Mapping the Large Scale Structure of the Universe

In this thesis I will be examining the study of the large scale structure of the universe. In particular, I will be looking at the role of peculiar motions of galaxies - ie motions that deviate from the uniform Hubble expansion of the universe. The study of these motions holds much promise for cosmology, but there are considerable problems with measuring and mapping them, a number of which I will be addressing in the course of this work. However, I will start in chapter 1 by giving a brief introduction to the theory behind the formation of structure in the universe and the current state of our knowledge about its form and history. The framework provided by the Big Bang theory of the origins of the universe, has withstood a number of observational tests with remarkable success and this has enabled cosmologists to extend and refine the theory. This enlarged model can also be tested by observations and in its success or failure, improve our understanding and narrow the limits of future research. In chapter 2, therefore, I will describe one of the most powerful set of observational tools - the measurement of the local velocity and density fields. Inherent in the Big Bang theory, and almost any other reasonable explanation of the universe, is the idea of formation and evolution of structure. The velocity and density fields tell us about the current state of that evolution and, therefore, will set very strong constraints on any theory. However, as I will show, the measurement and analysis of such fields is a complicated and difficult business. In particular, many methods rely on estimation of the distances to galaxies and this estimation is subject to very large errors. In the latter half of the chapter, I will be considering this problem and looking at some of the most popular distance estimators and some of the systematic errors or biases that they can introduce to field recoveries. The most commonly used estimators (Tully-Fisher and Dn-sigma techniques) rely on a strong correlation between two observable properties of each galaxy - one that varies with distance and another that is distance-independent. Therefore, by using the distance-independent quantity to estimate the absolute values of the dependent observable, the distance can be estimated. However, one of the major problems with such distance estimation techniques is calibration - exactly how are the two parameters of the chosen relation correlated? In chapter 3 I will describe a new method of calibration that attempts to combine a number of clusters of galaxies together into one large cluster that can then be used as a calibration yardstick. This is an extension of the standard technique where a single cluster is used. While describing this technique, I will also demonstrate its efficacy with several carefully devised tests that will show clearly how it improves over the single-cluster approach. Given a set of distance estimates, we now wish to derive some information about the local velocity field. One very successful method for doing this is described in chapter 4 - the POTENT method (Bertschinger and Dekel, 1989). With this technique, expansion of the universe is removed from the velocity field by comparing the recession velocities of galaxies with their estimated distances and the resultant smoothed peculiar velocity field recovered under the assumption of potential flow. I will describe my implementation of the method and go on to test it with a variety of different forms of distance estimator thereby demonstrating the large biases that can result (especially the so-called Malmquist bias). Although a number of "corrections" for this bias already exist, I will show that none are ideal and when applied in the wrong situation or without a full understanding of the properties of the chosen distance estimator, the results can be far worse than with no correction at all. Chapter 5, therefore, is concerned with a number of techniques I have developed during the course of this thesis to improve on this situation. The first of these attempts to minimise the problems by performing as much of the analysis as possible in redshift-space, thereby avoiding much of the use of distance estimates. However, although successful for simple tests, this proves to be inadequate when confronted with a realistically complicated situation. More successful is an iterative technique based around Monte Carlo error estimates that gradually adjusts an estimate of the velocity field until its recovery (with biases) matches the recovery with the actual data. This method has the particular advantage of making no assumptions about the causes of the various biases, but simply tries to estimate their effect and remove them. The results are noticeably better than any other method in all the tests performed. Finally in this chapter, an attack is made upon the random errors in POTENT recoveries. (Abstract shortened by ProQuest.)

