An Explanation for the Unexpected Diversity of Dwarf Galaxy Rotation Curves
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AN EXPLANATION FOR THE UNEXPECTED DIVERSITY OF DWARF GALAXY ROTATION CURVES by Kyle Oman B.Sc., University of Waterloo, 2011 M.Sc., University of Waterloo, 2013 A dissertation submitted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in the Department of Physics and Astronomy c Kyle Oman, 2017 University of Victoria All rights reserved. This dissertation may not be reproduced in whole or in part, by photocopying or other means, without the permission of the author. ii AN EXPLANATION FOR THE UNEXPECTED DIVERSITY OF DWARF GALAXY ROTATION CURVES by Kyle Oman B.Sc., University of Waterloo, 2011 M.Sc., University of Waterloo, 2013 Supervisory Committee Dr. J. F. Navarro, Supervisor (Department of Physics and Astronomy) Dr. F. Herwig, Departmental Member (Department of Physics and Astronomy) Dr. F. Diacu, Outside Member (Department of Mathematics and Statistics) iii ABSTRACT The cosmological constant + cold dark matter (ΛCDM) theory is the `standard model' of cosmology. Encoded in it are extremely accurate descriptions of the large scale structure of the Universe, despite a very limited number of degrees of freedom. The model struggles, however, to explain some measurements on galactic and smaller scales. The shape of the dark matter distribution toward the centres of galaxies is predicted to be steeply increasing in density (`cuspy') by the theory, yet observations of the rotation curves of some galaxies suggest that it instead reaches a central density plateau (a `core'). This discrepancy is termed the `cusp-core problem'. I propose a new way of quantifying this problem as a diversity in the central mass content of galaxies. This characterization does not distinguish between dark and ordinary (`baryonic') matter, but the apparent problem is so severe that the signature of the cusp-core discrepancy is still obvious. By formulating the problem in this way, several uncertain modelling steps are effectively removed from the discussion, allowing for a more narrowly focussed examination of remaining steps in the analysis. My subsequent comparison of recent results from galaxy formation simulations and observed galaxies in the space of the baryonic Tully-Fisher relation (BTFR) reveals some galaxies with an apparent anomalously low dark matter content not only in the centre, but out to the largest measurable radii. These objects are very difficult to explain within the ΛCDM framework; the most plausible interpretation which emerges is that the effect of systematic uncertainties in modelling the kinematics in these galaxies { particularly in the estimate of their inclinations { has been substantially underestimated. This motivates a re-examination of rotation curve measurement methods. I use a collection of simulated galaxies to demonstrate that, when these are syn- thetically `observed' and modelled analogously to real galaxies, non-circular motions present in the gas discs give the appearance of cores, even though all of the simulated galaxies have central cusps. The errors are large enough to reproduce the full width of the observed scatter in rotation curve shapes. Provided the simulations produce sufficiently faithful models of real galaxies, these modelling errors could constitute a solution to the cusp-core problem within the ΛCDM paradigm. Regardless, the kinematic models must be better understood before drawing any strong cosmological conclusions. iv Contents Supervisory Committee ii Abstract iii Table of Contents iv List of Tables viii List of Figures ix Acknowledgements xi Dedication xii 1 Introduction 1 1.1 The dark matter problem . 1 1.1.1 The ΛCDM model . 1 1.1.2 Successes on large scales . 2 1.1.3 The small-scale crisis . 4 1.2 The cusp-core problem . 6 1.2.1 The cusp prediction . 7 1.2.2 The core observation . 8 1.2.3 Supernova-driven core formation . 9 1.2.4 Clumpy infall & dynamical friction . 9 1.2.5 Modified dark matter physics . 10 1.2.6 Modified gravity . 10 1.2.7 Modelling errors . 11 1.3 Circular velocity and rotation curves . 12 1.3.1 Dwarf galaxies . 13 1.3.2 Pressure support corrections . 14 v 1.3.3 Kinematic modelling . 15 1.3.4 Mass modelling . 18 1.4 Outline . 19 1.5 Co-author contributions to published work . 20 2 The unexpected diversity of dwarf galaxy rotation curves 22 2.1 Introduction . 23 2.2 Circular Velocity Profiles of Simulated Galaxies . 29 2.2.1 The numerical simulations . 29 2.2.2 Circular velocity curves . 32 2.3 Observed Rotation Curves . 34 2.4 Observed vs Simulated Rotation Curves . 35 2.4.1 The similarity of simulated circular velocity curves . 35 2.4.2 The diversity of observed rotation curves . 