 2015). Regu-lar and frequent observations, at least once every few days,are necessary so that the variability pattern of the back-ground source can be observed in each of the multiple imagesand be matched up to obtain the time delays. Monitoringin the optical requires a long baseline or high photomet-ric precision to overcome systematic variations due to mi-crolensing by stars in the lensing galaxy that could be mis-taken as the background source intrinsic variability (e.g.,Tewes et al. 2013b; Sluse & Tewes 2014). Curve-shiftingmethods have been developed to measure the time delaysfrom the light curves (e.g., Press et al. 1992; Pelt et al.1996; Fassnacht et al. 2002; Harva & Raychaudhury 2008;Hirv et al. 2011; Morgan et al. 2008; Tewes et al. 2013a;Hojjati et al. 2013). A recent time-delay challenge showedthat some of the methods can recover accurately the timedelays in a blind test (Dobler et al. 2015; Liao et al. 2015),particularly the methods we use from the COSMOGRAILcollaboration (e.g., Tewes et al. 2013a; Bonvin et al. 2016).

(ii) Well-resolved lensed images. The strong lensing in-formation, such as the multiple image positions of the back-ground source, is needed to obtain the foreground lens massdistribution for converting the time delays into distances.Deep and high-resolution imaging of the strong lens systemreveal the “Einstein rings” that are the spatially extendedand lensed images of the background source, such as thehost galaxy of the AGN. In the past decade, methods havebeen developed to take advantage of the thousands of in-tensity pixels of the extended images to constrain preciselywithin a few percent the lens potential at the location of themultiple images (e.g., Kochanek et al. 2001; Warren & Dye2003; Treu & Koopmans 2004; Koopmans 2005; Dye et al.2008; Vegetti & Koopmans 2009; Suyu et al. 2009, 2013;Birrer et al. 2015b; Chen et al. 2016). The time-delay dis-tance is particularly sensitive to the radial profile of the lensgalaxy mass distribution (e.g., Kochanek 2002; Wucknitz2002; Wucknitz et al. 2004; Suyu 2012). Imaging with highsignal-to-noise-ratio and high angular resolution of the Ein-stein ring helps to constrain the lens radial profile in theregion of the ring, and hence the time-delay distance, up toa mass-sheet transformation (described below).

(iii) The lens environment. The distribution of mass ex-ternal to the lens galaxy, such as that associated with galax-ies which are close in projection to the lens system alongthe line of sight, affects the time delays between the mul-tiple images and hence our cosmological distance measure-ments. An external convergence κext can be absorbed bythe lens and source model leaving the fit to the lensed im-ages unchanged, but the predicted time delays altered by afactor of (1 − κext). To break this “mass-sheet degeneracy”(Falco et al. 1985), one can study the environment of thelens system to constrain κext within a few percent

1 throughspectroscopic/photometric observations of local galaxygroups and line-of-sight structures (e.g., Momcheva et al.2006; Fassnacht et al. 2006; Momcheva et al. 2015) incombination with ray-tracing through numerical N-bodysimulations (e.g., Hilbert et al. 2007, 2009; Suyu et al.2010; Greene et al. 2013; Collett et al. 2013). Furthermore,McCully et al. (2014, 2016) developed a new framework tomodel line-of-sight mass distributions efficiently and quan-tified the environment effects through realistic simulationsof lens fields. By reconstructing the 3-dimensional massdistribution of strong-lens sightlines, McCully et al. (2016)can obtain constraints on κext that are consistent withbut tighter than those from the aforementioned statisti-cal approach of combining galaxy number density observa-tions with N-body simulations (see also Collett et al. 2013whose sightline mass reconstruction also produces tighterconstraints on κext than the statistical approach). Re-cently, Collett & Cunnington (2016) have pointed out thatthe external convergence over an ensemble of lenses usu-ally does not average to zero – lenses, like typical mas-sive galaxies, preferentially live in locally over-dense regions(Holder & Schechter 2003; Treu et al. 2009; Fassnacht et al.2011) and are therefore slightly easier to detect and monitor.Nonetheless, this bias in detection and/or selection that isdue to overdensity is expected to have currently negligibleimpact on D∆t (< 1% impact). In contrast, measurements ofDd that come from combining delays with the lens velocitydispersion are impervious to κext (Jee et al. 2015).

(iv) The lens galaxy stellar velocity dispersion. Thecombination of lensing and stellar kinematics is apowerful probe of the lens galaxy mass distribution(e.g., Romanowsky & Kochanek 1999; Treu & Koopmans2002; Koopmans et al. 2003; Barnabè et al. 2009, 2011;Sonnenfeld et al. 2012) since the combination breaks degen-eracies that are inherent in each approach, and in particu-lar the mass-sheet degeneracy in lensing. Schneider & Sluse(2013) pointed out that the mass-sheet degeneracy can man-ifest as a lens mass profile degeneracy, which Xu et al. (2016)investigated using simulated galaxies. Moreover, the mass-sheet degeneracy is in fact a special case of a more general“source-position transformation” (Schneider & Sluse 2014;Unruh et al. 2016), although this latter transformation typ-ically does not leave the multiple time delays invariant.To break such lensing degeneracies, information from thelens galaxy stellar kinematics is crucial: Suyu et al. (2014)showed that the lens velocity dispersion substantially re-duced the dependence of the time-delay distance on lensmass profile assumptions. The lens velocity dispersion is also

1 in terms of its impact on D∆t

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H0LiCOW program overview 5

a key ingredient for measuring Dd, which is more sensitiveto dark energy properties than D∆t (Jee et al. 2015, 2016).

3 H0LICOW SAMPLE OF LENSES

In Figure 1, we show the images of the five lenses in oursample. The left four lenses are quadruply lensed quasarsystems (“quads”) and the right-most lens system is a dou-bly lensed quasar system (“double”). As described below, thefour quads span the three generic multiple image configura-tions we have in galaxy-scale strong lenses: symmetric, fold(with 2 merging images) and cusp (with 3 merging images).Therefore, our sample will allow us to explore to some extentthe optimal image configuration for cosmographic studies.

