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Neutral Hydrogen in Local Group Dwarf Galaxies

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Neutral Hydrogen in Local Group Dwarf Galaxies Neutral Hydrogen in Local Group Dwarf Galaxies Jana Grcevich Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Arts and Sciences COLUMBIA UNIVERSITY 2013 c 2013 Jana Grcevich All rights reserved ABSTRACT Neutral Hydrogen in Local Group Dwarfs Jana Grcevich The gas content of the faintest and lowest mass dwarf galaxies provide means to study the evolution of these unique objects. The evolutionary histories of low mass dwarf galaxies are interesting in their own right, but may also provide insight into fundamental cosmological problems. These include the nature of dark matter, the disagreement be- tween the number of observed Local Group dwarf galaxies and that predicted by ΛCDM, and the discrepancy between the observed census of baryonic matter in the Milky Way’s environment and theoretical predictions. This thesis explores these questions by studying the neutral hydrogen (HI) component of dwarf galaxies. First, limits on the HI mass of the ultra-faint dwarfs are presented, and the HI content of all Local Group dwarf galaxies is examined from an environmental standpoint. We find that those Local Group dwarfs within 270 kpc of a massive host galaxy are deficient in HI as compared to those at larger galactocentric distances. Ram- 4 3 pressure arguments are invoked, which suggest halo densities greater than 2-3 10− cm− × out to distances of at least 70 kpc, values which are consistent with theoretical models and suggest the halo may harbor a large fraction of the host galaxy’s baryons. We also find that accounting for the incompleteness of the dwarf galaxy count, known dwarf galaxies whose gas has been removed could have provided at most 2.1 108 M of HI gas to the Milky Way. × ⊙ Second, we examine the possibility of discovering unknown gas-rich ultra-faint galaxies in the Local Group using HI. The GALFA-HI Survey catalog is searched for compact, isolated HI clouds which are most similar to the expected HI characteristics of low mass dwarf galaxies. Fifty-one Local Group dwarf galaxy candidates are identified through column density, brightness temperature, and kinematic selection criteria, and their properties are explored. Third, we present hydrodynamic simulations of dwarf galaxies experiencing a constant velocity and density wind which emulates relative motion of the dwarf and the host’s hot halo. These simulations resolve instabilities which can contribute to gas loss, such as the effects of the Kelvin Helmholtz instability. The results of these simulations support the hypothesis that rapid gas loss occurs when the ram pressure stripping criterion is met, with complete stripping occurs with a timescale of about half a Gyr. This stripping would occur in less than an orbital period for ultra-faint like dwarfs. Models which do not meet the ram-pressure stripping criterion show slower but constant mass loss which does not depend on the residual dwarf’s gas mass. Extrapolating the stripping timescales, we show low-mass dwarf galaxies can be stripped on timescales between 1.1 and 3.3 Gyrs. These simulations are a first step towards accurate ram-pressure and dynamical mass loss rates for low mass Local Group galaxies orbiting within a hot halo. They suggest that the lack of low mass galaxies within 250 kpc of the Milky Way can be explained via ram-pressure and dynamical interactions between the satellite galaxy’s gas and the hot halo. (This page left intentionally blank.) Contents 1 Introduction 1 1.1 DwarfGalaxiesinaCosmologicalContext . ...... 1 1.2 Ultra-FaintDwarfs ............................... .. 2 1.3 GasinDwarfGalaxies .............................. 4 1.4 GalacticHotHalo................................. 6 1.5 DarkMatterinDwarfGalaxies . ... 7 1.6 ThesisSummary................................... 8 1.6.1 Chapter 2: Neutral Hydrogen in Known Local Group Dwarfs.... 8 1.6.2 Chapter3: LocalGroupDwarfGalaxyCandidatesintheGALFA-HI Survey.................................... 9 1.6.3 Chapter 4: Simulations of Mass Loss in Dwarf Satellite Galaxies. 9 2 NeutralHydrogeninKnownLocalGroupDwarfs 13 2.1 Introduction .................................... 13 2.2 Observations.................................... 15 2.3 Results ........................................ 16 2.3.1 Nondetections................................ 16 2.3.2 AmbiguousDetections. 20 2.3.3 ConfidentDetections. 21 2.3.4 HIMassandGalactocentricRadius . .. 22 2.4 Discussion ...................................... 23 i 2.4.1 DwarfGasLoss............................... 25 2.4.2 HaloDensity ................................ 29 2.4.3 ComparisontoHaloModels . 32 2.5 Summary....................................... 33 3 Local Group Dwarf Galaxy Candidates in the GALFA-HI Survey 47 3.1 Introduction .................................... 