We present the MUSE Hubble Ultra Deep Survey, a mosaic of nine MUSE fields covering 90% of the entire HUDF region with a 10-h deep exposure time, plus a deeper 31-h exposure in a single 1.15 arcmin 2 field. The improved observing strategy and advanced data reduction results in datacubes with sub-arcsecond spatial resolution (0 . 65 at 7000 Å) and accurate astrometry (0 . 07 rms). We compare the broadband photometric properties of the datacubes to HST photometry, finding a good agreement in zeropoint up to m AB = 28 but with an increasing scatter for faint objects. We have investigated the noise properties and developed an empirical way to account for the impact of the correlation introduced by the 3D drizzle interpolation. The achieved 3σ emission line detection limit for a point source is 1.5 and 3.1 × 10 −19 erg s −1 cm −2 for the single ultra-deep datacube and the mosaic, respectively. We extracted 6288 sources using an optimal extraction scheme that takes the published HST source locations as prior. In parallel, we performed a blind search of emission line galaxies using an original method based on advanced test statistics and filter matching. The blind search results in 1251 emission line galaxy candidates in the mosaic and 306 in the ultradeep datacube, including 72 sources without HST counterparts (m AB > 31). In addition 88 sources missed in the HST catalog but with clear HST counterparts were identified. This data set is the deepest spectroscopic survey ever performed. In just over 100 h of integration time, it provides nearly an order of magnitude more spectroscopic redshifts compared to the data that has been accumulated on the UDF over the past decade. The depth and high quality of these datacubes enables new and detailed studies of the physical properties of the galaxy population and their environments over a large redshift range.

The mass-function of dwarf satellite galaxies that are observed around Local Group galaxies substantially differs from simulations 1-5 based on cold dark matter: the simulations predict many more dwarf galaxies than are seen. The Local Group, however, may be anomalous in this regard 6, 7 . A massive dark satellite in an early-type lens galaxy at z = 0.222 was recently found 8 using a new method based on gravitational lensing 9, 10 , suggesting that the mass fraction contained in substructure could be higher than is predicted from simulations. The lack of very low mass detections, however, prohibited any constraint on their mass function. Here we report the presence of a 1.9 ± 0.1 × 10 8 M ⊙ dark satellite in the Einstein-ring system JVAS B1938+666 (ref. 11) at z = 0.881, where M ⊙ denotes solar mass. This satellite galaxy has a mass similar to the Sagittarius 12 galaxy, which is a satellite of the Milky Way. We determine the logarithmic slope of the mass function for substructure beyond the local Universe to be α = 1.1 +0.6 −0.4 , with an average mass-fraction of f = 3.3 +3.6 −1.8 %, by combining data on both of these recently discovered galaxies. Our results are consistent with the predictions from cold dark matter simulations 13-15 at the 95 per cent confidence level,and therefore agree with the view that galaxies formed hierarchically in a Universe composed of cold dark matter.The gravitational lens system JVAS B1938+666 (ref. 11) has a bright infrared background galaxy at redshift 2.059 (ref. 16), which is gravitationally lensed into an almost complete Einstein ring of diameter ∼ 0.9 arcseconds by a massive elliptical galaxy at redshift 0. 881 (ref. 17). The bright, highly-magnified Einstein ring made this system an excellent candidate in which to to search for surface brightness anomalies caused by very low mass (dark matter) substructure in the halo around the high redshift elliptical lens galaxy. The presence of a low-mass substructure (e.g. a luminous or dark satellite galaxy; also denoted as substructure hereafter) in the lens galaxy 1

We present an analysis of MUSE observations obtained on the massive Frontier Fields cluster Abell 2744. This new dataset covers the entire multiply-imaged region around the cluster core. The combined catalog consists of 514 spectroscopic redshifts (with 414 new identifications). We use this redshift information to perform a strong-lensing analysis revising multiple images previously found in the deep Frontier Field images, and add three new MUSE-detected multiply-imaged systems with no obvious HST counterpart. The combined strong lensing constraints include a total of 60 systems producing 188 images altogether, out of which 29 systems and 83 images are spectroscopically confirmed, making Abell 2744 one of the most well-constrained clusters to date. Thanks to the large amount of spectroscopic redshifts we model the influence of substructures at larger radii, using a parametrisation including two cluster-scale components in the cluster core and several group-scale in the outskirts. The resulting model accurately reproduces all the spectroscopic multiple systems, reaching an rms of 0.67 in the image plane. The large number of MUSE spectroscopic redshifts gives us a robust model, which we estimate reduces the systematic uncertainty on the 2D mass distribution by up to ∼ 2.5 times the statistical uncertainty in the cluster core. In addition, from a combination of the parametrisation and the set of constraints, we estimate the relative systematic uncertainty to be up to 9% at 200kpc.