Our sample of lenses was chosen based on three criteria:(1) availability of accurate and precise time delays, (2) ex-isting measurements of spectroscopic redshifts for both thelens and the background source, and (3) the lens system isnot located near a galaxy cluster (to avoid potentially largesystematic effects due to mass along the line of sight). Weprefer quads to doubles since quads provide more observa-tional constraints on the mass model (e.g., more time delays,image positions). The four quads in our sample were the onlyknown quad lenses that passed the above three criteria at thetime of our sample selection. There were a few doubles thatpass these criteria, and we chose HE1104−1805 as the firstdouble in this pilot program given its relative simplicity formass modeling with only one strong-lens galaxy (in contrastto other systems that have multiple massive lens galaxies).We describe in more detail each of the lenses below.

B1608+656. The lens system was discovered in theCosmic Lens All-Sky Survey (CLASS; Myers et al. 1995;Browne et al. 2003; Myers et al. 2003). The radio-loud AGNis lensed into four images that are relatively dim in the op-tical wavelength, thus showing clearly the extended Ein-stein ring of the AGN host galaxy in the HST imaging(Figure 1). Two of the four multiple images are close to-gether, making this a standard “fold” configuration. Thesystem contains two lens galaxies that appear to be inter-acting and resulting in dust extinction in the system (e.g.,Surpi & Blandford 2003; Koopmans et al. 2003; Suyu et al.2009). The lens and source redshifts are, respectively, zs =1.394 (Fassnacht et al. 1996) and zd = 0.6304 (Myers et al.1995). This system was the first quad lens with all threetime delays measured with uncertainties of only a few per-cent (Fassnacht et al. 1999, 2002).

RXJ1131−1231. Sluse et al. (2003) discoveredRXJ1131−1231 serendipitously during polarimetric imag-ing of a sample of radio quasars. This system shows aspectacular Einstein ring, with multiple arclets that are thelensed images of the AGN host galaxy containing a bulgeand a disk with spiral arms and star formation clumps.Three of the four quasar images are close to each other,forming the typical “cusp” configuration. The lens redshiftis at zd = 0.295 (Sluse et al. 2003, 2007), and the sourceredshift is at zs = 0.654 (Sluse et al. 2007)

2.

2 The source redshift of zs = 0.654 is based on the narrowemission lines, which is considered more accurate than the Hαand MgII lines (Hewett & Wild 2010) that yield zs = 0.657(Sluse et al. 2007). We note that a 0.003 change in zs corresponds

HE0435−1223. This lens system was found byWisotzki et al. (2002), originally selected in the Ham-burg/ESO survey (Wisotzki et al. 2000) as a highly probablequasar candidate. The background quasar is lensed into fourmultiple images that are nearly symmetrically positionedin the “cross” configuration. The background source is atredshift zs = 1.693 (Sluse et al. 2012)

3 and the foregroundstrong lens is at redshift zd = 0.4546 (Morgan et al. 2005;Eigenbrod et al. 2006). The HST image reveals an ellipticalring that connects the four images of the AGN. This ring isproduced by the extended lensed images of the AGN galaxy.

WFI2033−4723. Morgan et al. (2004) discovered thisquad lens system as part of an optical imaging survey usingthe MPG/ESO 2.2m telescope at La Silla, Chile that is op-erated by the European Southern Observatory (ESO). Thelens system exhibits a typical fold configuration, since it con-tains two merging quasar images. The quasar is at redshiftzs = 1.662 (Sluse et al. 2012), which is consistent with thefirst measurement by Morgan et al. (2004). The quasar im-ages are substantially brighter than the background quasarhost galaxy and the foreground lens galaxy. Morgan et al.(2004) identified the foreground lens galaxy, whose redshiftwas measured to be zd = 0.661 (Eigenbrod et al. 2006), con-sistent with an earlier measurmeent by Ofek et al. (2006).The high-resolution HST imaging shows several galaxies inthe vicinity of the lens system. Since these galaxies wouldlikely influence the lens potential, their redshifts will be ob-tained with our ancillary data (Section 4.3) in order to in-corporate them into the lens mass model.

HE1104−1805. This system was also discovered in theearly phase of the Hamburg/ESO survey by Wisotzki et al.(1993). The two lensed quasar images are separated by ∼ 3′′

and is unusual in having the brighter image as the one closerto the foreground lens galaxy, which was first identified byCourbin et al. (1998) and Remy et al. (1998). The source isat zs = 2.316 (Smette et al. 1995), and the lens is at a rela-tively high redshift of zd = 0.729 (Lidman et al. 2000). TheHST image shows multiple luminous structures/galaxiesaround the lens system.

4 OBSERVATIONAL FOLLOW-UP

In collaboration with the COSMOGRAIL team, we carryout an observational campaign in order to obtain each of thefour ingredients for distance measurements of the H0LiCOWlenses. We describe the monitoring in Section 4.1 to get thetime delays, deep HST imaging to constrain the lens galaxymass distribution in Section 4.2, wide-field spectroscopy andimaging to study the lens environment in Section 4.3 andspectroscopy of the foreground lens galaxy to measure thestellar velocity dispersion in Section 4.4.

to a < 0.4% change in D∆t for RXJ1131−1231, and even lesschange in D∆t for the other higher-redshift lens systems.3 based on Mg II emission line, which results in a slightly higherredshift value than the previous measurement of zs = 1.689(Wisotzki et al. 2002) from C IV line that is known to be proneto systematic blueshifts in many quasars

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6 S. H. Suyu et al.

B1608+656 RXJ1131−1231 HE0435−1223 WFI2033−4723 HE1104−1805

1" 1" 1" 1" 1"

Figure 1. H0LiCOW lens sample, consisting of four quadruply lensed quasar systems in various configurations and one doubly lensedquasar system. The lens name is indicated above each panel. The color images are composed using 2 (for B1608+656) or 3 (for otherlenses) HST imaging bands in the optical and near-infrared. North is up and east is left.

4.1 Time delays

Of the five H0LiCOW lenses, B1608+656 has been mon-itored previously by Fassnacht et al. (1999, 2002) usingthe Very Large Array, whereas the other four lenses arecurrently being monitored by the COSMOGRAIL andKochanek et al. (2006) collaborations using a network of1-2m optical telescopes, particularly the Euler telescope inChile.

