OBSERVATIONAL STUDIES OF THE GALAXY PECULIAR VELOCITY FIELD by Philip Andrew James Astrophysics Group Blackett Laboratory Imperial College of Science, Technology and Medicine London SW7 2BZ A thesis submitted for the degree of Doctor of Philosophy of the University of London and for the Diploma of Imperial College November 1988 1 ABSTRACT This thesis describes two observational studies of the peculiar velocity field of galaxies over scales of 50-100 Jr1 Mpc, and the consequences of these measurements for cosmological theories. An introduction is given to observational cosmology, emphasising the crucial questions of the nature of the dark matter and the formation of structure. The principal cosmological models are discussed, and the role of observations in developing these models is stressed. Consideration is given to those observations that are likely to prove good discriminators between the competing models, particular emphasis being given to studies of the coherent velocities of samples of galaxies. The first new study presented here uses optical photometry and redshifts, from the literature, for First Ranked Cluster Galaxies (FRCG’s). These galaxies are excellent standard candles, and thus ideal for peculiar velocity studies. A simple one­ dimensional analysis detects no relative motion between the Local Group of galaxies and 60 FRCG’s with redshifts of up to 15000 kms-1. This is shown to imply a streaming motion of the cluster galaxies of at least 600 kms_1 relative to the CBR. The second observational study is a reanalysis of the Rubin et al. (1976a,b) sample of Sc galaxies. Near-IR photometry is used in our reanalysis to minimise the effects of extinction and to facilitate the use of luminosity indicators in reducing the effects of selection biases. The velocity anisotropy found by Rubin et al. is confirmed with the new data, which exdudes the possibility that extinction causes this effect The near-IR Tully-Fisher relation and the B-H colour-magnitude relation are used, to investigate whether the effect is caused by true bulk motions. Both relations are shown to be subject to strong biases and uncertainties resulting from the galaxy selection, and thus do not add significant information on the basis of the measured effect Monte Carlo simulations of the galaxy selection procedure undertaken by Rubin et al. are then described. These show conclusive^ +hat Malmquist bias, combined with an anisotropic sampling depth, can explain the whole effect, which therefore does not provide evidence for large scale galaxy streaming. The consequences of these and other recent results are considered, with particular reference to the Hot Dark Matter vs. Cold Dark Matter controversy. A new study of the galaxy peculiar velotity field, using a new spiral galaxy sample, is then introduced. 2 ACKNOWLEDGEMENTS There are many people who have helped to make the three years of my Ph.D research very enjoyable and fulfilling, but there are three I would particularly like to thank here. The first is of course my supervisor, Bob Joseph, who was a constant source of encouragement and ideas, and who has tried to give me, as with all of his students, a sense of the breadth and richness of astrophysics and cosmology. His enthusiasm and dedication will long remain a source of inspiration for me. The other collaborator in the project described here, Chris Collins, must also receive my warmest thanks. He has spent many hours giving me the benefit of his deep understanding of cosmology, both on observing trips and during my many enjoyable visits to the Royal Observatory, Edinburgh. His friendship and hospitality, and that of his colleagues at ROE, are greatly appreciated. During August and September of 1987,1was lucky enough to be given the opportunity to work at the Joint Astronomy Centre in Hawaii. This was a most enjoyable stay, made possible through the kindness of Gillian Wright, who both provided a roof over my head and her considerable experience of observing at UKIRT. I also benefited greatly from her knowledge of the wider aspects of extragalactic astronomy and image processing, during this and other observing trips in Hawaii. I also thank the rest of the JAC staff for a most enjoyable stay, at a very exciting time. I would also like to acknowledge the day-to-day help and encouragement of my colleagues in the Imperial College Astrophysics group. Special mention should go to Martyn Wells, Simon Chase, Rene Doyon, Arvind Pattni and Jason Spyromilio for practical help and many interesting and informative discussions about physics. There are many others, too numerous to list, whose friendship and help is much appreciated, and from whom I beg forgiveness for not mentioning them individually. Finally, I must thank my family, and particularly my mother and sister Katy, for their continuing support and encouragement throughoutthis period. 