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Dark Matter in Dwarf Galaxies: Observational Tests of the Cold Dark Matter Paradigm on Small Scales

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Dark Matter in Dwarf Galaxies: Observational Tests of the Cold Dark Matter Paradigm on Small Scales Dark Matter in Dwarf Galaxies: Observational Tests of the Cold Dark Matter Paradigm on Small Scales by Joshua David Simon B.S. (Stanford University) 1998 M.A. (University of California, Berkeley) 2000 A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Astrophysics in the GRADUATE DIVISION of the UNIVERSITY OF CALIFORNIA, BERKELEY Committee in charge: Professor Leo Blitz, Chair Professor Chung-Pei Ma Professor William L. Holzapfel Spring 2005 The dissertation of Joshua David Simon is approved: Chair Date Date Date University of California, Berkeley Spring 2005 Dark Matter in Dwarf Galaxies: Observational Tests of the Cold Dark Matter Paradigm on Small Scales Copyright 2005 by Joshua David Simon 1 Abstract Dark Matter in Dwarf Galaxies: Observational Tests of the Cold Dark Matter Paradigm on Small Scales by Joshua David Simon Doctor of Philosophy in Astrophysics University of California, Berkeley Professor Leo Blitz, Chair Over the last decade, several crucial shortcomings of the Cold Dark Matter (CDM) model have been recognized on galaxy-size scales. In this thesis I present new observations ad- dressing two of these problems: the substructure problem and the central density problem. I describe results from our search for a connection between high-velocity clouds (HVCs) and the low-mass dark matter halos seen in CDM simulations that appear to be missing from the real universe. This survey demonstrates that HVCs do not have stellar counter- parts similar to other Local Group dwarf galaxies; if HVCs contain any stars at all, their surface brightnesses must be substantially lower than those of any known dwarfs. I also discuss observations of two candidate dark galaxies in the Local Group: Complex H and 7 HVC 127 41 330. Complex H is a massive, nearby HVC containing 2 10 M⊙ of H i − − ∼ × 5 and less than 3 10 M⊙ of stars, leaving it with an H i mass to light ratio several times ∼ × higher than any known galaxy. HVC 127 41 330 is a smaller HVC that appears to be − − interacting with the dwarf galaxy LGS 3, placing it at a distance of 700 kpc. The HVC is rotating, with a dynamical mass that makes it dark matter-dominated for any plausible distance. The existence of these two starless objects suggests that the substructure problem is related to the difficulty of forming stars in low-mass dark matter halos. However, many more dark galaxies must be located in order to resolve the substructure problem. In addition, I present a rotation curve analysis of five nearby dwarf galaxies, based on high-resolution two-dimensional velocity fields in Hα and CO. In contrast to previous studies, most of the galaxies in this sample do not seem to have a strong preference for 2 pseudo-isothermal density profiles. Instead, these objects exhibit the full range of dark matter density profiles between constant density and NFW halos. One galaxy, NGC 2976, definitively contains a large constant-density core that presents a significant challenge for CDM models. However, for another galaxy, NGC 5963, the derived density profile has a very steep central slope and is well fit by an NFW profile, confirming that some galaxies do contain cuspy profiles like the ones predicted by CDM. This observing program was designed to overcome some of the limitations of the traditional techniques of long-slit Hα spectroscopy and low-resolution H i observations. The results demonstrate that there is a distribution of central density slopes rather than the single universal density profile seen in CDM simulations. Four out of the five observed galaxies show evidence for noncircular motions, which are attributed to the triaxiality of their dark matter halos. These motions constrain the ellipticities of the halos in the planes of the disks to be larger than a few percent. Professor Leo Blitz Dissertation Committee Chair i To my parents, and Wendy ii Contents List of Figures vi List of Tables viii 1 Introduction: Cold Dark Matter 1 1.1 EvidenceforDarkMatter . 1 1.2 TheColdDarkMatterModel . 4 1.3 ProblemsonSmallScales . .. .. .. .. .. .. .. .. 6 1.3.1 TheCentralDensityProblem . 7 1.3.2 TheSubstructureProblem . .. .. .. .. .. .. 9 1.3.3 The Angular Momentum Problem . 10 1.4 Observational Tests of the Small-Scale Problems . ......... 11 2 The Absence of Stars in Compact High-Velocity Clouds 13 2.1 Introduction.................................... 14 2.2 Motivations .................................... 16 2.2.1 Are HVCs the Missing Dwarf Galaxies in the Local Group? ..... 16 2.2.2 RecentHVCResults ........................... 17 2.2.3 ANewTechnique............................. 18 2.3 SearchMethodology ............................... 