The black hole at the center of the Milky Way

Reviewing the fundamental instrumental techniques and current observational results, this book unveils the mysteries of the physical processes in the central parsec of our Milky Way: the super-massive black hole embedded in a central stellar cluster as well as the gas and dust in the circumnuclear region.The observations described cover the entire electromagnetic spectrum from decimeter radio-waves to high energy X-ray and γ-rays, and a comprehensive summary of up-to-date astrophysical interpretations is given.The emphasis is put on observational techniques, image processing aspects, and a detailed presentation of the most cutting-edge work carried out in the near-infrared wavelength regime. These recent results include both the first orbits of stars around the central black hole and the multiwavelength variability of the central source.

Preface	p. v
1 Observational Techniques	p. 1
1.1 The GC Across the Electromagnetic Spectrum	p. 1
1.1.1 Radio Wavelengths	p. 2
1.1.2 Far-Infrared Wavelengths	p. 7
1.1.3 Near- and Mid-Infrared Wavelengths	p. 12
1.1.4 Optical Wavelengths	p. 16
1.1.5 X-Ray Energies	p. 17
1.1.6 [gamma]-Ray Energies	p. 20
1.2 NIR Imaging Through the Atmosphere	p. 22
1.3 Speckle Imaging	p. 26
1.3.1 The Knox-Thompson Method	p. 30
1.3.2 The Bispectrum Analysis	p. 31
1.3.3 Seeing Calibration of Fourier Data	p. 32
1.3.4 The Shift-and-Add Algorithm	p. 33
1.3.5 Speckle Spectroscopy	p. 37
1.3.6 Speckle Instrumentation	p. 39
1.3.6.1 Sharp	p. 40
1.3.6.2 NIRC	p. 41
1.4 Single Telescope Adaptive Optics	p. 42
1.4.1 Adaptive Optics Imaging	p. 42
1.4.2 Adaptive Optics Spectroscopy	p. 49
1.4.3 Adaptive Optics Instrumentation	p. 51
1.4.3.1 NAOS/CONICA	p. 51
1.4.3.2 NIRC2	p. 54
1.4.3.3 PUEO	p. 54
1.4.3.4 ADONIS	p. 55
1.4.3.5 HOKUPA'A	p. 55
1.4.3.6 FASTTRAC	p. 56
1.5 Cleaning and Deconvolution	p. 57
1.5.1 Linear Deconvolution	p. 58
1.5.2 Lucy-Richardson Deconvolution	p. 60
1.5.3 Estimating the PSF	p. 61
1.5.4 Iterative Blind Deconvolution	p. 62
1.5.5 Comparison of Deconvolution Algorithms	p. 63
1.6 Future IR Interferometry	p. 66
1.6.1 VLTI	p. 67
1.6.2 Keck Interferometer	p. 69
1.6.3 LBT LINC/NIRVANA	p. 70
2 Observational Results	p. 71
2.1 The Discovery of Sagittarius A*	p. 71
2.2 Large-Scale Structures at the Galactic Center	p. 73
2.3 The Circum Nuclear Disk	p. 76
2.4 The Mini-Spiral	p. 77
2.5 Radio Filaments	p. 79
2.6 Near-Infrared Images of the Central Stellar Cluster	p. 80
2.6.1 A Brief History of NIR Imaging	p. 80
2.6.2 Diffraction Limited Images	p. 83
2.7 The Radio and Infrared Positional Reference Frames	p. 86
2.8 Number Density Counts	p. 88
2.8.1 Evidence for a Central Stellar Cusp	p. 92
2.8.2 K-band Luminosity Function	p. 95
2.9 Polarization of Filaments and Stars	p. 100
2.9.1 Extended Filaments in the Sgr A Complex	p. 100
2.9.2 Polarized Emission from Stars	p. 103
2.10 Stellar Velocities and Orbital Accelerations	p. 105
2.10.1 Measuring Stellar Proper Motions	p. 106
