H0 Revisited

I reanalyse the Riess et al. (2011, hereafter R11) Cepheid data using the revised geometric maser distance to NGC 4258 of Humphreys et al. (2013). I explore different outlier rejection criteria designed to give a reduced chi-squared of unity and compare the results with the R11 rejection algorithm, which produces a reduced chi-squared that is substantially less than unity and, in some cases, to underestimates of the errors on parameters. I show that there are sub-luminous low metallicity Cepheids in the R11 sample that skew the global fits of the period-luminosity relation. This has a small but non-negligible impact on the global fits using NGC 4258 as a distance scale anchor, but adds a poorly constrained source of systematic error when using the Large Magellanic Cloud (LMC) as an anchor. I also show that the small Milky Way (MW) Cepheid sample with accurate parallax measurements leads to a distance to NGC 4258 that is in tension with the maser distance. I conclude that H0 based on the NGC 4258 maser distance is H0 = 70.6 +/- 3.3 km/s/Mpc compatible within 1 sigma with the recent determination from Planck for the base six-parameter LCDM cosmology. If the H-band period-luminosity relation is assumed to be independent of metallicity and the three distance anchors are combined, I find H0 = 72.5 +/- 2.5 km/s/Mpc, which differs by 1.9 sigma from the Planck value. The differences between the Planck results and these estimates of H0 are not large enough to provide compelling evidence for new physics at this stage.Comment: 14 page

Statistical Inconsistencies in the KiDS-450 Dataset

The Kilo-Degree Survey (KiDS) has been used in several recent papers to infer constraints on the amplitude of the matter power spectrum and matter density at low redshift. Some of these analyses have claimed tension with the Planck $\Lambda \mathrm{CDM}$ cosmology at the $\sim 2-3\sigma$ level, perhaps indicative of new physics. However, Planck is consistent with other low redshift probes of the matter power spectrum such as redshift space distortions and the combined galaxy-mass and galaxy-galaxy power spectra. Here we perform consistency tests of the KiDS data, finding internal tensions for various cuts of the data at $\sim 2.2 - 3.5\sigma$ significance. Until these internal tensions are understood, we argue that it is premature to claim evidence for new physics from KiDS. We review the consistency between KiDS and other weak lensing measurements of $S_8$, highlighting the importance of intrinsic alignments for precision cosmology.Comment: Comments: 8 pages, 5 figures. Accepted for publication in MNRA

The APM cluster-galaxy cross-correlation function : Constraints on Omega and galaxy bias

(abridged) We estimate the cluster-galaxy cross-correlation function (Xi_cg), from the APM galaxy and galaxy cluster surveys, both in real space from the inversion of projected statistics and in redshift space using the galaxy and cluster redshift samples. The amplitude of Xi_cg is found to be almost independent of cluster richness. At large separations, r >~5 h^-1 Mpc, Xi_cg has a similar shape to the galaxy-galaxy and cluster-cluster autocorrelation functions. Xi_cg in redshift space can be related to the real space Xi_cg by convolution with an appropriate velocity field model. Here we apply a spherical collapse model, which we have tested against N-body simulations, finding that it provides a surprisingly accurate description of the averaged infall velocity of matter into galaxy clusters. We use this model to estimate beta (Omega^{0.6}/b) and find that it tends to overestimate the true result in simulations by only ~10-30%. Application to the APM results yields beta=0.43 with beta < 0.87 at 95% confidence. We also compare the APM Xi_cg and galaxy autocorrelations to results from popular cosmological models and derive two independent estimates of the galaxy biasing expected as a function of scale. Both low and critical density CDM models require anti-biasing by a factor ~2 on scales r <~ 2 h^-1Mpc and an MDM model is consistent with a constant biasing factor on all scales. We use the velocity fields predicted from the different models to distort the APM real space cross-correlation function. Comparison with the APM redshift space Xi_cg yields an estimate of the value of Omega^0.6 needed in each model. Only the low Omega model is fully consistent with observations, with MDM marginally excluded at the ~2 sigma level.Comment: Latex (mn.sty), 17 pages, 16 ps figs, submitted to MNRA

The Power Spectrum of Rich Clusters of Galaxies on Large Spatial Scales

We present an analysis of the redshift-space power spectrum, $P(k)$, of rich clusters of galaxies based on an automated cluster catalogue selected from the APM Galaxy Survey. We find that $P(k)$ can be approximated by a power law, P(k)\proptok^{n}, with $n\approx-1.6$ over the wavenumber range 0.04\hr. Over this range of wavenumbers, the APM cluster power spectrum has the same shape as the power spectra measured for optical and IRAS galaxies. This is consistent with a simple linear bias model in which different tracers have the same power spectrum as that of the mass distribution but shifted in amplitude by a constant biasing factor. On larger scales, the power spectrum of APM clusters flattens and appears to turn over on a scale k \sim 0.03\hmpcrev. We compare the power spectra estimated from simulated APM cluster catalogues to those estimated directly from cubical N-body simulation volumes and find that the APM cluster survey should give reliable estimates of the true power spectrum at wavenumbers k \simgt 0.02\hmpcrev. These results suggest that the observed turn-over in the power spectrum may be a real feature of the cluster distribution and that we have detected the transition to a near scale-invariant power spectrum implied by observations of anisotropies in the cosmic microwave background radiation. The scale of the turn-over in the cluster power spectrum is in good agreement with the scale of the turn-over observed in the power spectrum of APM galaxies.Comment: 9 pages, 7 ps figures, two style files, submitted to MNRAS. Un-xxx-ed version available at http://www-astro.physics.ox.ac.uk/research/preprints/aug97/cluspaper.ps.g

On the rotation and clustering of galaxies

The tidal torque theory for the origin of galactic rotation has been investigated using N-body computer simulations. The results show that this process is considerably less efficient than was previously thought, though consistent with recent observations of the rotation of giant elliptical galaxies. Spectroscopic observations of three elliptical galaxies along both major and minor axes are presented. Two galaxies were found to be slowly rotating, inconsistent with rotationally supported oblate spheroids. No convincing evidence for minor axis rotation was found. An investigation of the clustering of particles in Friedmann models of the Universe has been carried out using N-body simulations. The results of these computations have been analysed in terms of the two- and three- point correlation functions and various velocity statistics. It is found that the shapes of the two- and three-point functions are dependent upon the cosmological density parameter Ω, and that the shape of the two-point function is in rough agreement with simple analytic treatments based on the homogeneous spherical cluster model for the collapse of protoclusters. The effects of particle discreteness and two-body relaxation, which are particularly important in the N-body models are examined. The approach is compared to the detailed kinetic theory calculations of Davis and Peebles. The cosmological implications of the results are discussed. Other statistics, such as the multiplicity function are also considered