Merging Galaxies and Dark Matter Halos by Andrew Rodger Wetzel 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 Martin White, Chair Professor Bill Holzapfel Professor Eliot Quataert Spring 2010 Merging Galaxies and Dark Matter Halos Copyright 2010 by Andrew Rodger Wetzel 1 Abstract Merging Galaxies and Dark Matter Halos by Andrew Rodger Wetzel Doctor of Philosophy in Astrophysics University of California, Berkeley Professor Martin White, Chair Mergers between distinct objects are a natural part of hierarchical structure formation. Mergers are also one of the most critical elements in the evolution of both galaxies and halos. I use high-resolution, cosmological volume simulations to explore galaxy and halo evolution and merging activity in a cosmological context, including environmental dependence, merger rates and dynamics, and how these processes in halos connect with those of galaxies. I first explore halo merging and evolution, focusing on its interplay with large-scale environment. While halo spatial clustering has been thought to depend only on mass, I ex- amine how spatial clustering depends on secondary parameters such as halo formation time, concentration, and recent merger history, a phenomenon known as “assembly bias”. Next, I examine the extent to which close spatial pairs of objects can be used to predict mergers, finding limited utility to the pair-merger method arising from a competition between merger efficiency and completeness. I also explore the dependence of merging on environmental den- sity, discovering that merging is less efficient in overdense environments. I then investigate how a massive galaxy/halo population at high redshift connects to a massive population of the same number density today, finding that scatter in mass growth and mergers between massive objects preclude a direct population mapping either forward or backward in time. In the latter part of this work, I explore the dynamics and mergers of galaxies in groups and clusters. I first examine the orbital distributions of satellite halos/galaxies at the time of infall onto a more massive host halo, finding that satellite orbits become more radial and penetrate deeper at higher host halo mass and higher redshift. I then track the evolution of galaxies in groups directly, examining the merger rates of galaxies over time and finding that galaxy mergers do not simply trace halo mergers. I also examine the small-scale environments of galaxy mergers, discovering that recently merged galaxies exhibit enhanced small-scale spatial clustering for a short time after a merger. Finally, by using abundance matching to assign stellar mass to subhalos, I explore the importance of merging vs. disruption processes for satellite galaxy evolution. I rigorously test the connection of galaxies to subhalos by comparing simulations against observed galaxy spatial clustering, satellite fractions, and cluster satellite luminosity functions, finding agreement in all cases. i Contents List of Figures v List of Tables vii Acknowledgments viii 1 Introduction 1 1.1 ΛCDMCosmology ................................ 2 1.2 Density Fluctuations & Spatial Clustering . ........ 4 1.3 CosmologicalParameters. ... 5 1.4 DarkMatterHalos ................................ 5 1.4.1 HaloCollapse&MassFunction . 5 1.4.2 HaloBias ................................. 7 1.4.3 HaloStructure .............................. 7 1.4.4 HaloMergers ............................... 8 1.5 Galaxies...................................... 9 1.5.1 Subhalos&Galaxies ........................... 9 1.5.2 GalaxyMergers.............................. 10 1.5.3 TheSubhalo-GalaxyConnection. .. 11 2 The Clustering of Massive Halos 13 2.1 Introduction.................................... 13 2.2 Simulations .................................... 16 2.3 MeasuringClustering. .. 17 2.4 AssemblyBias................................... 19 2.5 MergerBias.................................... 22 2.5.1 Markedcorrelationfunction . .. 25 2.5.2 Integratedcorrelationfunction. ..... 28 2.5.3 Likelihood fit to r0 ............................ 30 2.6 Summary&Conclusion ............................. 31 Contents ii 3 ClosePairsasProxiesforGalaxyClusterMergers 35 3.1 Introduction.................................... 36 3.2 Simulations .................................... 38 3.3 ClosePairsasPredictorsofMergers. ...... 39 3.3.1 Pair Mergers at z =0........................... 40 3.3.2 RedshiftDependenceofPairMergers . ... 42 3.3.3 Mass & Linking Length Dependence of Pair Mergers . .... 42 3.3.4 Merger Timescale Dependence of Pair Mergers . ..... 46 3.4 Scatter in Mass, Redshift, & Redshift Space Distortions ............ 47 3.5 The Merger Kernel & the Density Dependence of Mergers . ........ 