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We present a new measurement of the Hubble Constant H0 and other cosmological parameters based on the joint analysis of three multiply imaged quasar systems with measured gravitational time delays. First, we measure the time delay of HE 0435-1223 from 13-yr light curves obtained as part of the COSMOGRAIL project. Companion papers detail the modelling of the main deflectors and line-of-sight effects, and how these data are combined to determine the time-delay distance of HE 0435-1223. Crucially, the measurements are carried out blindly with respect to cosmological parameters in order to avoid confirmation bias. We then combine the time-delay distance of HE 0435-1223 with previous measurements from systems B1608+656 and RXJ1131-1231 to create a Time Delay Strong Lensing probe (TDSL). In flat Λ cold dark matter (ΛCDM) with free matter and energy density, we find H0 = 71.9-3.0+2.4 km s-1 Mpc-1 and ΩΛ = 0.62-0.35+0.24. This measurement is completely independent of, and in agreement with, the local distance ladder measurements of H0. We explore more general cosmological models combining TDSL with other probes, illustrating its power to break degeneracies inherent to other methods. The joint constraints from TDSL and Planck are H0 = 69.2-2.2+1.4 km s-1 Mpc-1, ΩΛ = 0.70-0.01+0.01 and Ωk = 0.003-0.006+0.004 in open ΛCDM and H0 = 79.0-4.2+4.4 km s-1 Mpc-1, Ωde = 0.77-0.03+0.02 and w = -1.38-0.16+0.14 in flat wCDM. In combination with Planck and baryon acoustic oscillation data, when relaxing the constraints on the numbers of relativistic species we find Neff = 3.34-0.21+0.21 in NeffΛCDM and when relaxing the total mass of neutrinos we find ∑mν =0.182 eV in mνλCDM. Finally, in an open wCDM in combination with Planck and cosmic microwave background lensing, we find H0 = 77.9-4.2+5.0 km s-1 Mpc-1, Ωde = 0.77-0.03+0.03, Ωk = -0.003-0.004+0.004 and w = -1.37-0.23+0.18.

Concerted effort is currently ongoing to open up the Epoch of Reionization (EoR) (z∼15-6) for studies with IR and radio telescopes. Whereas IR detections have been made of sources (Lyman-α emitters, quasars and drop-outs) in this redshift regime in relatively small fields of view, no direct detection of neutral hydrogen, via the redshifted 21-cm line, has yet been established. Such a direct detection is expected in the coming years, with ongoing surveys, and could open up the entire universe from z∼6-200 for astrophysical and cosmological studies, opening not only the EoR, but also its preceding Cosmic Dawn (z∼30-15) and possibly even the later phases of the Dark Ages (z∼200-30). All currently ongoing experiments attempt statistical detections of the 21-cm signal during the EoR, with limited signal-to-noise. Direct imaging, except maybe on the largest (degree) scales at lower redshifts, as well as higher redshifts will remain out of reach. The Square Kilometre Array(SKA) will revolutionize the field, allowing direct imaging of neutral hydrogen from scales of arc-minutes to degrees over most of the redshift range z∼6-28 with SKA1-LOW, and possibly even higher redshifts with the SKA2-LOW. In this SKA will be unique, and in parallel provide enormous potential of synergy with other upcoming facilities (e.g. JWST). In this chapter we summarize the physics of 21-cm emission, the different phases the universe is thought to go through, and the observables that the SKA can probe, referring where needed to detailed chapters in this volume (Abridged).

We report the discovery of a transient source in the central regions of galaxy cluster A267. The object, which we call "PALS-1,'' was found in a survey aimed at identifying highly magnified Lyman break galaxies in the fields of intervening rich clusters. At discovery, the source had U(n)>24.7 (2 sigma; AB), g=21.96+/-0.12, and very blue g-r and r-i colors; i.e., PALS-1 was a "U-band dropout,'' characteristic of star-forming galaxies and quasars at zsimilar to3. However, 3 months later the source had faded by more than 3 mag. Further observations showed a continued decline in luminosity, to R>26.4 at 7 months after discovery. Although the apparent brightness suggests a supernova at roughly the cluster redshift, we show that the photometry and light curve argue against any known type of supernova at any redshift. The spectral energy distribution and location near the center of a galaxy cluster are consistent with the hypothesis that PALS-1 is a gravitationally lensed transient at zapproximate to3.3. If this interpretation is correct, the source is magnified by a factor of 4-7, and two counterimages are predicted. Our lens model predicts that there are time delays between the three images of 1-10 yr and that we have witnessed the final occurrence of the transient. The intense luminosity (M(AB)similar to-23.5 after correcting for lensing) and blue UV continuum (implying Tgreater than or similar to50,000 K) argue that the source may have been a flare resulting from the tidal disruption of a star by a 10(6)-10(8) M(circle dot) black hole. Regardless of its physical nature, PALS-1 highlights the importance of monitoring regions of high magnification in galaxy clusters for distant time-varying phenomena.