Using three seasons of monitoring of B1608+656, es-pecially the third season which showed significant variabil-ity that repeated in all four quasar images, Fassnacht et al.(2002) measured all three relative time delays between thefour quasar images with uncertainties of a few percent. Theimage fluxes were measured every 3–4 days during the mon-itoring. The time delays span ∼ 30−80 days, relative to thefirst image that varies.

The monitoring of RXJ1131−1231, HE0435−1223,WFI2033−4723 and HE1104−1805 by the COSMOGRAILand Kochanek et al. (2006) teams started in 2003, with aphotometric point every 2–4 days. The MCS deconvolutionmethod (Magain et al. 1998; Cantale et al. 2016) is usedto extract the photometry of the quasar images for build-ing the light curves. Tewes et al. (2013a) set up an auto-mated pipeline to reduce the images, build the light curves,and measure the time delays using a state-of-the-art curve-shifting algorithm that simultaneously models both intrinsicvariability of the AGNs and microlensing variations. Withthis pipeline, Bonvin et al. (2016) recovered the time delayswith a precision of ∼ 3% and negligible bias for simulatedlight curves mimicking COSMOGRAIL monitoring in theblind strong lens time delay challenge (Liao et al. 2015),demonstrating the robustness of their curve-shifting algo-rithms.

The monitoring and analysis yield time delays inRXJ1131−1231 with a 1.5% uncertainty on the longest de-lay (Tewes et al. 2013b). The light curve has been sepa-rately modeled by A. Hojjati and E. Linder using the Gaus-sian process technique (Hojjati et al. 2013), who have ob-tained delays that are consistent with the measurementsof Tewes et al. (2013b) (Eric Linder, private communica-tions). The monitoring and analysis of HE0435−1223 is de-scribed in H0LiCOW Paper V, with a relative uncertaintyof 6.5% on the longest delay (between images A and D).The measurement precision in the delays has improved bya factor of 2 compared to the previous measurements byCourbin et al. (2011) with the five additional years of mon-

itoring and improvements in the curve-shifting algorithms.For WFI2033−4723 and HE1104−1805, we expect to im-prove on the previous delay measurements by Vuissoz et al.(2008) and Poindexter et al. (2007), respectively, with thenew curve-shifting techniques, and estimate relative uncer-tainties of∼ 4% and∼ 2%, respectively, from the monitoringcampaign.

4.2 HST observations

Deep HST Advanced Camera for Surveys (ACS) obser-vations of B1608+656 were obtained in Program 10158(PI: C. D. Fassnacht) in two filters, F606W and F814W.Suyu et al. (2009) have described these observations in de-tail. Furthermore, Suyu et al. (2009) analyzed these dataand used a pixelated lens potential reconstruction tech-nique to model the lens mass distribution, which were subse-quently used for cosmographic analysis in Suyu et al. (2010).

Archival HST ACS observations of RXJ1131−1231(Program 9744; PI: C. S. Kochanek) are available in twofilters, F555W and F814W. Details of the observations aredescribed in, e.g., Claeskens et al. (2006). These have beenused to model the lens mass distribution for cosmography,accounting for uncertainties due to assumptions on the lensmass profile (Suyu et al. 2013, 2014). Recently, Birrer et al.(2015a) have also used these observations to model in-dependently the lens mass distribution of RXJ1131−1231for cosmography, obtaining results that are consistent withSuyu et al. (2013).

We have obtained new deep HST Wide Field Camera 3(WFC3) observations in Program 12889 (PI: S. H. Suyu) ofthe remaining three lenses (HE0435−1223, WFI2033−4723and HE1104−1805) in the infrared (IR) channel. The goalof these observations is to detect the Einstein rings of theAGN host galaxies at high signal-to-noise ratios, in orderto constrain the foreground lens mass distribution (previousHST observations had insufficient signal-to-noise ratios ofthe rings for our analysis). We use the F160W filter to op-timize the contrast between the AGN host galaxy and theAGN, since the host galaxy is brighter in the infrared com-pared to the optical, especially for HE1104−1805 where thequasar is at a high redshift.

We employ four-point dither patterns that trace outparallelograms with the lengths of the sides being non-integral numbers of pixels. For each lens, we use multipleparallelograms that are offset by non-integral pixels. Specif-ically, we use 2, 5 and 3 parallelograms for HE0435−1223,

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H0LiCOW program overview 7

Table 1. New HST WFC3/IR Observations of HE 0435−1223,WFI2033−4723 and HE1104−1805

Lens Date Number/type Time (s) perof exposures exposure

HE0435−1223 2012-10-28 8 short exp. 44

15 long exp.4 599

WFI2033−4723 2013-05-03 20 short exp. 74

to 2013-05-04 4 long exp.4 599

32 long exp.4 699

HE1104−1805 2013-03-18 12 short exp. 2624 long exp. 599

Notes. At each dither position, an exposure sequence ofshort-long-long exposure times is adopted in order to sample thelarge dynamical range of the AGN and its much fainter hostgalaxy4.

WFI2033−4723, HE1104−1805, respectively, depending onthe total exposure time needed to image the Einstein ring.We further ensure that the dithering points do not overlap toavoid IR persistence effects. This dithering strategy allowsus to recover effectively an angular resolution of ∼ 0.′′08 fromthe native 0.′′13 pixel scale.

Since the AGN host galaxy is substantially fainter thanthe AGN, we further adopt an exposure sequence of short-long-long at each of the dithering point4. The first short ex-posure allows us to characterize the AGN, whereas the longexposures would get the AGN host with possibly the pix-els near the bright AGN saturated. We note that there aremultiple non-destructive reads during each exposure withthe MULTIACCUM mode of the WFC3/IR detector, so wecan have a count-rate estimate on the AGN pixels even inthe long exposures if several non-destructive reads are avail-able before saturation. The short exposures are taken to en-sure that there are sufficient reads to characterize accuratelythe pixel count rates near the AGN positions, in case thelong exposures are indeed saturated with insufficient non-destructive reads. In essence, the combination of the shortand long exposures allows us to reconstruct in full the bright-ness distribution of both the lensed AGN and the lensed hostgalaxy. We summarize our observations in Table 1.