2.10.2 Motions in the Outer Cluster	p. 108
2.10.3 Motions in the Inner Cluster	p. 108
2.10.4 Accelerations of Stars Near Sgr A*	p. 112
2.10.5 Stellar Orbits	p. 113
2.11 Spectroscopy	p. 117
2.11.1 Stellar Populations	p. 118
2.11.2 The Upper Mass Cut-Off	p. 120
2.11.3 The Central Arcsecond	p. 122
2.11.4 Results of Speckle Spectroscopy	p. 123
2.11.4.1 Extraction and Calibration of Spectra	p. 123
2.11.4.2 Spectra in the Central Sgr A* Cluster	p. 124
2.11.5 Adaptive Optics Spectroscopy of the Central Cusp	p. 125
2.12 Star Formation at the Galactic Center?	p. 128
2.12.1 Embedded Objects	p. 130
2.12.2 IR-Excess Objects	p. 131
2.13 Sgr A* Across the Electromagnetic Spectrum	p. 136
2.13.1 Size and Spectrum of Sgr A* at Radio Wavelengths	p. 136
2.13.2 Polarization of Sgr A* at Radio Wavelengths	p. 141
2.13.3 Sgr A* as a Variable X-Ray Source	p. 143
2.13.4 Sgr A* as a Variable NIR Source	p. 145
2.13.5 How Likely are Strong NIR Flares?	p. 150
2.13.6 First Simultaneous X-Ray/NIR Detection of Sgr A*	p. 152
3 Astrophysical Results	p. 155
3.1 The Stellar Velocity Field	p. 156
3.2 Scenarios for Star Formation at the Galactic Center	p. 160
3.2.1 Infall of Stellar Clusters	p. 161
3.2.2 In Situ Formation from Dense Gas	p. 164
3.2.3 Formation of Stars in the Mini-Spiral	p. 165
3.2.4 Capture of Stars by Ejection of Stellar Remnants	p. 167
3.3 The Central Dark Mass	p. 167
3.3.1 First Order Estimates	p. 167
3.3.2 Mass Estimators	p. 168
3.3.3 The Jeans Method	p. 169
3.3.4 The Orbit of the Star S2	p. 173
3.3.5 Orbits of Other Stars Around Sgr A*	p. 174
3.3.6 A Lower Limit to the Mass	p. 174
3.4 Stability of the Enclosed Dark Mass	p. 175
3.5 Agglomerations of Exotic Particles?	p. 178
3.6 The Central Stellar Cusp	p. 181
3.6.1 Spatial Distribution of the Stars	p. 181
3.6.2 Velocity Distribution of the Stars	p. 183
3.6.3 Mass of the Cusp	p. 185
3.6.4 Young, Massive Stars in the Cusp	p. 185
3.6.5 Dynamics of the Cusp and Presence of Young Stars	p. 186
3.7 Analysis of Stellar Orbits Near the Central Black Hole	p. 189
3.7.1 Relativistic Periastron Shift	p. 190
3.7.2 Lense-Thirring Precession	p. 191
3.7.3 Newtonian Effects	p. 193
3.7.4 Orbits in a Uniform Density Sphere	p. 193
3.7.5 Stellar Deflections	p. 194
3.7.6 Constraints from non-Keplerian Orbits	p. 196
3.8 The Central Black Hole	p. 199
3.8.1 Standard Accretion Theory	p. 200
3.8.2 The Low Luminosity of Sgr A*	p. 201
3.8.3 Radiatively Inefficient Accretion Flow Models	p. 202
3.8.4 Sgr A* Flare Models	p. 207
3.8.5 A Spin Measurement of the Black Hole?	p. 210
3.8.6 Gravitational Disk Modes	p. 212
3.8.7 Imaging the Event Horizon of Sgr A*	p. 215
3.9 Comparison to Nuclei of Other Galaxies	p. 215
3.10 Massive Black Holes at High Redshifts	p. 218
Appendix A Color Versions of Selected Figures	p. 223
Appendix B Table of Individual Sources	p. 237
Appendix C Useful Quantities	p. 241
Appendix D Table of Abbreviations	p. 243
Bibliography	p. 249
Index	p. 273

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