We present the first unambiguous case of external variability of an extra-galactic radio source, the CLASS gravitational lens B1600+434, consisting of two images lensed by an edge-on disk galaxy. The VLA 8.5-GHz difference light curve of the lens images shows external variability at the 14.6-sigma confidence level. Although the current single-frequency VLA observations cannot conclusively exclude scintillation, several lines of evidence show that it does not dominate the short-term variability. This is supported by an ongoing three-frequency WSRT campaign. If scintillation is the dominant source of this external variability, it would require very different properties of the Galactic ionized ISM towards the two lens images. Microlensing of a superluminal jet component can explain both the rms and time-scale of variability, but requires the halo of the lens galaxy to be filled with greater than or similar to0.5-M-circle dot MACHOs. The current data appear to support microlensing as the dominant source of the observed external variability.

We present the first unambiguous case of external variability of a radio gravitational lens, CLASS B1600+434. The Very Lai-ge Array (VLA) 8.5-GHz difference light curve of the lensed images, taking the proper time-delay into account, shows the presence of external variability with 14.6-sigma confidence. We investigate two plausible causes of this external variability: scattering by the ionized component of the Galactic interstellar medium and microlensing by massive compact objects in the bulge/disk and halo of the lens galaxy. Based on the tight relation between the modulation-index (fractional rms variability) and variability time scale and the quantitative difference between the light curves of both lensed images, we conclude that the observed short-term variability characteristics of the lensed images are incompatible with scintillation in our Galaxy. This conclusion is strongly supported by multi-frequency Westerbork Synthesis Radio Telescope (WSRT) observations at 1.4 and 5 Gi;z, which are in disagreement with predictions based on the scintillation hypothesis. Several arguments against scintillation might need to be reevaluated if evidence is found for significant scatter-broadening of lensed image B seen through the lens galaxy. However, the frequency-dependence and time scale of variability from image A are not affected by this and remain strong arguments against scintillation. On the other hand, a single superluminal jet-component in the source, having an apparent velocity 9 less than or similar to(nu(app)/c)less than or similar to 26, a radius of 2-5 mu as and containing 5-11 % of the observed 8.5-GHz source flux density, can reproduce the observed modulation-indices and variability time scale at 8.5 GHz, when it is microlensed by compact objects in the lens galaxy. It also reproduces the frequency-dependence of the modulation-indices, determined from the independent WSRT 1.4 and 5-GHz observations. The difference between the modulation-indices of the lensed images (i.e. 2.8% and 1.6% at 8.5 GHz in 1998 for images A and B, respectively), if not affected by scatter-broadening of image B by the ionized ISM of the lens galaxy, can be explained through a different mass-function for the compact objects in the bulge/disk and halo of the lens galaxy. Comparing the observations with microlensing simulations, we place a tentative lower limit of greater than or similar to 0.5 M. on the average mass of compact objects in the halo line-of-sight. The above-mentioned set of mass-function and source parameters is consistent, although not unique, and should only be regarded as indicative. The only conclusion fully consistent with the data gathered thus far is that we have indeed detected radio microlensing. The far reaching consequence of this statement is that a significant fraction of the mass in the dark-matter halo at similar to 6kpc (h=0.65) above the lens-galaxy disk in B1600+434 consists of massive compact objects.