We reduce the images using DrizzlePac5. The imagesare drizzled to a final pixel scale of 0.′′08, without maskingthe bright AGN pixels as they are well characterized by theshort exposures. The uncertainty on the flux in each pixel isestimated from the science image and the drizzled exposuretime map by adding in quadrature the Poisson noise fromthe source and the background noise due to the sky anddetector readout.

4 For HE0435−1223 one long exposure was lost due to a satel-lite passing over the target. For one of the parallelogram ditherpattern for WFI2033−4723, we use an exposure sequence of short-long (rather than short-long-long) at each dither position to opti-mize target exposure time given overhead associated with obser-vations.5 DrizzlePac is a product of the Space Telescope Science Insti-tute, which is operated by AURA for NASA.

In Figure 2, we show the reduced HST WFC3 observa-tions of HE0435−1223, WFI2033−4723 and HE0435−1223in the top panels from left to right. In the bottom, we showthe images with the lens light subtracted withGlee6, reveal-ing the Einstein ring of the AGN host galaxy. In H0LiCOWPaper IV, we detail the modeling of HE0435−1223 us-ing multi-lens-plane ray tracing (e.g., Blandford & Narayan1986; Schneider et al. 1992; Blandford & Kochanek 2004)and point-spread-function reconstruction techniques devel-oped by Suyu et al. (in preparation). The subtraction of lenslight in WFI2033−4723 and HE1104−1805 (bottom-middleand bottom-right panels of Figure 2, respectively) is basedon an initial point spread function built from stars in thefield without any lens mass modeling or iterative PSF recon-struction, hence the lens-subtraction residuals. Furthermore,the lens galaxy of HE1104−1805 is on a diffraction spike ofthe brighter AGN image – an accurate PSF model wouldbe crucial for distinguishing the lens galaxy, the two AGNimages and the lensed host galaxy of the AGN. The full mod-eling and analysis of WFI2033−4723 and HE1104−1805 willappear in future publications.

4.3 Wide-field spectroscopy and imaging of lens

environment

We obtain wide-field spectroscopy to pinpoint the redshiftsof the bright galaxies in the fields of the H0LiCOW lenses,particularly the ones close to the strong lens. Redshifts ofnearby galaxies, especially those within a few arcsecondsfrom the strong lens, are crucial since the external conver-gence approximation is often insufficient for these galaxies(e.g., McCully et al. 2014) and they need to be incorpo-rated directly into the strong lens modeling. We use themulti-object spectrographs on the Very Large Telescope, theGemini Telescope and the W. M. Keck Telescope to targetour lens fields, as summarized in Table 2. The spectroscopicredshifts and galaxy group identifications are detailed inFassnacht et al. (2006), H0LiCOW Paper II and forthcom-ing publications.

To further characterize the lens environment and de-termine κext, we obtain wide-field multiband imaging usingthe Canada France Hawaii Telescope, Subaru Telescope, theVery Large Telescope, Gemini Telescope and Spitzer SpaceTelescope. Table 3 summarizes the follow-up imaging thatallow us to compute the photometric redshifts of structuresalong the line of sight as well as to estimate their stellarmasses. Details of the observations and inference on κext aredescribed in H0LiCOW Paper III and forthcoming publica-tions.

Williams et al. (2006, and in preparation) have inde-pendently obtained I and either V or R images of all fiveH0LiCOW lenses using the 4-m Cerro Tololo Inter-AmericanObservatory (CTIO) Blanco telescope for the sourthernfields and the 4-m Kitt Peak National Observatory (KPNO)Mayall telescope for the nothern fields. Using these imagesto select spectroscopic targets, Momcheva et al. (2015) haveobtained spectroscopic observations of the five H0LiCOW

6 A lens modeling software package developed by A. Halkola andS. H. Suyu (Suyu & Halkola 2010; Suyu et al. 2012b)

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8 S. H. Suyu et al.

HE0435−1223 WFI2033−4723 HE 1104−1805

Figure 2. HST WFC3 F160W observation of HE0435−1223, WFI2033−4723 and HE1104−1805 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 foraccurate and precise lens mass modeling. The full modeling of HE0435−1223 is detailed in H0LiCOW Paper IV. The lens subtractionfor WFI2033−4723 and HE1104−1805 in the bottom-middle and bottom-right panels, respectively, is based on an initial PSF modelwithout PSF reconstruction (which we defer to future work), hence the visible residuals. In each of the panels, north is up, and east isleft.

Table 2. Wide-field spectroscopy of H0LiCOW lenses as part ofthe H0LiCOW program

Lens Facility/instrument Proposal PI

B1608+656 W. M. Keck / LRIS C. .D. FassnachtW. M. Keck / ESI C. .D. Fassnacht

RXJ1131−1231 W. M. Keck / LRIS C. .D. FassnachtHE0435−1223 W. M. Keck / LRIS C. D. Fassnacht

VLT / FORS2 D. SluseGemini / GMOS T. Treu

WFI2033−4723 VLT / FORS2 D. SluseGemini / GMOS T. Treu

HE1104−1805 VLT / FORS2 D. SluseGemini / GMOS T. Treu

Notes. Abbreviations are LRIS (Low Resolution ImagingSpectrometer; Oke et al. 1995; Rockosi et al. 2010), ESI(Echellete Spectrograph and Imager; Sheinis et al. 2002), VLT(Very Large Telescope), FORS2 (FOcal Reducer and lowdispersion Spectrograph; Appenzeller et al. 1998), and GMOS(Gemini Multi-Object Spectrographs; Hook et al. 2004). Detailsof the observations for B1608+656 are in Fassnacht et al.(2006), and for the other four lenses are in H0LiCOW Paper IIand forthcoming publications. Additional integral fieldspectroscopy of the central 30′ around WFI2033−4723 has beenrecently obtained with the Multi Unit Spectroscopic Explorer(MUSE; Bacon et al. 2012) on the VLT.

lenses using the 6.5m Magellan telescopes. In H0LiCOW Pa-per II, we merge the spectroscopic catalog from the multiplespectroscopic campaigns on HE0435−1223.