19 2.3.1 POSSImageProcessing . .. .. .. .. .. .. .. 19 2.3.2 HVCCatalogs .............................. 21 2.3.3 SkySurveyImages ............................ 23 2.4 POSSSearchResults............................... 25 2.5 Optical Followup Observations . 30 2.5.1 ObservingSetup ............................. 30 2.5.2 Data Reduction and Analysis . 31 2.5.3 Results .................................. 32 2.5.4 Comments on Individual Candidates . 35 2.6 Discussion..................................... 39 2.6.1 Expected Red Giant Populations . 39 2.6.2 Distance and Surface Brightness Limits . 41 2.6.3 Implications................................ 42 iii 2.6.4 Compatibility with Previous and Subsequent Work . ...... 43 2.7 Conclusions .................................... 45 3 The Cosmological Significance of High-Velocity Cloud Complex H 47 3.1 Introduction.................................... 48 3.2 Search for Recent Star Formation in Complex H . ..... 52 3.2.1 MSXData ................................ 52 3.2.2 TargetSelection ............................. 52 3.2.3 COObservations ............................. 53 3.2.4 Millimeter Search Results . 56 3.2.5 Limits on the Star Formation Rate in Complex H . 62 3.3 Search for a Distant Stellar Population Behind the Milky Way . 62 3.3.1 2MASSData ............................... 63 3.3.2 SearchTechnique............................. 63 3.3.3 DetectionLimits ............................. 64 3.4 Discussion..................................... 68 3.4.1 WhatIsComplexH?........................... 68 3.4.2 Implications for the Nature of HVCs . 70 3.4.3 Implications for the Substructure Problem . ...... 70 3.4.4 Starless Gas Clouds in the Local Group . 72 3.5 Conclusions .................................... 73 4 21 cm Imaging of LGS 3 and an Apparently Interacting High-Velocity Cloud 76 4.1 Introduction.................................... 77 4.1.1 HVC 127 41 330 ............................ 77 − − 4.2 Observations and Data Reduction . 78 4.3 Results....................................... 79 4.3.1 NewAreciboObservations. 84 4.4 Discussion..................................... 84 4.5 Conclusions .................................... 88 5 High-Resolution Measurements of the Dark Matter Halo of NGC 2976: Evidence for a Shallow Density Profile 90 5.1 Introduction.................................... 91 5.2 Target, Observations, and Data Reduction . ....... 94 5.2.1 PropertiesofNGC2976 ......................... 94 5.2.2 Hα ObservationsandReductions . 94 5.2.3 COObservationsandReductions . 100 5.2.4 Optical and Near-IR Imaging and Reductions . 101 5.2.5 SurfaceBrightnessProfiles . 102 5.3 Baryonic Components of NGC 2976 . 107 5.3.1 TheStellarDisk ............................. 107 5.3.2 TheGasDisk............................... 110 5.4 Rotation Curve and Dark Matter Halo of NGC 2976 . 111 iv 5.4.1 Rotation Curve of NGC 2976 . 113 5.4.2 Limits on the Dark Matter Halo . 120 5.5 Discussion..................................... 124 5.5.1 Comparison to Cold Dark Matter Simulations . 124 5.5.2 ComparisontoNGC4605 . 127 5.5.3 Are All Dwarf Galaxies Dark Matter-Dominated? . 128 5.5.4 Are the Kinematics of NGC 2976 Affected By M81? . 129 5.5.5 Possible Origins of Noncircular Motions . 129 5.6 Systematics .................................... 131 5.6.1 TheProblemofSystematics. 131 5.6.2 Rotation Curve Fitting Systematics . 133 5.6.3 Comparing Velocities Derived From Different Tracers . ....... 137 5.6.4 Simulated Long-slit Observations of NGC 2976 . 140 5.6.5 Positioning Errors and Slit Offsets . 142 5.6.6 Barred And Highly Inclined Galaxies . 144 5.7 Conclusions .................................... 144 6 High-Resolution Measurements of the Halos of Four Low-Mass Galaxies: Deviations from a Universal Density Profile 148 6.1 Introduction.................................... 149 6.2 Observations, Data Reduction, and Methodology . ........ 150 6.2.1 Target Galaxies and Observations . 151 6.2.2 Isophotal fits and Stellar Disk Rotation Curves . 153 6.2.3 Velocity Field Fitting . 155 6.3 Results....................................... 161 6.3.1 RemovingtheBaryons. 161 6.3.2 Power Law Density Profile Fits . 164 6.3.3 Alternative Density Profile Fits . 169 6.3.4 Comparison to Previously Published Rotation Curve Data . 171 6.3.5 Comments on Individual Galaxies . 174 6.4 Discussion..................................... 178 6.4.1 Is There a Universal Density Profile? . 178 6.4.2 The Significance of NGC 5963 . 184 6.4.3 NFW and Pseudo-Isothermal Fits to Rotation Curves . 185 6.4.4 ImplicationsforCDM . .. .. .. .. .. .. .. 186 6.5 The Effects of Halo Triaxiality . 187 6.5.1 HaloOblateness ............................. 188 6.5.2 DiskEllipticity .............................. 189 6.6 SummaryandConclusions. 193 7 Conclusions 196 7.1 TheSubstructureProblem. 196 7.2 TheCentralDensityProblem . 199 Bibliography 202 v A Rotation Curve Fitting Algorithms
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