We present Very Large Array (VLA) 8.5-GHz light curves of the two lens images of the Cosmic Lens All Sky Survey (CLASS) gravitational lens B1600+434. We find a nearly linear decrease of 18-19% in the flux densities of both lens images over a period of eight months (February-October) in 1998. Additionally, the brightest image A shows modulations up to 11% peak-to-peak on scales of days to weeks over a large part of the observing period. Image B varies significantly less on this time scale. We conclude that most of the short-term variability in image A is not intrinsic source variability, but is most likely caused by microlensing in the lens galaxy. The alternative, scintillation by the ionized Galactic ISM, is shown to be implausible based on its strong opposite frequency dependent behavior compared with results from multi-frequency WSRT monitoring observations (Koopmans & de Bruyn 1999). From these VLA light curves we determine a median time delay between the lens images of 47^{+5}_{-6} d (68%) or 47^{+12}_{-9} d (95%). We use two different methods to derive the time delay; both give the same result within the errors. We estimate an additional systematic error between -8 and +7 d. If the mass distribution of lens galaxy can be described by an isothermal model (Koopmans, de Bruyn & Jackson 1998), this time delay would give a value for the Hubble parameter, H_0=57^{+14}_{-11} (95% statistical) ^{+26}_{-15} (systematic) km/s/Mpc (Omega_m=1 and Omega_Lambda=0). Similarly, the Modified-Hubble-Profile mass model would give H_0=74^{+18}_{-15} (95% statistical) ^{+22}_{-22} (systematic) km/s/Mpc. For Omega_m=0.3 and Omega_Lambda=0.7, these values increase by 5.4%. ... (ABRIDGED), 14 pages, 6 figures, accepted for publication in Astronomy & Astrophysics (Figs 1 and 3 with degraded resolution)

We present mass models of the four-image gravitational lens system B1608 + 656, based on information obtained through VLBA imaging, VLA monitoring, and Hubble Space Telescope (HST) WFPC2 and NICMOS imaging. We have determined a mass model for the lens galaxies that reproduces (1) all image positions within the observational errors, (2) two out of three flux-density ratios within about 10% from the observed ratios, and (3) the model time delays within 1% from their observed values, given our best estimate of the Hubble parameter. Using the time delays determined in a companion paper, we also find that the best isothermal mass model gives H-0 = 59(-6)(+7) km s(-1) Mpc(-1) for Omega(m) = 1 and Omega(Lambda) = 0.0, or H-0 = (65-63)(-6)(+7) km s(-1) Mpc(-1) for Omega(m) = 0.3 and Omega(Lambda) = 0.0-0.7. The statistical errors indicate the 95.4% (2 sigma) confidence interval. A systematic error of +/-15 km s(-1) Mpc(-1) is estimated from a 20% (1 sigma) uncertainty in the steepness of radial mass profile. This cosmological determination of H-0 agrees well with determinations from three other gravitational lens systems (i.e., B0218 + 357, Q0957 + 561, and PKS 1830-211), Type Ia supernovae, the Sunyaev-Zeldovich effect and local determinations. The current agreement on H-0-within the 1 sigma statistical errors-from four of five gravitational lens systems (1) emphasizes the reliability of its determination from isolated gravitational lens systems and (2) suggests that a close-to-isothermal mass profile can describe disk galaxies (e.g., B0218 + 357 and possibly PKS 1830-211), ellipticals (e.g., B1608 +/- 656), and central cluster ellipticals (e.g., 00957 + 561). The average of H-0 from B0218 + 357, Q0957 + 561, B1608 + 656, and PKS 1830-211, gives H-0(GL) = 69 +/- 7 km s(-1) Mpc(-1) for a flat universe with Omega(m) = 1 or H-0(GL) = 74 +/- 8 km s(-1) Mpc(-1) for Omega(m) = 0.3 and Omega(Lambda) = 0.0-0.7. When including PG 1115 + 080, these values decrease to 64 +/- 11 km s(-1) Mpc(-1) and 68 +/- 13 km s(-1) Mpc(-1), respectively. The errors are the estimated 2 a errors on the average. The Hubble parameter from gravitational lenses seems to agree best with local determinations of H-0 for a low-density universe, under the assumption that all lenses are nearly isothermal.