4.4 Lens galaxy spectroscopy for lens velocity

dispersion

For B1608+656 and RXJ1131−1231, we have obtained long-slit spectra of the lens systems with the Low-ResolutionImaging Spectrometer (LRIS; Oke et al. 1995) at the KeckObservatory for measuring the lens stellar velocity disper-sion (Suyu et al. 2010, 2013). For HE0435−1223, we ob-serve the lens system with LRIS in multi-object mode toobtain spectra of both the foreground lens galaxy for lensvelocity dispersion measurement (see H0LiCOW Paper IV)and also of nearby galaxies (see H0LiCOW Paper II). BothWFI2033−4723 and HE1104−1805 have bright AGNs rel-ative to the lens galaxy, making the lens velocity disper-sion measurement challenging. We have new observationsof WFI2033−4723 with MUSE (Bacon et al. 2012) at theVLT, which we expect will allow us to reduce the uncer-tainty on the current lens velocity dispersion by a factorof 2, to ∼ 5 − 7% precision. The velocity dispersion is akey ingredient to break the MSD/lensing degeneracies (e.g.,Suyu et al. 2014). For HE1104−1805, we obtained one-sixthof our proposed observations with XSHOOTER on the VLTin priority B, which is not sufficient to measure the velocity

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H0LiCOW program overview 9

Table 3. Wide-field imaging obtained as part of the H0LiCOW program

Lens Facility/instrument Wavelength bands Proposal PI

B1608+656 CFHT / MegaCam u S. H. SuyuSubaru / Suprime-Cam g, r, i C. D. FassnachtSubaru / MOIRCS J, H, Ks C. D. FassnachtGemini / NIRI J, Ks C. D. FassnachtSpitzer / IRAC 3.6µm, 4.5µm C. E. Rusu

RXJ1131−1231 CFHT / MegaCam u S. H. SuyuSubaru / Suprime-Cam g, r, i C. D. FassnachtSubaru / MOIRCS J, H, Ks C. D. FassnachtGemini / NIRI J, Ks C. D. Fassnacht

HE0435−1223 CFHT / MegaCam u S. H. SuyuSubaru / Suprime-Cam g, r, i C. D. FassnachtSubaru / MOIRCS H C. D. FassnachtGemini / NIRI J, Ks C. D. Fassnacht

WFI2033−4723 CTIO Blanco / DECam u C. E. RusuVLT / HAWK-I J, H, K C. D. Fassnacht

HE1104−1805 CFHT / MegaCam u S. H. SuyuSubaru / Suprime-Cam g, r, i C. D. FassnachtSubaru / MOIRCS J, H, Ks C. D. FassnachtGemini / NIRI J, Ks C. D. Fassnacht

Notes. Abbreviations and references for the instruments are CFHT (Canada-France-Hawaii Telescope) MegaCam (Boulade et al. 2003),Suprime-Cam (Miyazaki et al. 2002), MOIRCS (Multi-Object InfraRed Camera and Spectrograph; Suzuki et al. 2008; Ichikawa et al.2006), NIRI (Near InfraRed Imager and Spectrometer; Hodapp et al. 2003), IRAC (Infrared Array Camera; Fazio et al. 2004), CTIO(Cerro Tololo Inter-American Observatory) DECam (Dark Energy Camera; Diehl & Dark Energy Survey Collaboration 2012), VLT(Very Large Telescope) HAWK-I (High Acuity Wide field K-band Imager; Pirard et al. 2004; Casali et al. 2006; Kissler-Patig et al.2008). Details of the observations are in H0LiCOW Paper III and forthcoming publications. WFI2033−4723 is in the footprint of theDark Energy Survey with observations in g, r, i, z and Y bands, so we did not target WFI2033−4723 in these bands. We observed onlyB1608+656 with Spitzer since the other four lenses have archival Spitzer/IRAC observations (PI: C. S. Kochanek).

dispersion. We have time on OSIRIS (OH-Suppressing Infra-Red Imaging Spectrograph; Larkin et al. 2006) on Keck toobserve HE1104−1805, RXJ1131−1231, and HE0435−1223with adaptive optics. Because OSIRIS is an integral fieldspectrograph, these observations have the goal of obtain-ing two-dimensional kinematic data of the foreground lens,which will then be used to further constrain the lens massmodels. We summarize the spectroscopic observations forlens velocity dispersion measurement in Table 4.

5 COSMOGRAPHY AND ASTROPHYSICS

WITH HE0435−1223: KEY COMPONENTS

We summarize the key ingredients and analysis ofHE0435−1223 that are described in upcoming publicationsof the H0LiCOW project (H0LiCOW Papers II-V). Thetitles of the papers begin with “H0LiCOW”, followed by thespecific titles written below.

II. Spectroscopic survey and galaxy-group identifica-tion of the strong gravitational lens systems HE0435−1223(Sluse et al. 2017). From our spectroscopic campaign ofthe lens environment, we present the measured spectro-scopic redshifts, focussing in particular on the massiveand nearby objects to the strong lens system that arenecessary ingredients for lens mass modeling and distancemeasurement. By combining with the spectroscopic catalogof independent efforts (Momcheva et al. 2015), we identifypotential galaxy groups towards HE0435−1223 in orderto control the systematic effect due to the galaxies along

Table 4. Spectroscopy of foreground lens as part of theH0LiCOW program

Lens Facility/instrument Proposal PI

B1608+656 W. M. Keck / LRIS C. D. FassnachtRXJ1131−1231 W. M. Keck / LRIS C. D. Fassnacht

W. M. Keck / OSIRIS T. TreuHE0435−1223 W. M. Keck / LRIS C. D. Fassnacht

W. M. Keck / OSIRIS T. TreuWFI2033−4723 VLT / MUSE D. SluseHE1104−1805 VLT / X-shooter C. Spiniello

W. M. Keck / OSIRIS T. Treu

Notes. OSIRIS is the OH-Suppressing Infra-Red ImagingSpectrograph (Larkin et al. 2006). Details of the LRISobservations for B1608+656 are in Suyu et al. (2010), for

RXJ1131−1231 are in Suyu et al. (2013), and for HE0435−1223are in H0LiCOW Paper IV; other observations are inforthcoming publications. Only one-sixth of the HE1104−1805observations with X-shooter (Vernet et al. 2011) were obtained,which were insufficient for measuring the lens velocitydispersion. The observations with OSIRIS are pending.

the line of sight. We use the flexion-shift7 introduced by

7 The flexion-shift corresponds to the shift in the image positionsdue to the flexion (third order derivatives of the lens potential) ofa line-of-sight perturber. McCully et al. (2016) find through theirstudy of simulated lens fields that perturbers with flexion-shiftslarger than ∼ 10−4 arcseconds should be incorporated explicitly

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10 S. H. Suyu et al.