49 3.5.1 DensityDependenceoftheMergerKernel . ... 50 3.5.2 Density Dependence of Close Pair Mergers . .... 52 3.6 TheClusteringofClosePairs&MergerBias . ...... 55 3.7 Summary&Conclusion ............................. 57 4 Connecting Populations of Fixed Number Density across CosmicTime 61 4.1 Introduction.................................... 61 4.2 Methods...................................... 62 4.3 ScatterinMassEvolution . .. 63 4.4 Connecting Populations of Fixed Number Density . ....... 63 4.5 Summary&Discussion.............................. 68 5 On the Orbits of Infalling Satellite Halos 70 5.1 Introduction.................................... 70 5.2 Methods...................................... 72 5.2.1 Simulations&HaloTracking. 72 5.2.2 EjectedHalos&Re-mergers . 73 5.2.3 CalculatingOrbits ............................ 73 5.2.4 CalculatingOrbitalDistributions . ..... 74 5.3 Orbital Distributions at z =0.......................... 78 5.4 MassDependence................................. 79 5.5 RedshiftEvolution ............................... 83 5.6 FitstoOrbitalDistributions . ..... 89 5.7 Summary&Discussion.............................. 91 6 Galaxy Merger Rates, Counts, & Types 94 6.1 Introduction.................................... 94 6.2 NumericalTechniques .............................. 96 6.2.1 Simulations ................................ 96 6.2.2 SubhaloTracking ............................. 97 6.2.3 SubhaloMassAssignment . 101 6.2.4 Stellar Mass & Gas Content of Subhalo Galaxies . ..... 103 Contents iii 6.2.5 SubhaloMass&CircularVelocity . 103 6.2.6 SatelliteSubhaloMassFunction . 107 6.2.7 SatelliteFraction . 110 6.3 MergerCriteria&Rates ............................ 110 6.3.1 MergerCriteria .............................. 110 6.3.2 FitstoSimulation............................. 112 6.3.3 Subhalovs.HaloMergerRates . 113 6.3.4 ResolvingtheDiscrepancy . 114 6.4 Satellitevs.CentralMergers . ..... 115 6.5 Galaxy&HaloMergerCounts. 117 6.5.1 CountsofRecentMergers . 117 6.5.2 Fraction“On”............................... 117 6.6 EvolutionoftheSatelliteHaloOccupation . ....... 119 6.6.1 SatelliteHaloOccupationinSimulation . .... 120 6.6.2 Analytic Estimate of Satellite Halo Occupation . ..... 120 6.6.3 Comparison to Other Work on Satellite Occupation Evolution . 123 6.7 Summary&Discussion.............................. 124 7 The Clustering & Host Halos of Galaxy Mergers at High Redshift 127 7.1 Introduction.................................... 127 7.2 NumericalTechniques&MergerDefinitions . ..... 129 7.2.1 Simulations&SubhaloTracking. 129 7.2.2 MergerCriteria .............................. 130 7.3 Small-ScaleSpatialClustering . ...... 130 7.4 Halo Occupation Distribution & Radial Profile . ....... 132 7.4.1 HODofSubhalos ............................. 133 7.4.2 Central & Satellite Cross-Correlation . ...... 134 7.4.3 Central&SatelliteHOD . 135 7.4.4 RadialDistributionProfile . 138 7.5 MergerPairs.................................... 140 7.6 Summary&Discussion.............................. 140 8 Whatdeterminessatellitegalaxydisruption? 143 8.1 Introduction.................................... 143 8.2 NumericalMethods................................ 146 8.2.1 Simulations&SubhaloTracking. 146 8.2.2 TheStellarMassofSubhalos . 148 8.2.3 AssigningInfallMass&StellarMass . 149 8.3 ImpactofSatelliteRemoval . 152 8.3.1 HaloOccupationDistribution . 152 8.3.2 SatelliteRadialDensityProfile . 154 8.3.3 Radius at Removal: Merger vs. Disruption . .... 157 Contents iv 8.3.4 AnalyticalModelforSatelliteRemoval . .... 158 8.4 ComparisonswithObservations . .... 160 8.4.1 SpatialClustering. 160 8.4.2 SatelliteFraction . 164 8.4.3 Cluster Satellite Luminosity Function . ...... 167 8.5 HighRedshift ................................... 167 8.5.1 HOD&AnalyticRemoval . 167 8.5.2 Spatial Clustering at z 1........................ 170 8.5.3 EvolutionoftheSatelliteFraction∼ . ..... 172 8.6 ImpactofSimulationSize&Cosmology . .... 172 8.7 ComparisonswithOtherRemovalCriteria . ...... 176 8.8 Summary&Conclusion ............................. 179 Bibliography 182 v List of Figures 2.1 Spatialclusteringbiasvs. halomass . ........ 18 2.2 Influence of halo formation and structure on spatial clustering .......... 20 2.3 Halomassconservationduringamerger . ...... 23 2.4 Halo marked auto-correlation function for recent merger activity. 26 2.5 Halo integrated auto-correlation function for recent mergeractivity . 27 2.6 Likelihood fit to halo merger auto-correlation function .............. 29 2.7 Halomergerbiasvs.redshift . .... 32 2.8 Halomergerbiasvs. mergermassratio . ...... 33 3.1 Distribution of halo merger progenitor separations . .............. 39 3.2 Fraction