We present the results of a program to monitor the four-image gravitational lens B1608 + 656 with the VLA. The system was observed over a 7 month period from 1996 October to 1997 May. The 64 epochs of observation have an average spacing of 3.6 days. The light curves of the four images of the background source show that the flux density of the background source has varied at the similar to 5% level. We measure time delays in the system based on common features that are seen in all four light curves. The three independent time delays in the system are found to be Delta t(BA) = 31 +/- 7 days, At-BC = 36 +/- 7 days, and Delta t(BD) = 76(-10)(+9) days at 95% confidence. The uncertainties on the time delays are determined by Monte Carlo simulations that use faire light curves that have the characteristics of the observed light curves. This is the first gravitational lens system for which three independent time delays have been measured. A companion paper presents a mass model for the lensing galaxy that correctly reproduces the observed image positions, flux density ratios, and time delay ratios. The last condition is crucial for determining H-0 with a four-image lens. We combine the time delays with the model to obtain a value for the Hubble constant of H-0 = 59(-7)(+8) km s(-1) Mpc(-1) at 95% confidence (statistical) for (Omega(M), Omega(Lambda)) = (1,0). In addition, there is an estimated systematic uncertainty of +/-15 km s(-1) Mpc(-1) from uncertainties in modeling the radial mass profiles of the lensing galaxies. The value of H-0 presented in this paper is comparable to recent measurements of H-0 from the gravitational lenses 0957 + 561, PG 1115 + 080, B0218 + 357, and PKS 1830-211.

We have discovered a new gravitational lens in the Cosmic Lens All-Sky Survey (CLASS). The lens B2045+265 is a four-image system with a maximum separation of 1".9. A fifth radio component is detected, but its radio spectrum and its positional coincidence with infrared emission from the lensing galaxy strongly suggest that it is the radio core of the lensing galaxy. This implies that the B2045+265 lens system consists of a flat-spectrum radio source that is being lensed by another flat-spectrum radio source. Infrared images taken with the Hubble Space Telescope and the Keck I Telescope detect the lensed images of the background source and the lensing galaxy. The lensed images have relative positions and flux densities that are consistent with those seen at radio wavelengths. The lensing galaxy has magnitudes of J = 19.2, m(F160W) = 18.8, and K = 17.6 mag in a 1".9 diameter aperture, which corresponds to the size of the Einstein ring of the lens. Spectra of the system taken with the Keck I Telescope reveal a lens redshift of z(l) = 0.8673 and a source redshift of z(s) = 1.28. The lens spectrum is typical of an Sa galaxy. The image splitting and system redshifts imply that the projected mass inside the Einstein radius of the lensing galaxy is M(E) = 4.7 x 10(11) h(-1) M(.). An estimate of the light emitted inside the Einstein radius from the K magnitude gives a mass-to-light ratio in the rest-frame B band of (M/L(B))(E) = 20 h (M/L(B))(.). Both the mass and mass-to-light ratio are higher than what is seen in nearby Sa galaxies. In fact, the implied rotation velocity for the lensing galaxy is 2-3 times higher than what is seen in nearby spiral galaxies. The large projected mass inside the Einstein ring radius may be the result of a significant amount of dark matter in the system, perhaps from a compact group of galaxies associated with the primary lensing galaxy; however, it may also arise from a misidentification of the source redshift. A simple model of the gravitational potential of the lens reproduces the image positions well, but further modeling is required to satisfy the constraints from the image flux density ratios. With further observations and modeling, this lens may yield an estimate of H(o).

We report the discovery of a new double image gravitational lens system B1030+074 which was found during the Jodrell Bank - VLA Astrometric Survey (JVAS). We have collected extensive radio data on the system using the VLA, MERLIN, the EVN and the VLBA as well as HST WFPC2 and NICMOS observations. The lensed images are separated by 1.56 arcseconds and their flux density ratio at centimetric wavelengths is approximately 14:1 although the ratio is slightly frequency dependent and the images appear to be time variable. The HST pictures show both the lensed images and the lensing galaxy close to the weaker image. The lensing galaxy has substructure which could be either part of the galaxy or a companion object. We have modeled B1030+074 using a Singular Isothermal Ellipsoid that yielded a time delay of 156/h(50) days. This lens is likely to be suitable for the measurement of the Hubble constant.

New HST and NOT observations of the gravitational lens B1600+434(1) suggest that the lensing galaxy is an edge-on spiral galaxy.(3) We have used these observations to constrain the velocity dispersion (sigma(parallel to) > 150 km/s) and oblateness (q(halo) = (c/a)(rho) > 0.5) of dark matter halo around the luminous galaxy and to calculate the time-delay between the two lens images. The time delay is found to be (54+/-3)/h(50) days using a non-singular isothermal ellipsoid (NIE) halo and (70 +/- 4)/h(50) days using a modified Hubble profile (MHP) halo model around the spiral galaxy. As B1600+434 has shown to be strongly variable in the radio (WSRT) and optical, the system is potentially useful in determining a value for the Hubble constant, once the time delay has been measured.

