We present the results of a deep Hubble Space Telescope (HST) exposure of the nearby globular cluster NGC 6397, focussing attention on the cluster's white dwarf cooling sequence. This sequence is shown to extend over 5 magnitudes in depth, with an apparent cutoff at magnitude F 814W ∼ 27.6. We demonstrate, using both artificial star tests and the detectability of background galaxies at fainter magnitudes, that the cutoff is real and represents the truncation of the white dwarf luminosity function in this cluster. We perform a detailed comparison between cooling models and the observed distribution of white dwarfs in colour and magnitude, taking into account uncertainties in distance, extinction, white dwarf mass, progenitor lifetimes, binarity and cooling model uncertainties. After marginalising over these variables, we obtain values for the cluster distance modulus and age of µ 0 = 12.02 ± 0.06 and T c = 11.47 ± 0.47 Gyr (95% confidence limits). Our inferred distance and white dwarf initial-final mass relations are in good agreement with other independent determinations, and the cluster age is consistent with, but more precise than, prior determinations made using the main sequence turnoff method. In particular, within the context of the currently accepted ΛCDM cosmological model, this age places

We explore the dark matter distribution in ms1224.7+2007 using the gravitational distortion of the images of faint background galaxies. Projected mass image reconstruction reveals a highly significant concentration coincident with the X-ray and optical location. The concentration is seen repeatably in reconstructions from independent subsamples and the azimuthally averaged tangential shear pattern is also clearly seen in the data. The projected mass within a 2.76 ′ radius aperture is ≃ 3.5 × 10 14 h −1 M ⊙ . This is ≃ 3 times larger than that predicted if mass traces light with M/L = 275h as derived from virial analysis. It is very hard to attribute the discrepancy to a statistical fluctuation, and a further indication of a significant difference between the mass and the light comes from a second mass concentration which is again seen in independent subsamples but which is not seen at all in the cluster light. We find a mass per galaxy visible to I = 22 of ≃ 8 × 10 12 h −1 M ⊙ which, if representative of the universe, implies a density parameter Ω ∼ 2. We find a null detection of any net shear from large-scale structure with a precision of 0.9% per component. This is much smaller than the possible detection in a recent comparable study, and the precision here is comparable to to the minimum level of rms shear fluctuations implied by observed large-scale structure.

We present the white dwarf sequence of the globular cluster M4, based on a 123 orbit Hubble Space Telescope exposure, with limiting magnitude V = 30, I = 28. The white dwarf luminosity function rises sharply for I >25.5, consistent with the behaviour expected for a burst population. The white dwarfs of M4 extend to approximately 2.5 magnitudes fainter than the peak of the local Galactic disk white dwarf luminosity function. This demonstrates a clear and significant age difference between the Galactic disk and the halo globular cluster M4. Using the same standard white dwarf models (Hansen 1999) to fit each luminosity function yields ages of 7.3 +/- 1.5 Gyr for the disk and 12.7 +/- 0.7 Gyr for M4 (2-sigma statistical errors).Comment: 14 pages, 4 diagrams. Accepted for publication in the Astrophysical Journal Letter