Aims. We test cosmological models of structure formation using the rotation curve of the nearest spiral galaxy, M 31, determined using a recent deep, full-disk 21-cm imaging survey smoothed to 466 pc resolution. Methods. We fit a tilted ring model to the HI data from 8 to 37 kpc and establish conclusively the presence of a dark halo and its density distribution via dynamical analysis of the rotation curve. Results. The disk of M 31 warps from 25 kpc outwards and becomes more inclined with respect to our line of sight. Newtonian dynamics without a dark matter halo provide a very poor fit to the rotation curve. In the framework of modified Newtonian dynamic (MOND) however the 21-cm rotation curve is well fitted by the gravitational potential traced by the baryonic matter density alone. The inclusion of a dark matter halo with a density profile as predicted by hierarchical clustering and structure formation in a ΛCDM cosmology makes the mass model in newtonian dynamic compatible with the rotation curve data. The dark halo concentration parameter for the best fit is C = 12 and its total mass is 1.2 × 10 12 M . If a dark halo model with a constant-density core is considered, the core radius has to be larger than 20 kpc in order for the model to provide a good fit to the data. We extrapolate the best-fit ΛCDM and constant-density core mass models to very large galactocentric radii, comparable to the size of the dark matter halo. A comparison of the predicted mass with the M 31 mass determined at such large radii using other dynamical tracers, confirms the validity of our results. In particular the ΛCDM dark halo model which best fits the 21-cm data well reproduces the mass of M 31 traced out to 560 kpc. Our best estimate for the total mass of M 31 is 1.3 × 10 12 M , with 12% baryonic fraction and only 6% of the baryons in the neutral gas phase.

We have searched for star‐forming galaxies at z≈ 7–10 by applying the Lyman‐break technique to newly released Y‐, J‐ and H‐band images (1.1, 1.25 and 1.6 μm) from Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. By comparing these images of the Hubble Ultra Deep Field with the Advanced Camera for Surveys (ACS) z′‐band (0.85 μm) images, we identify objects with red colours, (z′−Y)AB > 1.3, consistent with the Lyman α forest absorption at z≈ 6.7–8.8. We identify 12 of these z′‐drops down to a limiting magnitude YAB < 28.5 (equivalent to a star formation rate of 1.3 M⊙ yr−1 at z= 7.1), all of which are undetected in the other ACS filters. We use the WFC3 J‐band image to eliminate contaminant low‐mass Galactic stars, which typically have redder colours than z≈ 7 galaxies. One of our z′‐drops is probably a T‐dwarf star. The z≈ 7 z′‐drops appear to have much bluer spectral slopes than Lyman‐break galaxies at lower redshift. Our brightest z′‐drop is not present in the NICMOS J‐band image of the same field taken 5 years before, and is a possible transient object. From the 10 remaining z≈ 7 candidates we determine a lower limit on the star formation rate density of 0.0017 M⊙ yr−1 Mpc−3 for a Salpeter initial mass function, which rises to 0.0025–0.004 M⊙ yr−1 Mpc−3 after correction for luminosity bias. The star formation rate density is a factor of ≈10 less than that of Lyman‐break galaxies at z= 3–4, and is about half the value at z≈ 6. We also present the discovery of seven Y‐drop objects with (Y−J)AB > 1.0 and JAB < 28.5 which are candidate star‐forming galaxies at higher redshifts (z≈ 8–9). We find no robust J‐drop candidates at z≈ 10. While based on a single deep field, our results suggest that this star formation rate density would produce insufficient Lyman continuum photons to reionize the Universe unless the escape fraction of these photons is extremely high (fesc > 0.5), and the clumping factor of the Universe is low. Even then, we need to invoke a large contribution from galaxies below our detection limit (a steep faint‐end slope). The apparent shortfall in ionizing photons might be alleviated if stellar populations at high redshift are of low metallicity or have a top‐heavy initial mass function.

The acquisition of deep near‐IR imaging with Wide Field Camera 3 on the Hubble Space Telescope has provided the opportunity to study the very high redshift Universe. For galaxies up to z≈ 7.7 sufficient wavelength coverage exists to probe the rest‐frame ultraviolet (UV) continuum without contamination from either Lyman α emission or the Lyman α break. In this work we use near‐infrared (near‐IR) imaging to measure the rest‐frame UV continuum colours of galaxies at 4.7 < z < 7.7. We have carefully defined a colour–colour selection to minimize any inherent bias in the measured UV continuum slope for the drop‐out samples. For the highest redshift sample (6.7 < z < 7.7), selected as zf850lp‐band drop‐outs, we find mean UV continuum colours approximately equal to zero (AB), consistent with a dust‐free, solar metallicity, star‐forming population (or a moderately dusty population of low metallicity). At lower redshift we find that the mean UV continuum colours of galaxies (over the same luminosity range) are redder, and that galaxies with higher luminosities are also slightly redder on average. One interpretation of this is that lower redshift and more luminous galaxies are dustier; however, this interpretation is complicated by the effects of the star formation history and metallicity and potentially the initial mass function on the UV continuum colours.

Following our previous spectroscopic observations of z > 7 galaxies with Gemini/GNIRS and VLT/XSHOOTER, which targeted a total of 8 objects, we present here our results from a deeper and larger VLT/FORS2 spectroscopic sample of Wide Field Camera 3 selected z > 7 candidate galaxies. With our FORS2 setup we cover the 737-1070nm wavelength range, enabling a search for Lyman-α in the redshift range spanning 5.06 -7.80. We target 22 z-band dropouts and find no evidence of Lyman-α emission, with the exception of a tentative detection (< 5σ, which is our adopted criterion for a secure detection) for one object. The upper limits on Lyman-α flux and the broad-band magnitudes are used to constrain the rest-frame Equivalent Widths for this line emission. We analyse our FORS2 observations in combination with our previous GNIRS and XSHOOTER observations, and suggest that a simple model where the fraction of high rest-frame Equivalent Width emitters follows the trend seen at z = 3 − 6.5 is inconsistent with our non-detections at z ∼ 7.8 at the 96% confidence level. This may indicate that a significant neutral HI fraction in the intergalactic medium suppresses Lyman-α, with an estimated neutral fraction χ HI ∼ 0.5, in agreement with other estimates.