7.4 Infrared and submillimeter astronomy

Infrared and submillimeter astronomy lets us peek into dusty cosmic regions invisible to regular telescopes. These wavelengths reveal hidden star nurseries, molecular clouds, and even the heart of our galaxy.

By studying light from cool dust and molecules, we can learn about the chemistry of space. This helps us understand how stars and planets form, and maybe even how life began in the universe.

Principles of Infrared and Submillimeter Astronomy

Wavelength Ranges and Penetration of Interstellar Dust

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• Infrared astronomy studies electromagnetic radiation with wavelengths longer than visible light but shorter than radio waves, typically in the range of 1 to 100 microns
• Submillimeter astronomy focuses on wavelengths between far-infrared and microwave regions, usually 0.3 to 1 millimeter
• Infrared and submillimeter radiation can penetrate interstellar dust clouds, allowing the observation of regions obscured at visible wavelengths
  • Enables the study of star-forming regions, molecular clouds, and the galactic center

Sources of Infrared and Submillimeter Emission

• Infrared and submillimeter emission arises from various sources
  • Thermal emission from dust grains
  • Molecular rotational and vibrational transitions
  • Fine-structure lines of atoms and ions
• The Earth's atmosphere is largely opaque to infrared and submillimeter radiation due to absorption by water vapor and other molecules
  • Observations are conducted using high-altitude telescopes, airborne observatories (SOFIA), or space-based telescopes (Herschel, JWST) to minimize atmospheric interference

Resolution and Techniques in Infrared and Submillimeter Astronomy

• The resolution of infrared and submillimeter telescopes is typically lower than that of visible-light telescopes due to the longer wavelengths
• Larger apertures and interferometric techniques are employed to improve angular resolution
  • Interferometry combines signals from multiple telescopes to achieve higher resolution (ALMA)
• Spectroscopy is widely used to study molecular transitions and chemical compositions
  • Provides information about abundances, temperatures, and densities of interstellar gas

Key Molecules in Infrared and Submillimeter Astronomy

Molecular Transitions and Chemical Composition

• Infrared and submillimeter astronomy is particularly sensitive to the rotational and vibrational transitions of molecules
  • Provides valuable information about the chemical composition, temperature, and density of interstellar gas
• Carbon monoxide (CO) is one of the most abundant and easily observable molecules in the interstellar medium
  • Its rotational transitions, especially the J=1-0 transition at 2.6 mm, are used to trace molecular gas and study the structure and kinematics of molecular clouds
• Water (H2O) is another important molecule observed in the infrared and submillimeter
  • Its rotational transitions serve as a tracer of shock-heated gas and a probe of the physical conditions in star-forming regions and circumstellar envelopes

Complex Organic Molecules and Prebiotic Chemistry

• Complex organic molecules, such as methanol (CH3OH), formaldehyde (H2CO), and methyl cyanide (CH3CN), are detected through their rotational and vibrational transitions in the infrared and submillimeter
  • Provides insights into the chemistry of star-forming regions and potential pathways for the formation of prebiotic molecules
• Polycyclic aromatic hydrocarbons (PAHs) exhibit characteristic emission features in the mid-infrared, typically between 3 and 20 microns, arising from their vibrational modes
  • These features are ubiquitous in the interstellar medium and are used to study the properties and evolution of PAHs

Dust Emission and Grain Properties

• Infrared and submillimeter observations also probe the thermal emission from interstellar dust grains, which peaks in the far-infrared and submillimeter regions
• The spectral energy distribution of dust emission provides information about the temperature, composition, and size distribution of dust grains
  • Helps understand the role of dust in interstellar chemistry and star formation processes
• Dust grains can act as catalysts for chemical reactions and provide surfaces for molecule formation
  • Studying dust properties is crucial for understanding the chemical evolution of the interstellar medium

