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Kinematics and Stellar Populations in Brightest Cluster Galaxies

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Kinematics and Stellar Populations in Brightest Cluster Galaxies KINEMATICS AND STELLAR POPULATIONS IN BRIGHTEST CLUSTER GALAXIES Susan Ilani Loubser A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Jeremiah Horrocks Institute for Astrophysics and Supercomputing School of Computing, Engineering and Physical Sciences University of Central Lancashire June 2009 uclan Iinlvenity of Central Lancashire Student Declaration Concurrent registration for two or more academic awards I declare that while registered as a candidate for the research degree. I have not been a registered candidate or enrolled student for another award of the University or other academic or professional Institution Material submitted for another award I declare that no material contained in the thesis has been used In any other submission for an academic award and Is solely my own work Collaboration Where a candidates research programme is part of a collaborative project, the thesis must indicate In addition clearly the candidate's individual contribution and the extent of the collaboration. Please state below: The entire thesis is the work of the candidate, Susan Ilani Loubser. Signature of Candidate Type of Award Doctor of Philosophy School School of Computing, Engineering and Physical Sciences Abstract This thesis is devoted to the investigation of a new, large sample of brightest cluster galaxies (BOGs), their kinematic and stellar population properties and the relationships between these and the properties of the host clusters. Some of the questions addressed are: how the kinematic and stellar population properties differ from those of ordinary giant elliptical galaxies; and whether these properties are more influenced by the internal parameters of the BOGs or the properties of the host clusters. In order to do this, high signal-to-noise, long-slit spectra of 49 BCOs (concen- trating on those classified as cD galaxies) in the nearby Universe were obtained with the Gemini and WHT telescopes. The radial velocity and velocity disper- sion profiles were measured, and the Lick/IDS system of absorption indices was used to derive Single Stellar Population (SSP)-equivalent ages, metallicities and a-abundance ratios. A systematic comparison was made between the indices and derived parameters for this sample of BOGs and those of large samples of or- dinary elliptical galaxies in the same mass range. The derived properties were tested for possible correlations with the internal properties of the galaxies (mass and luminosity) and the properties of the host clusters (density, mass, distance to X-ray peak and the presence of cooling flows). Clear rotation curves were found for a number of BOGs. In particular, 3 N006034 and N007768 are rapidly rotating (> 100 km r1 as measured from their major axis spectra), indicating that it is unlikely that they formed through dissipationless mergers. Velocity substructure in the form of kinematically de- coupled cores was detected in 15 BOGs, and five BOGs were found with velocity dispersion increasing with radius. In general, the amount of rotation and the velocity substructure detected in this sample, and the position of the BOGs as a class of objects on the anisotropy—luminosity diagram, are similar to those of ordinary giant ellipticals in high density environments. No significant discrep- ancies between the index—velocity dispersion relations of this sample and those of normal ellipticals were found, but subtle differences between the derived SSP- parameters do exist. The BOGs show, on average, higher metallicity ([Z/H]) and a-abundance ([E/Fe]) values. The SSP-parameters show very little dependence on the mass or luminosity of the galaxies, or the mass or density of the host clus- ters. The derived ages of these massive galaxies are consistent with being old, as expected. Overall, the star formation histories in BOGs, and the connection to the processes in the cluster centres, are very complex. 4 Contents Declaration 2 Abstract 3 Acronyms and abbreviations 17 Publications 19 Acknowledgements 20 1 Introduction 22 1.1 Galaxy formation ........................... 23 1.1.1 Galactic archaeology ..................... 25 1.2 Brightest cluster galaxies ....................... 25 1.2.1 Formation mechanisms .................... 28 1.3 Investigating stellar populations ................... 35 1.3.1 Lick indices .......................... 36 1.3.2 SSP-models .......................... 42 1.3.3 Previous work ......................... 44 1.4 Scientific motivation and thesis outline ............... 45 5 2 BCGs: sample and observations 48 2.1 Galaxy sample ............................ 