We present a new measurement of the Hubble Constant H0 and other cosmological parameters based on the joint analysis of three multiply imaged quasar systems with measured gravitational time delays. First, we measure the time delay of HE 0435-1223 from 13-yr light curves obtained as part of the COSMOGRAIL project. Companion papers detail the modelling of the main deflectors and line-of-sight effects, and how these data are combined to determine the time-delay distance of HE 0435-1223. Crucially, the measurements are carried out blindly with respect to cosmological parameters in order to avoid confirmation bias. We then combine the time-delay distance of HE 0435-1223 with previous measurements from systems B1608+656 and RXJ1131-1231 to create a Time Delay Strong Lensing probe (TDSL). In flat Λ cold dark matter (ΛCDM) with free matter and energy density, we find H0 = 71.9-3.0+2.4 km s-1 Mpc-1 and ΩΛ = 0.62-0.35+0.24. This measurement is completely independent of, and in agreement with, the local distance ladder measurements of H0. We explore more general cosmological models combining TDSL with other probes, illustrating its power to break degeneracies inherent to other methods. The joint constraints from TDSL and Planck are H0 = 69.2-2.2+1.4 km s-1 Mpc-1, ΩΛ = 0.70-0.01+0.01 and Ωk = 0.003-0.006+0.004 in open ΛCDM and H0 = 79.0-4.2+4.4 km s-1 Mpc-1, Ωde = 0.77-0.03+0.02 and w = -1.38-0.16+0.14 in flat wCDM. In combination with Planck and baryon acoustic oscillation data, when relaxing the constraints on the numbers of relativistic species we find Neff = 3.34-0.21+0.21 in NeffΛCDM and when relaxing the total mass of neutrinos we find ∑mν =0.182 eV in mνλCDM. Finally, in an open wCDM in combination with Planck and cosmic microwave background lensing, we find H0 = 77.9-4.2+5.0 km s-1 Mpc-1, Ωde = 0.77-0.03+0.03, Ωk = -0.003-0.004+0.004 and w = -1.37-0.23+0.18.

The degeneracy among the disk, bulge and halo contributions to galaxy rotation curves prevents an understanding of the distribution of baryons and dark matter in disk galaxies. In an attempt to break this degeneracy, we present an analysis of the spiral galaxy strong gravitational lens SDSS J2141-0001, discovered as part of the SLACS survey. We present new Hubble Space Telescope multicolor imaging, gas and stellar kinematics data derived from long-slit spectroscopy, and K-band LGS adaptive optics imaging, both from the Keck telescopes. We model the galaxy as a sum of concentric axisymmetric bulge, disk and halo components and infer the contribution of each component, using information from gravitational lensing and gas kinematics. This analysis yields a best-fitting total (disk plus bulge) stellar mass of log_{10}(Mstar/Msun) = 10.99(+0.11,-0.25). The photometric data combined with stellar population synthesis models yield log_{10}(Mstar/Msun) = 10.97\pm0.07, and 11.21\pm0.07 for the Chabrier and Salpeter IMFs, respectively. Accounting for the expected gas fraction of \simeq 20% reduces the lensing plus kinematics stellar mass by 0.10\pm0.05 dex, resulting in a Bayes factor of 11.9 in favor of a Chabrier IMF. The dark matter halo is roughly spherical, with minor to major axis ratio q_{halo}=0.91(+0.15,-0.13). The dark matter halo has a maximum circular velocity of V_{max}=276(+17,-18) km/s, and a central density parameter of log_{10}\Delta_{V/2}=5.9(+0.9,-0.5). This is higher than predicted for uncontracted dark matter haloes in LCDM cosmologies, log_{10}\Delta_{V/2}=5.2, suggesting that either the halo has contracted in response to galaxy formation, or that the halo has a higher than average concentration. At 2.2 disk scale lengths the dark matter fraction is f_{DM}=0.55(+0.20,-0.15), suggesting that SDSS J2141-0001 is sub-maximal., Comment: 24 pages, 20 figures, accepted to MNRAS, minor changes