Prof Gavin Dalton

Abstract:

We measure moments of the galaxy count probability distribution function in the Two-degree Field Galaxy Redshift Survey (2dFGRS). The survey is divided into volume-limited subsamples in order to examine the dependence of the higher-order clustering on galaxy luminosity. We demonstrate the hierarchical scaling of the averaged p-point galaxy correlation functions, ξ̄^p, up to p = 6. The hierarchical amplitudes, S^p = S2S^p-1, are approximately independent of the cell radius used to smooth the galaxy distribution on small to medium scales. On larger scales we find that the higher-order moments can be strongly affected by the presence of rare, massive superstructures in the galaxy distribution. The skewness S3 has a weak dependence on luminosity, approximated by a linear dependence on log luminosity. We discuss the implications of our results for simple models of linear and non-linear bias that relate the galaxy distribution to the underlying mass.

Abstract:

The Oxford-Dartmouth Thirty Degree Survey (ODTS) is a deep, wide, multiband imaging survey designed to cover a total of 30 deg² in BV Ri′Z, with a subset of U- and K-band data, in four separate fields of 5-10 deg² centred at 00:18:24 +34:52, 09:09:45 +40:50, 13:40:00 +02:30 and 16:39:30 +45:24. Observations have been made using the Wide Field Camera on the 2.5-m Isaac Newton Telescope (INT) in La Palma to average limiting depths (5σ Vega, aperture magnitudes) of U = 24.8, B = 25.6, V = 25.0, R = 24.6 and i′ = 23.5, with observations taken in ideal conditions reaching the target depths of U = 25.3, B = 26.2, V = 25.7, R = 25.4 and i′ = 24.6. The INT Z-band data were found to be severely effected by fringing and, consequently, are now being obtained at the MDM observatory in Arizona. A complementary K-band survey has also been carried out at MDM, reaching an average depth of K5σ ≈ 18.5. At present, approximately 23 deg² of the ODTS have been observed, with 3.5 deg² of the K-band survey completed. This paper details the survey goals, field selection, observation strategy and data reduction procedure, focusing on the photometric calibration and catalogue construction. Preliminary photometric redshifts have been obtained for a subsample of the objects with R ≤ 23. These results are presented alongside a brief description of the photo-metric redshift determination technique used. The median redshift of the survey is estimated to be z ≈ 0.7 from a combination of the ODTS photometric redshifts and comparison with the redshift distributions of other surveys. Finally, galaxy number counts for the ODTS are presented which are found to be in excellent agreement with previous studies.

Abstract:

We measure the redshift-space reduced void probability function (VPF) for 2dFGRS volume-limited galaxy samples covering the absolute magnitude range MbJ -5log10 h = -18 to -22. Theoretically, the VPF connects the distribution of voids to the moments of galaxy clustering of all orders, and can be used to discriminate clustering models in the weakly nonlinear regime. The reduced VPF measured from the 2dFGRS is in excellent agreement with the paradigm of hierarchical scaling of the galaxy clustering moments. The accuracy of our measurement is such that we can rule out, at a very high significance, popular models for galaxy clustering, including the lognormal distribution. We demonstrate that the negative binomial model gives a very good approximation to the 2dFGRS data over a wide range of scales, out to at least 20 h-1 Mpc. Conversely, the reduced VPF for dark matter in a A cold dark matter ( ACDM) universe does appear to be lognormal on small scales but deviates significantly beyond ∼4 h-1 Mpc. We find little dependence of the 2dFGRS reduced VPF on galaxy luminosity. Our results hold independently in both the North and South Galactic Pole survey regions.