3 CONTENTS ABSTRACT 2 ACKNOWLEDGEMENTS 3 LIST OF TABLES 7 LIST OF FIGURES 8 CHAPTER ONE INTRODUCTION I Cosmological models 1.1 Description of structures of cosmological interest 12 1.2 The gravitational collapse of primordial density fluctuations 15 1.3 Non-gravitational collapse processes 19 1.4 Modifications to the standard (baryon dominated model) 21 1.5 The dark matter 24 II Observational tests of cosmological models n.l Galaxy catalogues and redshift surveys 28 H.2 CBR anisotropies 31 H.3 Structure athighredshift-quasars and primeval galaxies 32 H.4 Density enhancements and streaming motions 32 n. 5 Observational searches for deviations from isotropic Hubble flow n.5a The local velocity field 36 H.5b The large scale velocity field i Studies which support significant streaming motions 38 ii Studies which find no significant streaming motions 41 iii Conclusions from previous streaming motion studies 44 CHAPTER TWO MEASUREMENT OF GALAXY STREAMING USING FIRST RANKED CLUSTER ELLIPTICALS I The cluster galaxy sample and data 4 1.1 Suitability of first ranked cluster galaxies for streaming studies 45 1.2 The data 46 II Data analysis 50 m Solutions for the Local Group motion 53 IV Interpretation of the Local Group velocity relative to First Ranked Cluster Galaxies IV. 1 Comparison with the Local Group motion relative to the CBR 56 IV.2 Comparison with previous studies 58 IV.3 The effect of choice of frame on streaming solutions 60 CHAPTER THREE GALAXY STREAMING FROM AN ALL-SKY SAMPLE OF SCI SPIRALS I The Rubin et aL (1976a,b) study 1.1 Galaxy sample selection 62 1.2 Rubin et al. data and analysis 63 II Criticisms of the Rubin et al. result n.l The Fall and Jones interpretation 63 n.2 Weaknesses in the Rubin et al. data and analysis 64 IQ A reanalysis of the Rubin et al. sample using near-IR photometry 65 m.l Advantages of near-IR photometry 66 m.2 Newdata-near-DR galaxy photometry 68 m.3 Photometric corrections 72 m.4 Galactic and internal extinction corrections 75 m.5 Luminosity indicators HI. 5a The B-H colour-magnitude relation 7 8 HI. 5b The InfraredTully-Fisher relation 80 HI. 6 Solutions for galaxy streaming velocities 83 m.7 The importance of choice of <HM> 86 IV Estimating the likely impact of selection effects 5 IV. 1 A ’Monte Carlo’ simulation of selection effects 88 IV.2 Directionality in the Malmquist bias 97 IV. 3 Conclusions from simulations of selection bias 101 CHAPTER FOUR COMPARISON OF RESULTS WITH PREVIOUS STUDIES, AND CONSEQUENCES FOR COSMOLOGICAL MODELS I Related observational results 1.1 A comparison of recent studies of galaxy peculiar velocities 103 1.2 Other recent results of importance to cosmology 107 II Consequences for cosmological models 108 n.l Neutrino-dominated cosmologies 109 H.2 The cold dark matter model 110 H. 3 Other models-strings and isocurvature fluctuations 111 m Conclusions 112 CHAPTER FIVE FURTHER STUDIES OF GALAXY STREAMING MOTIONS I A new study of the peculiar velocities of field spirals I. 1 Scientific justification 113 1.2 Use of modem instrumentation 114 1.3 Selection criteria 114 1.4 Observational methods: Photometry and luminosity indicators 119 1.5 Observations 123 1.6 Analysis of the new galaxy sample 123 II Further analysis of galaxy streaming using First Ranked Cluster Ellipticals 128 HI Future work 130 REFERENCES 132 APPENDIX 139 PUBLICATIONS 150 6 LIST OF TABLES TableNumber Title Page 2.1 Photometric and redshiftdatafor 48 60 First Ranked Cluster Galaxies. 2.2 Solutions for Local Group motion, 54 relative to the sample of 60 cluster galaxies. 2.3 Local Group motions derived from 59 previous streaming studies. 3.1 Photometric andredshift data for 86 70 Sc galaxies from the Rubin et al. Minimum Bias Subset 3.2 CBR-frame streaming velocities for the 85 86 MBS galaxies, using the Rubin et al. ’Hubble Modulus’ technique. 3.3 CBR-frame streaming velocities for the 87 86 MBS galaxies, with <HM> varied to minimise streaming amplitude or errors. 5.1 Names, coordinates, approximate diameters 115 and inclinations for the sample of 218 Sc galaxies. 7 LIST OF FIGURES Figure Number Title Page 2.1 Sky distribution of 60 cluster galaxies from 51 Sandage and Hardy (galactic coordinates). 2.2 CBR-frame velocity residuals for the 60 57 cluster galaxies, projected onto the galactic plane. The position of the’Great Attractor’ is also shown. 3.1 Sky distribution of 86 Sc galaxies from 69 Rubin etal. (1976) (galactic coordinates). 3.2 Malmquist bias shown as a trend in absolute B & 76 H magnitude with redshift for 86 Sc galaxies. 3.3 B-H colour-magnitude relation for 86 Sc galaxies. 79 3.4 H-band Tully-Fisher relation for 6 8 Sc galaxies. 82 3.5 Distribution of apparent B magnitudes for the 90 86 Rubin et al.