McCully et al. (2016) to determine which mass structures(galaxies/groups) need to be incorporated explicitly in thelens mass model and which could be well approximatedby an external shear/convergence field. The flexion-shiftanalysis presented in H0LiCOW Paper II shows that themost significant line-of-sight perturber is the galaxy G1 thatis closest to the lens system, which justifies our inclusionof this particular galaxy in all of our strong lensing modelsin H0LiCOW Paper IV. Furthermore, the next four nearestperturbers from the lens system may also produce higherorder perturbations on the lens potential, and we accountfor the effects of these four additional galaxies in one of oursystematic tests in H0LiCOW Paper IV.

III. Quantifying the effect of mass along the line ofsight to the gravitational lens HE0435−1223 throughweighted galaxy counts (Rusu et al. 2017). Using thewide-field photometry and spectroscopy in Section 4.3,we compute photometric redshifts and stellar masses forobjects in the field up to 120′′ from the strong lens, andwith i

H0LiCOW program overview 11

Table 5. Prior for “uniform” cosmological models

Cosmology Prior ranges

open ΛCDM H0 ∈ [0, 120] km s−1 Mpc−1

Ωm ∈ [0, 0.5]ΩΛ ∈ [0.5, 1]Ωk = 1− Ωm − ΩΛ

flat wCDM H0 ∈ [0, 120] km s−1 Mpc−1

Ωm ∈ [0, 1]ΩDE = 1− Ωmw ∈ [−2.5, 0]

flat NeffΛCDM H0 ∈ [0, 120] km s−1 Mpc−1

Ωm ∈ [0, 1]ΩΛ = 1−ΩmNeff ∈ [0, 10]

marginalized H0 distributions of the panels to the left), asindicated in the legend. We list in Table 5 the prior rangesfor the uniform background cosmologies. The WMAP 9-yearand Planck chains have a prior with H0 < 100 kms

−1 Mpc−1

imposed. The cosmographic constraints of our lenses shownin Figure 3 (from the forecasted measurements of D∆t andDd) mostly stem from the D∆t measurements as a resultsof the substantially smaller uncertainties of D∆t than thatof Dd. In fact, the cosmographic constraints from D∆t alonewould increase the H0 1σ uncertainties shown in Figure 3by at most 0.8 km s−1 Mpc−1 (depending on the backgroundcosmology). The additional cosmographic information fromDd would become more significant when the Dd uncertain-ties are reduced to ∼ 5− 10% (Jee et al. 2016).

As seen in the left column, the time-delay lenses primar-ily constrain H0, and depend weakly (if at all) on other pa-rameters. Nonetheless, the time-delay distances D∆t and thelenses’ angular diameter distances Dd provide some informa-tion on w, as the constraint contours are tilted rather thanbeing vertical. With more lenses or smaller uncertaintieson Dd measurements, the constraints on cosmology becomemore prominent (Jee et al. 2016). However, the H0LiCOWlenses provide strong cosmographic constraints when com-bined with the CMB measurements since they help to breakparameter degeneracies in the CMB. Thus, we should beable to place substantially better constraints on, for exam-ple, the spatial curvature, w and Neff (middle two columns),compared to constraints from CMB alone. In particular, weexpect better than 3.5% precision on H0 for the two cos-mologies with w = −1 (open ΛCDM and flat NeffΛCDM)

9;when w is allowed to vary, this constraint weakens to ∼ 11%without CMB priors and ∼ 5% with CMB priors in thewCDM cosmology, as visible in the right-most panel in themiddle row. By combining our five H0LiCOW lenses withPlanck, we expect to achieve the following precisions: Ωk to0.004 in open ΛCDM, w to 0.14 in flat wCDM, and Neffto 0.2 in flat NeffΛCDM (all 1σ uncertainties). These preci-sions are a factor of ∼ 15, ∼ 2 and ∼ 1.5, respectively tighterthan Planck on its own. Our H0LiCOW sample provides notonly an independent check of systematics, but also a great

9 relative to H0 = 72 km s−1 Mpc−1

complement to other cosmological probes for pinning downcosmological parameters.

7 SUMMARY AND OUTLOOK

We present the H0LiCOW program that aims to measure H0to < 3.5% in precision and accuracy (in most backgroundcosmological models) with a sample of five time-delay lenses,completely independent of the cosmic distance ladder andother direct measurements of H0. Our cosmographic infor-mation comes from measuring the distances to the lens sys-tems, specifically D∆t and Dd.

To achieve our goal, we have obtained almost all the keyingredients for our lens sample10: (1) the time delays fromthe COSMOGRAIL and Very Large Array monitoring, (2)high-resolution HST imaging for modeling the lens mass dis-tributions, (3) wide-field imaging and spectroscopy to quan-tify the effects of the lens environment, and (4) lens velocitydispersion measurements to augment our lensing mass mod-els. Our new HST observations reveal Einstein rings in thelens systems that allow us to perform precision lens massmodeling.

