An Optical and Infrared Analysis of Blue Compact Dwarf Galaxies
Total Page：16
File Type：pdf, Size：1020Kb
An Optical and Infrared Analysis of Blue Compact Dwarf Galaxies Bethan Lesley James Thesis submitted for the Degree of Doctor of Philosophy of University College London Department of Physics & Astronomy UNIVERSITY COLLEGE LONDON August 2009 I, Bethan James, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. Specifically: • The VLT-VIMOS data discussed and analysed in Chapters 2 and 3 were provided by the principal investigator of the ESO proposal, Y.G. Tsamis. • The work presented in Chapter 2 has been published by James et al. (2009a) and incorporated comments from all co-authors. Figure 2.9 was created by Y.G. Tsamis (the second author of the publication), as were the final calculations of SFRs listed in Table 2.11. • The work presented in Chapter 3 has been published by James et al. (2009b) and incorporated comments from all co-authors. Figure 3.19 was derived from helium abundance maps created by J. R. Walsh (the fifth author of the publication). To my parents, whose endless love and support made this possible. There is nothing in the dark that isn’t there when the lights are on. — Rod Serling (creator of The Twilight Zone, 1924-1975) ABSTRACT An understanding of Blue Compact Dwarf galaxies (BCDs) and the processes occurring within their chemically un-evolved environments is fundamental in our understanding of the early uni- verse. This thesis presents an investigation into their physical conditions, kinematics, chemical abundances and dust compositions. An optical integral field spectroscopy investigation of two perturbed BCDs, UM420 and UM462, is presented. Emission line maps show that both galaxies display signs of ongoing perturbation and/or interaction. Electron temperatures, densities and chemical abundances are computed from spectra integrated over the whole galaxies and for each area of star formation. A similar yet more complicated analysis is undertaken of the BCD Mrk996, which displays multi-component emission lines. The high excitation energy [O III] λ4363 and [N II] λ5755 lines are detected only in the inner regions and purely in broad component form, implying unusual excitation conditions. A separate physical analysis of the broad and narrow emission line regions is undertaken, yielding a revised metallicity and N/O ratio typical for the galaxy’s metallicity. The mid-IR properties of 19 BCDs are studied through Spitzer spectral and imaging data. The depeletion of PAH emission in BCDs is investigated and found to be due to formation and destruc- tion effects. The [S III] flux ratio is used as a density diagnostic, showing typically low-densities. Maps of PAH emission and radiation field hardness are derived from IRS spectral mapping data. Blackbody fits to IR photometric SEDs typically reveal two dust components. The observed physical and chemical properties of Mrk 996 are successfully reproduced using the photoionisation code MOCASSIN. The best-fit model involved the inclusion of a filling factor and an amorphous carbon dust component with a two-zone dust distribution. A STARBURST99 input spectrum was used, yielding ages consistent with the known young WR stars and old super star clusters within Mrk 996. v CONTENTS Abstract v Table of Contents vi List of Figuresx List of Tables xx 1 Introduction1 1.1 What are blue compact dwarf galaxies?......................1 1.1.1 The history of blue compact dwarf studies.................2 1.1.2 Old or young?...............................3 1.1.3 The spectra of BCDs............................8 1.1.4 Ionising Sources within BCDs.......................9 1.2 Physical processes in BCD H II regions...................... 10 1.2.1 Photoexcitation............................... 10 1.2.2 Radiative recombination and line emission................ 11 1.2.3 Collisionally excited line emission..................... 13 1.3 Interstellar Extinction................................ 15 1.3.1 Derivation of c(Hβ)............................ 17 1.4 What can emission lines tell us about BCDs?................... 17 1.4.1 Te and Ne determinations......................... 18 1.4.2 Abundance determinations......................... 20 1.5 Integral Field Spectroscopy............................. 22 1.5.1 Integral Field Units (IFUs)......................... 23 vi Contents vii 1.5.2 IFS and BCDs............................... 24 1.6 Structure of this Thesis............................... 25 2 An IFU study of perturbed blue compact galaxies: UM 420 and UM 462 27 2.1 A VLT-VIMOS Investigation............................ 27 2.2 Introduction..................................... 28 2.3 VIMOS IFU Observations and Data Reduction.................. 30 2.3.1 Observations................................ 