Terahertz Astronomy

A Powerful Window into Cosmic Evolution Terahertz (THz) observations of interstellar atoms, molecules, and dust serve as powerful probes of the conditions within the interstellar medium that permeates our galaxy, providing insights into the origins of stars, planets, galaxies, and the Universe. Taking a cross-disciplinary approach to the subject, Terahertz Astronomy explores THz astrophysics and the technologies that make this rapidly evolving field possible. The first four chapters of the book discuss the origin and interpretation of THz light in astrophysical sources. The remaining five chapters present an overview of the technologies used to collect and detect THz light. Every chapter contains worked-out examples and exercises. The author explains each topic as intuitively as possible and includes the equations needed for real-life astrophysical applications. In just a few years, the number of active THz researchers has substantially grown due to increased interest in terrestrial remote sensing at THz frequencies. This book provides researchers with both the background science and technology to interpret THz observations and design, build, and deploy THz astronomical instrumentation.

The Interstellar Medium at Terahertz Frequencies Introduction ISM Components of the Milky Way Lifecycle of the ISM Probing the Lifecycle of the ISM THz SEDsConclusion THz Radiative Transfer Basics and Line Radiation Equation of Radiative Transfer Solution to the Equation of Radiative Transfer under LTE (Local Thermodynamic Equilibrium) Radiative Transfer of Rotational Transitions of Linear Molecules in LTE Non-LTE ApproachConclusion THz Continuum Emission Introduction THz Spectral Energy Distributions Deriving Dust Optical Depth, Column Density, and Mass Temperature and Density Distributions Dust Energy Balance in Clouds Dust–Gas Coupling Dust Polarization: Origin and Measurement Conclusion Simple Radiative Transfer Model Introduction Geometry Source Physical Conditions Lines of Sight Model Equation of Transfer LTE Radiative Transfer with Hydrodynamic SimulationsNon-LTE Radiative Transfer with Hydrodynamic Simulations Going Further THz Optical Systems Introduction: Source-Beam Coupling QED and Maxwell Origin of a Single-Aperture Diffraction Pattern (QED) Gaussian Beam Optics Focusing Gaussian Beams Intercepting Gaussian Beams Illuminating THz Telescopes Conclusion THz Coherent Detection Systems Introduction Superheterodyne Receivers Receiver Noise Temperature Noise Temperature of THz Optical SystemsTHz Mixer Architectures and NoiseIF Amplifiers Effective Temperature Measurement of THz Components THz Mixers THz Local Oscillators Receiver Back-Ends Receiver Stability and Allan TimeHeterodyne Array Considerations Summary Incoherent DetectorsIntroduction Bolometer BasicsSemiconductor Bolometers Superconducting Incoherent Background Noise Limited OperationInstrument Noise Limited Operation Sensitivity Requirements Comparing Heterodyne and Incoherent Detector Sensitivity Incoherent Array Considerations Conclusion THz Observing Techniques Introduction Observing Strategies Receiver Calibration Estimating Atmospheric Optical Depth THz Brightness Temperature of Planetary Bodies Conclusion THz Interferometry Introduction Simple Adding Interferometer Phase Switched Interferometer Correlation Interferometer Phasor Equation for Interferometry Aperture Synthesis of Extended Sources Filling-in uv Space Transforming the Visibility Function Map Noise Level Phase Closure or Self-Calibration Phase Error at THz Frequencies Answers to Problems Appendix 1: Timeline of Thz Technology Appendix 2: More Thz Transitions of Atoms and Molecules Appendix 3: Commonly Used Physical and Astronomical Quantities Appendix 4: Useful Radiative Transfer Expressions Appendix 5: Commonly Used Quasi-Optical Expressions Appendix 6: Useful Heterodyne Receiver Expressions References appear at the end of each chapter.

Christopher K. Walker is a Professor of Astronomy and an Associate Professor of Optical Sciences and Electrical & Computer Engineering at the University of Arizona. He has over 30 years of experience designing, building, and using state-of-the-art receiver systems for THz astronomy. He is the author of numerous papers on star formation and protostellar evolution, a topical editor for IEEE Transactions on TeraHertz Science and Technology, and the principal investigator of the long-duration balloon project "The Stratospheric THz Observatory (STO)" funded by NASA.