The results of our recent blind analysis ofHE0435−1223 will appear in the companion H0LiCOWpublications. H0LiCOWPaper II (Sluse et al. 2017) presentsthe spectroscopic campaign on the HE0435−1223 field andidentifies galaxy groups in the light cone containing the lens.H0LiCOW Paper III (Rusu et al. 2017) combines the spec-troscopy, the wide-field imaging data, and the MillenniumSimulation to derive the external convergence of the line-of-sight mass distributions. H0LiCOW Paper IV (Wong et al.2017) models the lens mass distribution using the HSTdata, the time delays and the lens velocity dispersion toinfer the time-delay distance, that is blinded throughoutthe analysis. H0LiCOW Paper V (Bonvin et al. 2017)presents the COSMOGRAIL monitoring of HE0435−1223and investigates the cosmological implications based on thethree lenses (B1608+656, RXJ1131−1231, HE0435−1223)that we have so far analyzed.

With our sample of five lenses, we expect to measure H0to < 3.5% in precision and accuracy for the non-flat ΛCDMcosmology or flat NeffΛCDM cosmology, with w = −1.When w is allowed to vary, the constraint on H0 degrades to∼ 11% with time-delay data only, and to ∼ 5% when aug-mented with CMB data. Our independent strong-lensing dis-tances significantly improve cosmological constraints fromthe Planck data: the precisions on Ωk, w and Neff improveby a factor of ∼ 15, ∼ 2 and 1.5 respectively when we com-bine our lenses with Planck. Time-delay lenses are thereforehighly complementary to other cosmological probes.

Our data set provides an excellent opportunity to study,in addition to cosmography, galaxy formation and evolu-tion. For example, we can study the distribution of darkmatter in the lens galaxies by combining lensing and kine-matics data, and also infer the stellar mass of the lensgalaxies (e.g., Treu & Koopmans 2004; Barnabè et al. 2011;

10 with spectroscopic observations of HE 1104−1805 pending forlens velocity dispersion measurement

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12 S. H. Suyu et al.

Uniform Prior WMAP9 Prior Planck Prior Marginalized H0

40 60 80 100-0.

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.2-0

.10.

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Figure 3. Forecasted cosmographic constraints from the H0LiCOW lens sample through measurements of D∆t and Dd. Columns fromleft to right are, respectively, the constraints from the H0LiCOW lenses alone (with uniform prior on cosmological parameters), lenses incombination with WMAP 9-year results, lenses in combination with Planck 2015 results, and marginalized constraints on H0 from the

previous three columns. The H0LiCOW lenses primarily constrain H0, which in turn break CMB parameter degenercies to elucidate thespatial curvature of universe (Ωk, top row), dark energy equation of state (w, middle row) and effective number of relativistic species(Neff , bottom row). H0LiCOW lenses provide an independent, complementary and competitive probe of cosmology.

Suyu et al. 2012b; Sonnenfeld et al. 2012, 2015). By sepa-rately determining the stellar mass based on either (1) stel-lar population synthesis using multiband photometry (e.g.,Auger et al. 2009; Treu et al. 2010; Oguri et al. 2014), or(2) identification/characterization of spectral features (e.g.,van Dokkum & Conroy 2010; Conroy & van Dokkum 2012;Barnabè et al. 2013; Spiniello et al. 2012, 2014, 2015), andcomparing this stellar mass to that obtained from lensingand dynamics, we can study properties of the stellar popu-lation and infer the stellar initial-mass-function (IMF) slope(e.g., Grillo et al. 2009; Auger et al. 2010; Treu et al. 2010;Spiniello et al. 2011; Barnabè et al. 2013; Spiniello et al.2015). There are about a dozen early-type lens galaxiesthat have been studied in detail for constraining the stel-lar IMF slope individually (e.g., Sonnenfeld et al. 2012;

Barnabè et al. 2013; Spiniello et al. 2015; Newman et al.2016), and these galaxies are all at redshifts below 0.35.Four of our H0LiCOW lens galaxies are at redshifts between0.45 and 0.73, which would allow us to explore the stellarIMF with comparable precisions per lens galaxy as previ-ous studies, but at substantially higher redshifts. Given thecurrent tension in the IMF measurement between nearby(zd < 0.06) lens galaxies and zd ∼ 0.2 − 0.3 lens galax-ies (e.g., Smith & Lucey 2013; Newman et al. 2016), ourH0LiCOW lenses would help assess whether the tensionsare just limited to those particular objects or if they reflecta more general problem in our understanding of stellar pop-ulations. In addition, our lenses are natural telescopes thatmagnify the background sources, allowing us to study thehost galaxies of the AGNs in detail and probe the origin of

MNRAS 000, 1–15 (2016)

H0LiCOW program overview 13

the co-evolution between supermassive black holes and theirhost galaxies (Peng et al. 2006; Rusu et al. 2016; Ding et al.2017).

Our H0LiCOW program aims to establish gravitationallens time delays as an independent and competitive probeof cosmology, and paves the way for determining H0 to 1%in the future. Given the hundreds, if not thousands, of time-delay lens systems that are expected to be discovered inongoing and future surveys such as the Sloan Digital SkySurvey (e.g., Oguri et al. 2006; Inada et al. 2012; More et al.2016b), the Dark Energy Survey (e.g., Agnello et al. 2015),the Hyper Suprime-Cam Survey (e.g., Chan et al. 2016),the Kilo-Degree Survey (e.g., Napolitano et al. 2015), Euclidand the Large Synoptic Survey Telescope (Oguri & Marshall2010), and continuous advances in high-resolution imagingand spectroscopy in the current and next generation of tele-scopes for observational follow-up (Meng et al. 2015; Linder2015), the H0LiCOW program will provide the basis for ex-tracting cosmological information from the wealth of stronglensing data sets. In particular, we expect the combinationof facilities at different wavelengths such as the HST in theoptical/near-infrared, James Webb Space Telescope in theinfrared, large and extremely large telescopes with adaptiveoptics, the Atacama Large Millimeter/submilliter Array inthe sub mm waveband, and the Square Kilometer Array inthe radio, will be of great synergistic value for studying thesefruitful lenses.

