## Determination of the local group gravitational acceleration using all-sky red-shift surveys

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2002##### Author

D'Mellow, Kenton J.

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##### Abstract

A consolidation of the Local Group velocity as inferred from measurements of the Cosmic Microwave Background, and that inferred from gravitational arguments, is of major importance to modern cosmology. Agreement between the two lends strength to modern ideas of structure formation, and can in principle, be used to estimate the matter density parameter, Ωm. Measurement of the change of the Local Group velocity with respect to increasingly distant objects can also be used as a probe of the mass of nearby structures. Modern methods of velocity determination employ information about the local matter density to construct an estimate of the local gravitational field. The most effective current method of probing the local matter density is by using all-sky galaxy redshift surveys to map the 3-dimensional distribution of nearby galaxies, and then use these galaxies to infer the distribution of matter throughout the local Universe. However, any practical determination of the Local Group velocity is fraught with errors. Among these are: distortion effects on the data introduced by using an galaxy’s redshift as a measure of its distance; sampling effects caused by the approximate representation of a continuous mass distribution by a sample of discrete galaxies; intrinsic uncertainty caused by only sampling a nearby finite subset of the (effectively infinite) mass distribution; and the uncertainty in the determination of a correction for non-linear effects caused by nearby massive structures. This thesis aims to give a definitive measure of the velocity of the Local Group from gravitational arguments while attempting to minimise errors in the calculation. Iterative techniques are used to converge on a self consistent solution to the Local Group velocity and surrounding spatial galaxy distribution. To minimise intrinsic survey error, a new dataset— the Behind The Plane (B T P ) galaxy redshift survey has been completed and analysed. The BTP is the low-latitude extension to the previously completed Point Source Catalogue redshift (PSCz) survey of galaxies. Near-infrared and radio techniques were used to identify and measure optically obscured galaxies that were excluded from the PSCz, increasing the overall sky coverage from 84% to 93%. This high degree of sky coverage makes the PSCz + BTP the best available dataset for dynamical studies of the local Universe. The reduction of the gap in sky coverage significantly reduces uncertainty in dynamical predictions, especially as the missing strip behind the Milky Way is known to include the Great Attractor. The major result of this work is the possible discovery o f an unexpectedly large mass concentration beyond the Great Attractor, at an approximate distance of cz = 20,000kms-1 in the direction l = 300,b = 0. The misalignment between the inferred Local Group velocity vector and the CMB temperature dipole significantly increases, and β parameter estimates yield inconsistency with many other current and reliable estimates in the literature, if this structure is excluded. Upon the inclusion of this concentration, estimates for the Local Group velocity center upon I = 245, b = 30, and yield a value of βIRAS = 0-65 ± 0.01. Directional misalignment is consistent to within 2-σ, but is robust across significant variation in both the data and calculation method applied.