30 2.3.2 Data Reduction............................... 31 2.3.3 Emission Line Profile Fitting........................ 32 2.4 Mapping Line Fluxes................................ 33 2.4.1 Line fluxes and reddening corrections................... 33 2.4.2 Hα line properties............................. 33 2.4.3 [O III] and [N II].............................. 40 2.5 Electron Temperature and Density Diagnostics.................. 40 2.6 Chemical Abundances............................... 42 2.6.1 UM 420................................... 45 2.6.2 UM 462................................... 46 2.6.3 Stellar Properties.............................. 53 2.7 Emission Line Galaxy Classification........................ 56 2.8 Conclusions..................................... 59 3 An IFU study of the anomalous BCD Mrk 996 62 3.1 Introduction..................................... 62 3.2 VIMOS IFU Observations and Data Reduction.................. 64 3.2.1 Observations................................ 64 3.2.2 Data Reduction............................... 65 3.2.3 Emission Line Profile Fitting........................ 66 3.2.4 Cube alignment and correction for differential atmospheric refraction.. 69 3.3 Mapping of line fluxes and kinematics....................... 69 3.3.1 Line fluxes and reddening correction.................... 69 3.3.2 The Balmer line properties......................... 72 3.3.3 The [O III] nebular line properties..................... 76 3.3.4 Broad emission lines: [O III] λ4363 and [N II] λ5755........... 77 Contents viii 3.4 Multi-Component diagnoses: electron temperature and density.......... 81 3.4.1 The Narrow Component Gas........................ 81 3.4.2 [Fe III] line diagnostics........................... 83 3.4.3 The Broad Component Gas......................... 84 3.5 Chemical Abundances............................... 86 3.5.1 Narrow Component Abundances...................... 86 3.5.2 Broad component abundances....................... 89 3.5.3 Elemental Helium Abundance....................... 91 3.6 Wolf-Rayet Stars and Stellar Age.......................... 92 3.7 Discussion...................................... 96 3.7.1 Diagnosing the ionisation mechanisms................... 96 3.7.2 What is the origin of the broad line emission?............... 100 3.8 Summary and conclusions............................. 102 4 Infrared Spectroscopic and Photometric Analysis 104 4.1 Spitzer Space Telescope.............................. 104 4.1.1 The Infrared Spectrograph: IRS...................... 105 4.1.2 Multiband Imaging Photometer: MIPS.................. 105 4.1.3 Infra-Red Array Camera: IRAC...................... 106 4.2 What can Infrared data tell us about BCDs?.................... 106 4.2.1 Polycyclic Aromatic Hydrocarbons: PAHs................ 107 4.2.2 IR observations of PAHs within BCDs................... 108 4.2.3 Infra-Red Forbidden Lines......................... 109 4.3 IRS Spectroscopy.................................. 110 4.3.1 Reduction and Analysis Techniques.................... 113 4.3.2 Emission Line Measurements....................... 114 4.3.3 Correlations with PAH emission...................... 122 4.3.4 IRS Spectral Mapping........................... 126 4.4 Infrared Photometry: MIPS & IRAC........................ 135 4.4.1 Methods of Photometry.......................... 137 4.4.2 Blackbody Fitting............................. 137 4.5 Summary...................................... 143 Contents ix 5 3D Photoionisation Modelling of Mrk 996 147 5.1 Modelling BCDs.................................. 147 5.2 Modelling Mrk 996................................. 149 5.2.1 Previous Attempts at Modelling Mrk 996: CLOUDY modelling..... 149 5.2.2 MOCASSIN: a 3-D Photoionisation Code................. 151 5.2.3 Modelling the Narrow Component Flux.................. 153 5.2.4 Modelling with STARBURST99...................... 159 5.2.5 The Addition of Dust............................ 164 5.2.6 Modelling the Broad Emission Line Component Flux........... 169 5.3 Summary...................................... 172 6 Conclusions & Future Work 174 6.1 Conclusions..................................... 174 6.2 Future Work..................................... 177 6.2.1 VLT-FLAMES 3D Spectroscopy of UM 448 & Haro 11......... 180 6.2.2 HST Imaging of DDO 68.......................... 182 6.2.3 Abundance Determinations in Local SF Galaxies using COS....... 184 Bibliography 186 Acknowledgements 200 LIST OF FIGURES 1.1 HST ACS and WFPC2 V - and I-band images of I Zw 18. Credit: NASA, Y. Izotov (Main Astronomical Observatory, Kyiv, UA) and T. Thuan (University of Virginia). North is up, east is left..........................5 1.2 SBS 0335-052: left panel shows 20 × 2000 (5:2 × 5:2 kpc) HST WFPC2 V-band images of eastern component, cf. Thuan