ACKNOWLEDGMENTS

We thank M. Barnabè, J. Chan, Y. Hezaveh, E. Komatsu,E. Linder, J. McKean, D. Paraficz, P. Schneider, and S. Veg-etti for helpful discussions, and the anonymous referee fordetailed comments that improved the presentation of thiswork. H0LiCOW and COSMOGRAIL are made possiblethanks to the continuous work of all observers and tech-nical staff obtaining the monitoring observations, in partic-ular at the Swiss Euler telescope at La Silla Observatory.Euler is supported by the Swiss National Science Founda-tion. S.H.S. is supported by the Max Planck Society throughthe Max Planck Research Group. This work is supported inpart by the Ministry of Science and Technology in Taiwanvia grant MOST-103-2112-M-001-003-MY3. V.B., F.C. andG.M. acknowledge the support of the Swiss National Sci-ence Foundation (SNSF). C.D.F. and C.E.R. were fundedthrough the NSF grant AST-1312329, “Collaborative Re-search: Accurate cosmology with strong gravitational lenstime delays,” and C.D.F. and N.R. were funded by the HSTgrant GO-12889. D.S. acknowledges funding support from aBack to Belgium grant from the Belgian Federal Science Pol-icy (BELSPO). T.T. thanks the Packard Foundation for gen-erous support through a Packard Research Fellowship, theNSF for funding through NSF grant AST-1450141, “Collab-orative Research: Accurate cosmology with strong gravita-tional lens time delays”. K.C.W. is supported by an EACOAFellowship awarded by the East Asia Core Observatories As-sociation, which consists of the Academia Sinica Instituteof Astronomy and Astrophysics, the National AstronomicalObservatory of Japan, the National Astronomical Observa-tories of the Chinese Academy of Sciences, and the Korea As-tronomy and Space Science Institute. X.D. is supported by

the China Scholarship Council. S.H. acknowledges supportby the DFG cluster of excellence ‘Origin and Structure ofthe Universe’ (www.universe-cluster.de). P.J.M. acknowl-edges support from the U.S. Department of Energy undercontract number DE-AC02-76SF00515. M.T. acknowledgessupport by a fellowship of the Alexander von HumboldtFoundation and the DFG grant Hi 1495/2-1. L.V.E.K. issupported in part through an NWO-VICI career grant(project number 639.043.308). Based on observations madewith the NASA/ESA Hubble Space Telescope, obtained atthe Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observa-tions are associated with programs #12889, #10158, #7422and #9744. Support for program #12889 was provided byNASA through a grant from the Space Telescope Science In-stitute, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under NASA contract NAS5-26555. Some of the data presented herein were obtained atthe W.M. Keck Observatory, which is operated as a scientificpartnership among the California Institute of Technology,the University of California and the National Aeronauticsand Space Administration. The Observatory was made pos-sible by the generous financial support of the W.M. KeckFoundation. The authors wish to recognize and acknowl-edge the very significant cultural role and reverence thatthe summit of Mauna Kea has always had within the indige-nous Hawaiian community. We are most fortunate to havethe opportunity to conduct observations from this mountain.Access to the CFHT was made possible by the Institute ofAstronomy and Astrophysics, Academia Sinica, National Ts-ing Hua University, and National Science Council, Taiwan.Based in part on data collected at Subaru Telescope, whichis operated by the National Astronomical Observatory ofJapan. Based on observations obtained at the Gemini Obser-vatory, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under a cooperative agree-ment with the NSF on behalf of the Gemini partnership:the National Science Foundation (United States), the Na-tional Research Council (Canada), CONICYT (Chile), Min-isterio de Ciencia, Tecnoloǵıa e Innovación Productiva (Ar-gentina), and Ministério da Ciência, Tecnologia e Inovação(Brazil). These observations are associated with programsGN-2012B-Q-11, GN-2013A-Q-72, GS-2013A-Q-2 and GS-2013B-Q-28. Based on observations made with ESO Tele-scopes at the La Silla Paranal Observatory under pro-gramme ID 090.A-0531(A), P91.B-0346(B), 091.A-0642(A),092.A-0515(A), 092.A-0515(B), 60.A-9306(A), and 097.A-0454(A). This work is based in part on observations andarchival data obtained with the Spitzer Space Telescope,which is operated by the Jet Propulsion Laboratory, Cali-fornia Institute of Technology under a contract with NASA.This project used data obtained with the Dark Energy Cam-era (DECam), which was constructed by the Dark EnergySurvey (DES) collaboration. Funding for the DES Projectshas been provided by the U.S. Department of Energy, theU.S. National Science Foundation, the Ministry of Scienceand Education of Spain, the Science and Technology Fa-cilities Council of the United Kingdom, the Higher Edu-cation Funding Council for England, the National Centerfor Supercomputing Applications at the University of Illi-nois at Urbana-Champaign, the Kavli Institute of Cosmo-

MNRAS 000, 1–15 (2016)

http://www.universe-cluster.de

14 S. H. Suyu et al.

logical Physics at the University of Chicago, the Center forCosmology and Astro-Particle Physics at the Ohio StateUniversity, the Mitchell Institute for Fundamental Physicsand Astronomy at Texas A&M University, Financiadorade Estudos e Projetos, Fundação Carlos Chagas Filho deAmparo à Pesquisa do Estado do Rio de Janeiro, Con-selho Nacional de Desenvolvimento Cient́ıfico e Tecnológicoand the Ministério da Ciência, Tecnologia e Inovacão, theDeutsche Forschungsgemeinschaft, and the CollaboratingInstitutions in the Dark Energy Survey. The CollaboratingInstitutions are Argonne National Laboratory, the Univer-sity of California at Santa Cruz, the University of Cam-bridge, Centro de Investigaciones Enérgeticas, Medioambi-entales y Tecnológicas-Madrid, the University of Chicago,University College London, the DES-Brazil Consortium,the University of Edinburgh, the Eidgenössische TechnischeHochschule (ETH) Zürich, Fermi National Accelerator Lab-oratory, the University of Illinois at Urbana-Champaign, theInstitut de Ciències de l’Espai (IEEC/CSIC), the Institut deF́ısica d’Altes Energies, Lawrence Berkeley National Labo-ratory, the Ludwig-Maximilians Universität München andthe associated Excellence Cluster Universe, the Universityof Michigan, the National Optical Astronomy Observatory,the University of Nottingham, the Ohio State University, theUniversity of Pennsylvania, the University of Portsmouth,SLAC National Accelerator Laboratory, Stanford Univer-sity, the University of Sussex, and Texas A&M University.

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