48 2.2 Observations and data reduction ................... 54 2.2.1 WHT observations ...................... 58 2.2.2 Gemini observations ..................... 59 2.2.3 WHT data reduction ..................... 64 2.2.4 Gemini data reduction .................... 71 3 Kinematics 75 3.1 Measurements ............................. 75 3.1.1 Lick star spectra ....................... 75 3.1.2 BCG spectra ......................... 76 3.2 Central radial velocity and velocity dispersion values ....... 80 3.3 Spatially resolved kinematics ..................... 83 3.4 Results and discussion ........................ 89 4 Stellar populations 108 4.1 Calibration to the Lick system .................... 108 4.1.1 Correction to the Lick spectral resolution and velocity dis- persion corrections ...................... 109 4.1.2 Correction to the Lick flux scale ............... 118 4.2 Emission-line contamination ..................... 120 4.2.1 Emission-line measurement and correction ......... 120 4.2.2 The nature of BCG emission lines .............. 126 4.3 Indices .................................. 130 4.3.1 Comparison with previous measurements .......... 133 4.3.2 Correlations between kinematics and indices ........ 140 4.4 5SF-equivalent parameters ...................... 148 4.4.1 Comparison with ellipticals . 161 4.4.2 Correlations between derived properties and velocity dis- persion ............................. 167 4.4.3 Age—metallicity ........................ 170 4.4.4 Correlation between derived properties and magnitude 171 4.5 Reconstructing star formation histories using full spectrum fitting 173 4.6 Results and discussion ........................ 181 5 Cluster environment 186 5.1 Cluster X-ray luminosity ....................... 192 5.1.1 Velocity dispersion versus log L .............. 192 5.1.2 Indices versus log Lx ..................... 193 5.2 Cooling-flow clusters ......................... 194 5.3 Log Lx - log Tx relation ....................... 198 5.4 BCG offset from the X-ray peak ................... 202 5.5 Cluster velocity dispersion ...................... 205 5.6 Results and discussion ........................ 205 6 Conclusions and future work 209 6.1 Conclusions .............................. 209 6.2 General recommendations for future work and limitations of the present study ............................. 211 6.3 Specific areas of future work ..................... 213 6.3.1 Radial gradients of indices and SSP-parameters ...... 213 6.3.2 Surface brightness profiles and isophotes .......... 215 6.3.3 Radio observations ...................... 216 A Notes on individual galaxies au 7 B Radial kinematics 236 C Central index measurements 266 D Publications appended 275 Bibliography 276 List of Tables 1.1 The Lick indices . 38 2.1 BCG sample. ............................. 57 2.2 Exposure times for WHT BCGs....................60 2.3 Spectrophotometric standard stars used in WHT observations. 60 2.4 Lick calibration stars used in WilT observations ..........61 2.5 Galaxies observed with Gemini ...................65 2.6 Spectrophotometric standard stars used in Gemini observations 66 2.7 Lick calibration stars used in Gemini observations .........67 3.1 Radial velocity measurements of Lick calibration stars . 77 3.2 Gentral velocity dispersions and radial velocities of the galaxies. 84 4.1 Values for the calibration of the WilT data to Lick/IDS system. 116 4.2 Values for the calibration of the Gemini data to Lick/IDS system 117 4.3 S/N ratios in the central apertures ................. 134 4.4 Parameters of the indices versus velocity dispersion linear relation- ships.................................. 147 4.5 Scatter of the index measurements . . . . . . . . . . . . . . . . . 149 4.6 Central values for the SSP-equivalent parameters . 159 4.7 Scatter of the SSP-parameters . 160 4.8 Kolmogorov—Smirnov tests on the SSP-parameter distributions 166 4.9 SSP-parameters: correlations with velocity dispersion . 168 4.10 Comparison between the ages derived using full spectrum fitting and using only four indices . 182 5.1 X-ray properties and velocity dispersions of the host clusters. 191 A.1 SSP-parameters of the Coma BCC galaxies . 229 B.1 Radial kinematics of ES0146-028 . . . . . . . . . . . . . . . . . 236 B.2 Radial kinematics of ES0202-043 . . . . . . . . . . . . . . . . 237 B.3 Radial kinematics of ES0303-005 . . . . . . . . . . . . . . 237 8.4 Radial kinematics of ES0346-003 . . . . . . . . . . . . . . . 237 B.5 Radial kinematics of ES0349-010 . . . . . . . . . . . . . . . . 238 B.6 Radial kinematics of ES0444-046 . . . . . . . . . . . . . . . . . 238 B.? Radial kinematics of ES0488-027 . . . . . . . . . . . . . . . 239 B.8 Radial kinematics of ES0541-013 . . . . . . . . . . . . . . . . . 239 B.9 Radial kinematics of ES0552-020
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