Prof Gavin Dalton

Abstract:

We present CCD (charge-coupled device) photometry for galaxies around 204 bright (mZ < 15.5) Zwicky galaxies in the equatorial extension of the APM Galaxy Survey, sampling an area over 400 deg2, which extends 6 h in right ascension. We fit a best linear relation between the Zwicky magnitude system, mZ, and the CCD photometry, BCCD, by doing a likelihood analysis that corrects for Malmquist bias. This fit yields a mean scale error in Zwicky of 0.38 mag mag-1: i.e. ΔmZ ≃ (0.62 ± 0.05)ΔBCCD and a mean zero-point of 〈BCCD - mZ〉 = -0.35 ± 0.15 mag. The scatter around this fit is about 0.4 mag. Correcting the Zwicky magnitude system with the best-fitting model results in a 60 per cent lower normalization and 0.35-mag brighter M* in the luminosity function. This brings the CfA2 luminosity function closer to the other low-redshift estimations (e.g. Stromlo-APM or LCRS). We find a significant positive angular correlation of magnitudes and position in the sky at scales smaller than about 5 arcmin, which corresponds to a mean separation of 120h-1 kpc. We also present colours, sizes and ellipticities for galaxies in our fields, which provides a good local reference for the studies of galaxy evolution.

Abstract:

We estimate the cluster-galaxy cross-correlation function (ξcg), from the APM galaxy and galaxy cluster surveys. We obtain estimates both in real space from the inversion of projected statistics and in redshift space using the galaxy and cluster redshift samples. The amplitude of ξcg is found to be almost independent of cluster richness. At large separations, r ≳ 5 h-1 Mpc (h = H0/100 km s-1 Mpc-1, where H0 is the Hubble constant), ξcg has a similar shape to the galaxy-galaxy and cluster-cluster autocorrelation functions. ξcg in redshift space can be related to the real-space ξcg 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 an accurate description of the averaged infall velocity of matter into galaxy clusters. We use this model to estimate β(β = Ω0.6/b, where b is the linear bias parameter), and find that it tends to overestimate the true result in simulations by only ∼10-30 per cent. Application to the APM results yields β = 0.46 with β < 0.73 at 95 per cent confidence. This measure is complementary to the estimates made of the density parameter from larger scale bulk flows and from the virialized regions of clusters on smaller scales. We also compare the APM ξcg and galaxy autocorrelations directly with the mass correlation and cluster-mass correlations in COBE-normalized simulations of popular cosmological models, and derive two independent estimates of the galaxy biasing expected as a function of scale. This analysis reveals that both low-density and critical-density COBE-normalized cold dark matter (CDM) models require anti-biasing by a factor ∼2 on scales r ≤ 2 h-1 Mpc, and that the mixed dark matter (MDM) model is consistent with a constant biasing factor on all scales. The critical-density CDM model also suffers from the usual deficit of power on large scales (r ≳ 20 h-1 Mpc). 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 ξcg yields an estimate of the value of Ω0.6 needed in each model. We find that only the low-Ω model is fully consistent with observations, with MDM marginally excluded at the ∼2σ level.