47 3.2 TheGALFA-HISurveyandCompactCloudCatalog . ..... 49 3.3 DwarfCandidateSelection. .... 51 3.4 Results ........................................ 54 3.4.1 SpatialandKinematicCharacteristics . ...... 54 3.4.2 ComparisontoLeoT............................ 56 3.4.3 Search for Associated Stellar Populations with SDSS . ........ 57 3.5 Discussion ...................................... 59 3.5.1 TwoCandidateswithOpticalCounterparts . ..... 60 3.5.2 TheCCClouds ............................... 62 3.5.3 M31SatelliteCandidates. .. 63 3.5.4 RemainingGroup1Candidates . 64 3.5.5 RemainingGroup2Candidates . 65 3.6 Summary....................................... 67 4 Simulations of Mass Loss in Dwarf Satellite Galaxies 86 4.1 Introduction .................................... 86 4.2 SimulationSetup ................................. 90 4.2.1 TheCode .................................. 90 4.2.2 BasicCharacteristics . .. 90 4.2.3 GravitationalPotential. ... 91 4.2.4 GasProfile.................................. 92 4.3 Results ........................................ 95 ii 4.3.1 StrippingTimescales . 95 4.3.2 GasLossTrends............................... 96 4.3.3 ResolutionStudy .............................. 97 4.3.4 Discussion.................................. 98 4.3.5 RamPressureStripping . 98 4.3.6 Kelvin-HelmholtzInstability . .100 4.3.7 ModelsasComparedtoObservations . 102 4.3.8 ImplicationsforDwarfGalaxyEvolution . .103 4.4 Summary.......................................104 5 Conclusion 113 5.1 FutureWork .....................................113 5.1.1 Searching for Unknown Local Group Dwarf Galaxies . .113 5.1.2 DwarfGalaxySimulations. 114 5.2 Conclusion...................................... 115 iii List of Figures 1.1 Number of known Local Group member galaxies over time . ....... 11 1.2 Thestructuralpropertiesofgalaxies . ........ 12 2.1 Neutral Hydrogen maps for the Tucana, Sculptor, and Fornaxdwarfs . 41 2.2 NeutralHydrogenmassversusdistance . ...... 42 2.3 Mass fraction of neutral Hydrogen versus distance . .......... 43 2.4 HI mass divided by V-band luminosity versus distance . ......... 44 2.5 Halodensityasafunctionofdistance . ...... 45 2.6 Satellite velocities required for ram-pressure stripping............. 46 3.1 Integrated brightness temperature maps for group 1 dwarfcandidates . 75 3.2 Integrated brightness temperature maps for group 1 dwarf candidates, continued ...................................... 76 3.3 Integrated brightness temperature maps for group 2 candidates . 77 3.4 Integrated brightness temperature maps for group 2 candidates, continued 78 3.5 Spatial and kinematic distribution of compact clouds . ........... 79 3.6 Velocityversusangletoapexofsolarmotion . ........ 80 3.7 Spatialprofilesofcandidateclouds . ....... 81 3.8 Massestimationmethod . .. 82 3.9 SloanDigitalSkySurveyimageofcandidate22. ........ 83 3.10 SloanDigitalSkySurveyimageofcandidate23 . ......... 84 3.11 Sloan Digital Sky Survey identified stars in the vicinityofcandidate22 . 85 iv 4.1 Fractionalgasmassinthedwarf’soriginalextent . ..........107 4.2 DensitymapsformodelsA1andA2 . 108 4.3 DensitymapsformodelsB1andB2 . 109 4.4 DensitymapsformodelsC1andC2 . 110 4.5 Masslossinresolutionstudy . .111 4.6 Ram pressure strength versus initial central density . ............112 v (This page left intentionally blank.) vi List of Tables 2.1 HIMassofNewlyDiscoveredSatellites . ...... 36 2.2 HIMassofAndromedaSatellites . .... 37 2.3 HI Mass of Additional Local Group Satellite Galaxies . .......... 38 2.3 HI Mass of Additional Local Group Satellite Galaxies . .......... 39 2.4 OrbitalCharacteristicsandHotHaloDensities . .......... 40 3.1 Coordinates and Kinematics for Group 1 Dwarf Galaxy Candidates . 68 3.2 Observational Characteristics for Group 1 Dwarf Galaxy Candidates . 69 3.3 Coordinates and Kinematics for Group 2 Galaxy Candidates......... 70 3.3 Coordinates and Kinematics for Group 2 Galaxy Candidates......... 71 3.4 Observational Characteristics for Group 2 Galaxy Candidates ........ 72 3.4 Observational Characteristics for Group 2 Galaxy Candidates ........ 73 3.5 Range and Mean Values for Candidate Clouds as Compared to LeoT. 74 4.1 ParametersforModels .. .. .. .. .. .. .. .. 106 vii (This page left intentionally blank.) viii DEDICATION I dedicate this work to my father and the memory of my mother. Of all that I have learned, their lessons of kindness, love, and generosity are the ones I most hope to remember. ix ACKNOWLEDGEMENTS I am truly fortunate to have so many supportive people in my life. Without their encouragement this work would not exist. My deepest thanks go to my advisor Mary Putman. Not only was her guidance and knowledge invaluable, but she also lent me her confidence when I found my own lacking. I cannot thank you enough
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