We present the measurement of the Hubble Constant, H 0 , with three strong gravitational lens systems. We describe a blind analysis of both PG 1115+080 and HE 0435−1223 as well as an extension of our previous analysis of RXJ 1131−1231. For each lens, we combine new adaptive optics (AO) imaging from the Keck Telescope, obtained as part of the SHARP AO effort, with Hubble Space Telescope (HST ) imaging, velocity dispersion measurements, and a description of the line-of-sight mass distribution to build an accurate and precise lens mass model. This mass model is then combined with the COSMOGRAIL measured time delays in these systems to determine H 0 . We do both an AO-only and an AO+HST analysis of the systems and find that AO and HST results are consistent. After unblinding, the AO-only analysis gives H 0 = 82.8 +9.4 −8.3 km s −1 Mpc −1 for PG 1115+080, H 0 = 70.1 +5.3 −4.5 km s −1 Mpc −1 for HE 0435−1223, and H 0 = 77.0 +4.0 −4.6 km s −1 Mpc −1 for RXJ 1131−1231. The joint AOonly result for the three lenses is H 0 = 75.6 +3.2 −3.3 km s −1 Mpc −1 . The joint result of the AO+HST analysis for the three lenses is H 0 = 76.8 +2.6−2.6 km s −1 Mpc −1 . All of the above results assume a flat Λ cold dark matter cosmology with a uniform prior on Ω m in [0.05, 0.5] and H 0 in [0, 150] km s −1 Mpc −1 . This work is a collaboration of the SHARP and H0LiCOW teams, and shows that AO data can be used as the high-resolution imaging component in lens-based measurements of H 0 . The full time-delay cosmography results from a total of six strongly lensed systems are presented in a companion paper.

Context. Spectroscopic surveys of massive galaxy clusters reveal the properties of faint background galaxies thanks to the magnification provided by strong gravitational lensing. Aims. We present a systematic analysis of integral-field-spectroscopy observations of 12 massive clusters, conducted with the Multi Unit Spectroscopic Explorer (MUSE). All data were taken under very good seeing conditions (∼0″​​.6) in effective exposure times between two and 15 h per pointing, for a total of 125 h. Our observations cover a total solid angle of ∼23 arcmin2 in the direction of clusters, many of which were previously studied by the MAssive Clusters Survey, Frontier Fields (FFs), Grism Lens-Amplified Survey from Space and Cluster Lensing And Supernova survey with Hubble programmes. The achieved emission line detection limit at 5σ for a point source varies between (0.77–1.5) × 10−18 erg s−1 cm−2 at 7000 Å. Methods. We present our developed strategy to reduce these observational data, detect continuum sources and line emitters in the datacubes, and determine their redshifts. We constructed robust mass models for each cluster to further confirm our redshift measurements using strong-lensing constraints, and identified a total of 312 strongly lensed sources producing 939 multiple images. Results. The final redshift catalogues contain more than 3300 robust redshifts, of which 40% are for cluster members and ∼30% are for lensed Lyman-α emitters. Fourteen percent of all sources are line emitters that are not seen in the available HST images, even at the depth of the FFs (∼29 AB). We find that the magnification distribution of the lensed sources in the high-magnification regime (μ = 2–25) follows the theoretical expectation of N(z) ∝ μ−2. The quality of this dataset, number of lensed sources, and number of strong-lensing constraints enables detailed studies of the physical properties of both the lensing cluster and the background galaxies. The full data products from this work, including the datacubes, catalogues, extracted spectra, ancillary images, and mass models, are made available to the community.

We present a strong lensing analysis on the massive cluster Abell 370 (A370; z = 0.375), using a combination of deep multi-band Hubble Space Telescope (HST) imaging and Multi-Unit Spectroscopic Explorer (MUSE) spectroscopy. From only two hours of MUSE data, we are able to measure 120 redshifts in the Southern BCG area, including several multiply-imaged lens systems. In total, we increase the number of multiply-imaged systems with a secure redshift from 4 to 15, nine of which are newly discovered. Of these, eight are located at z > 3, greatly extending the redshift range of spectroscopically-confirmed systems over previous work. Using these systems as constraints, we update a parametric lens model of A370, probing the mass distribution from cluster to galaxy scales. Overall, we find that a model with only two clusterscale dark matter halos (one for each BCG) does a poor job of fitting these new image constraints. Instead, two additional mass clumps -a central "bar" of mass located between the BCGs, and another clump located within a "crown" of galaxies in the Northern part of the cluster field -provide significant improvements to the fit. Additional physical evidence suggests these clumps are indeed real features of the system, but with relatively few image constraints in the crown region, this claim is difficult to evaluate from a modeling perspective. Additional MUSE observations of A370 covering the entire strong-lensing region will greatly help these efforts, further improving our understanding of this intriguing cluster.

A young star-forming galaxy at z = 3.5 with an extended Lyman α halo seen with MUSE ABSTRACTSpatially resolved studies of high redshift galaxies, an essential insight into galaxy formation processes, have been mostly limited to stacking or unusually bright objects. We present here the study of a typical (L * , M = 6 ×10 9 M ) young lensed galaxy at z = 3.5, observed with MUSE, for which we obtain 2D resolved spatial information of Ly α and, for the first time, of C iii] emission. The exceptional signal-to-noise of the data reveals UV emission and absorption lines rarely seen at these redshifts, allowing us to derive important physical properties (T e ∼15600 K, n e ∼300 cm −3 , covering fraction f c ∼ 0.4) using multiple diagnostics. Inferred stellar and gas-phase metallicities point towards a low metallicity object (Z stellar = ∼ 0.07 Z and Z ISM < 0.16 Z ). The Ly α emission extends over ∼10 kpc across the galaxy and presents a very uniform spectral profile, showing only a small velocity shift which is unrelated to the intrinsic kinematics of the nebular emission. The Ly α extension is ∼4 times larger than the continuum emission, and makes this object comparable to low-mass LAEs at low redshift, and more compact than the Lyman-break galaxies and Lyα emitters usually studied at high redshift. We model the Ly α line and surface brightness profile using a radiative transfer code in an expanding gas shell, finding that this model provides a good description of both observables.

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