35 2.4.3 The challenge to ΛCDM . 38 2.4.4 The challenge to baryon-induced core formation . 40 2.4.5 The challenge to alternative dark matter models . 40 2.4.6 The `inner mass deficit’ problem . 42 2.5 Summary and conclusions . 44 3 Missing dark matter in dwarf galaxies? 47 3.1 Introduction . 48 3.2 The APOSTLE project . 51 3.2.1 The numerical simulations . 51 3.2.2 The simulated galaxy sample . 52 3.3 The observed galaxy sample . 53 3.3.1 THINGS and LITTLE THINGS . 54 3.3.2 Adams et al. (2014) . 54 3.3.3 McGaugh (2012) . 55 3.4 Galaxy baryonic mass and dark halo mass . 58 3.4.1 The baryonic Tully-Fisher relation . 58 3.4.2 Galaxy formation efficiency . 60 3.5 Rotation curves and halo masses . 63 3.5.1 Rising rotation curves? . 63 3.5.2 The effects of baryon-induced dark matter `cores' . 63 vi 3.5.3 Missing dark matter? . 66 3.5.4 Observational and modelling uncertainties . 67 3.6 Summary and Conclusions . 71 4 Apparent cores and non-circular motions in the H i discs of simu- lated galaxies 74 4.1 Introduction . 75 4.2 Simulations . 78 4.2.1 The APOSTLE simulations . 78 4.2.2 Sample selection . 81 4.2.3 Creation of synthetic H i data cubes . 82 4.2.4 Kinematic properties of simulated galaxies . 85 4.3 Kinematic modelling . 89 4.3.1 Parameter choices . 89 4.3.2 Model fitting . 91 4.3.3 Correction for pressure support . 91 4.4 The importance of orientation . 93 4.4.1 Applicability to observed galaxies . 97 4.5 The inner mass deficit problem . 102 4.5.1 Diagnosing the influence of non-circular motions . 106 4.6 Discussion . 111 4.6.1 Inclination errors . 113 4.7 Summary and Conclusions . 114 5 Conclusions 116 5.1 Prospects for the cusp-core problem . 116 5.1.1 Numerical galaxy formation models . 116 5.1.2 The particle nature of dark matter . 118 5.1.3 Kinematic modelling of observations . 118 5.2 An explanation for the unexpected diversity of dwarf galaxy rotation curves . 120 APPENDICES 121 A Observed Rotation Curve Compilation 122 A.1 Kauffmann et al. (2015) . 122 vii A.2 Oh et al. (2015) . 123 A.3 Adams et al. (2014) . 123 A.4 de Blok at al. (2008) and Oh et al. (2011) . 124 A.5 Reyes et al. (2011) . 124 A.6 Kuzio de Naray et al. (2008) . 125 A.7 de Blok et al. (2004) . 125 A.8 Swaters et al. (2003) . 125 A.9 de Blok & Bosma (2002) . 126 A.10 de Blok et al. (2001) . 126 B Additional rotation curve examples 140 C Additional properties of mock-observed simulated galaxies and ob- served comparison sample 144 C.1 Properties of simulated and observed galaxies . 144 C.2 Velocity field symmetry diagnostics . 151 C.3 3Dbarolo configuration . 151 Bibliography 157 viii List of Tables Table 2.1 Summary of the key parameters of the EAGLE and LOCAL GROUPS simulations. 31 Table 3.1 Summary of the key parameters of the APOSTLE simulations. 52 Table 3.2 Summary of properties of galaxies with radially extended rotation curves. 56 Table 4.1 Summary of the key parameters of the APOSTLE simulations. 80 Table A.1 Summary of properties of a compilation of galaxies with rotation curve measurements. 127 Table C.1 Properties of synthetically observed APOSTLE galaxies. 146 Table C.2 Properties of THINGS and LITTLE THINGS galaxies for com- parison with synthetically observed sample. 148 Table C.3 3Dbarolo configuration parameters. 154 ix List of Figures 1.1 Schematic density and circular velocity profiles of cusps and cores. 6 2.1 Rotation curves of IC 2574 and simulated galaxies. 25 2.2 Comparison of simulated circular velocity curves with and without baryonic contribution for several mass bins. 27 2.3 As Fig. 2.2, but on linear axes and comparing different simulation resolutions. 30 2.4 Four examples of observed galaxies of different masses whose rotation curves agree well with the `cuspy' mass profile prediction from simu- lations. 36 2.5 Comparison of NGC 1560 and individual simulated rotation curves. 37 2.6 Four examples of observed galaxies at ∼fixed mass whose rotation curves deviate from simulation predictions and are typical of a `cored' mass profile. 39 2.7 Characterization of the `inner mass deficits’ of simulated and observed galaxies, parameterized by the maximum circular velocity and circular velocity at 2 kpc. 41 3.1 Baryonic Tully-Fisher relation for observed and simulated galaxies. 59 3.2 Galaxy formation efficiency of observed and simulated galaxies. 61 3.3 Six examples of galaxies with radially extended rotation curve mea- surements with various rotation curve shapes. 64 3.4 Baryonic Tully-Fisher relation, but using the circular velocity at twice the stellar half mass radius rather than at maximum. 65 3.5 `Mass discrepancy' as a function of the inclination angle for a subset of observed galaxies. 69 4.1 H i scaling relations for APOSTLE, THINGS, LITTLE THINGS and SPARC galaxies.