Potent and Max-Flow Algorithms

Although Potent purports to use only radial velocities in retrieving the potential velocity field of galaxies, the derivation of transverse components is implicit in the smoothing procedures. Thus the possibility of using nonradial line integrals to derive the velocity field arises. In the case of inhomogeneous distributions of galaxies, the optimal path for integration need not be radial, and can be obtained by using max-flow algorithms. In this paper we present the results of using Dijkstra's algorithm to obtain this optimal path and velocity field.Comment: 9 pages includeing 5 figures, uuencoded compressed PostScript, for Cosmic Velocity Fields, IAP Paris July 1993. UG-COS-JFLS-00

Optical Polarimetry of the May 2022 Lunar Eclipse

The sunlight reflected from the Moon during a total lunar eclipse has been transmitted through the Earth's atmosphere on the way to the Moon. The combination of multiple scattering and inhomogeneous atmospheric characteristics during that transmission can potentially polarize that light. A similar (although much smaller) effect should also be observable from the atmosphere of a transiting exoplanet. We present the results of polarization observations during the first 15 minutes of totality of the lunar eclipse of 2022 May 16. We find degrees of polarization of 2.1 +/- 0.4 per cent in B, 1.2 +/- 0.3 per cent in V, 0.5 +/- 0.2 per cent in R and 0.2 +/- 0.2 per cent in I. Our polarization values lie in the middle of the range of those reported for previous eclipses, providing further evidence that the induced polarization can change from event to event. We found no significant polarization difference (<0.02 per cent) between a region of dark Mare and nearby bright uplands or between the lunar limb and regions closer to the disk centre due to the different angle of incidence. This further strengthens the interpretation of the polarization's origin being due to scattering in the Earth's atmosphere rather than by the lunar regolith.Comment: Accepted for publication in MNRA

Optical Polarimetry of the May 2022 Lunar Eclipse

The sunlight reflected from the Moon during a total lunar eclipse has been transmitted through the Earth's atmosphere on the way to the Moon. The combination of multiple scattering and inhomogeneous atmospheric characteristics during that transmission can potentially polarize that light. A similar (although much smaller) effect should also be observable from the atmosphere of a transiting exoplanet. We present the results of polarization observations during the first 15 minutes of totality of the lunar eclipse of 2022 May 16. We find degrees of polarization of 2.1 +/- 0.4 per cent in B, 1.2 +/- 0.3 per cent in V, 0.5 +/- 0.2 per cent in R and 0.2 +/- 0.2 per cent in I. Our polarization values lie in the middle of the range of those reported for previous eclipses, providing further evidence that the induced polarization can change from event to event. We found no significant polarization difference (<0.02 per cent) between a region of dark Mare and nearby bright uplands or between the lunar limb and regions closer to the disk centre due to the different angle of incidence. This further strengthens the interpretation of the polarization's origin being due to scattering in the Earth's atmosphere rather than by the lunar regolith.Comment: Accepted for publication in MNRA

The Liverpool Telescope: Rapid follow-up observation of Targets of opportunity with a 2 m robotic telescope

The Liverpool Telescope, situated at Roque de los Muchachos Observatory, La Palma, Canaries, is the first 2-m, fully instrumented robotic telescope. It recently began observations. Among Liverpool Telescope's primary scientific goals is to monitor variable objects on all timescales from seconds to years. An additional benefit of its robotic operation is rapid reaction to unpredictable phenomena and their systematic follow up, simultaneous or coordinated with other facilities. The Target of Opportunity Programme of the Liverpool Telescope includes the prompt search for and observation of GRB and XRF counterparts. A special over-ride mode implemented for GRB/XRF follow-up enables observations commencing less than a minute after the alert, including optical and near infrared imaging and spectroscopy. In particular, the moderate aperture and rapid automated response make the Liverpool Telescope excellently suited to help solving the mystery of optically dark GRBs and for the investigation of currently unstudied short bursts and XRFs.Comment: 4 pages, 1 figure. To appear in the Proceedings of The Restless High-Energy Universe, 5-8 May 2003, Amsterdam, E.P.J. van den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers (eds.

The National Schools’ Observatory

Andrew Newsam, NSO Director, describes the rationale and running of the National Schools’ Observatory, designed to inspire and inform schoolchildren by asking them to make the same sort of observing decisions as professional astronomers