Telescopes for Infrared and Submillimeter Astronomy

Ground-Based Telescopes and Interferometers

• The Atacama Large Millimeter/submillimeter Array (ALMA) is a groundbreaking interferometric telescope located in the Atacama Desert in Chile
  • Consists of 66 high-precision antennas operating at wavelengths from 0.32 to 3.6 mm
  • Provides unprecedented sensitivity and angular resolution for studying the chemistry of molecular clouds, protoplanetary disks, and distant galaxies
• Ground-based submillimeter telescopes, such as the Submillimeter Array (SMA) in Hawaii and the NOrthern Extended Millimeter Array (NOEMA) in the French Alps, provide high-resolution observations of molecular lines and dust continuum emission
  • Complements the capabilities of ALMA and space-based observatories

Space-Based Observatories

• The James Webb Space Telescope (JWST) is an infrared-optimized space observatory launched in 2021
  • Has a 6.5-meter primary mirror and advanced instruments covering wavelengths from 0.6 to 28.5 microns
  • Poised to revolutionize our understanding of the chemical evolution of the Universe, from the formation of the first galaxies to the birth of stars and planets
• The Herschel Space Observatory, which operated from 2009 to 2013, was a space-based telescope that observed the Universe in the far-infrared and submillimeter wavelengths (55 to 672 microns)
  • Its instruments, such as the Heterodyne Instrument for the Far-Infrared (HIFI) and the Photodetector Array Camera and Spectrometer (PACS), provided detailed observations of the chemistry in various astrophysical environments

Airborne Observatories

• The Stratospheric Observatory for Infrared Astronomy (SOFIA) is an airborne observatory consisting of a 2.7-meter telescope mounted on a modified Boeing 747 aircraft
  • Flies at altitudes up to 45,000 feet, observing the infrared universe (0.3 to 1,600 microns) above 99% of the Earth's atmospheric water vapor
  • Enables high-resolution spectroscopy and imaging of astrochemical targets
• Airborne observatories provide a unique platform for infrared and submillimeter observations, combining the advantages of reduced atmospheric absorption with the flexibility of a mobile telescope

Importance of Infrared and Submillimeter Observations

Studying Star and Planet Formation

• Infrared and submillimeter observations are essential for studying the chemical composition and physical conditions of the cold, dense regions where stars and planets form
  • These wavelengths probe the rotational and vibrational transitions of molecules, which are sensitive to the gas temperature and density
• Observations of molecular lines in the infrared and submillimeter provide crucial information about the abundances, excitation, and kinematics of interstellar molecules
  • This data is used to constrain astrochemical models, test theories of molecular formation and destruction, and understand the chemical evolution of the Universe
• Infrared and submillimeter observations can penetrate the dusty envelopes surrounding young stellar objects and protoplanetary disks
  • Allows the study of the chemical processes that lead to the formation of complex organic molecules and the building blocks of life

High-Resolution Mapping and Chemical Substructures

• The high angular resolution achieved by interferometric observatories like ALMA enables the detailed mapping of molecular emission on scales comparable to protoplanetary disks
  • Allows the identification of chemical substructures within these disks, providing insights into the chemical environment in which planets form
• Resolving chemical substructures helps understand the distribution and evolution of molecules during the planet formation process
  • Provides constraints on the initial chemical conditions for planetary systems and the potential for the emergence of life

Galactic and Extragalactic Astrochemistry

• Infrared and submillimeter observations of external galaxies allow the investigation of the chemical properties and evolution of galaxies across cosmic time
  • These studies shed light on the interplay between chemistry and galaxy evolution, star formation, and the build-up of heavy elements in the Universe
• Studying the astrochemistry of different galactic environments (e.g., spiral arms, central regions, halos) helps understand the role of chemistry in regulating star formation and galactic structure
• Observations of high-redshift galaxies in the infrared and submillimeter provide insights into the chemical evolution of the early Universe
  • Allows the study of the first generations of stars and the formation of heavy elements

Infrared and Submillimeter Astronomy vs Other Wavelength Ranges

Comparison with Visible-Light Astronomy

• Compared to visible-light astronomy, infrared and submillimeter observations can penetrate interstellar dust clouds, revealing the hidden regions of star formation and the galactic center
• However, the angular resolution of infrared and submillimeter telescopes is typically lower than that of visible-light telescopes due to the longer wavelengths
  • Requires larger apertures or interferometric techniques to achieve high resolution

Comparison with Radio Astronomy

• Radio astronomy, which focuses on wavelengths longer than 1 mm, is also sensitive to molecular transitions and can probe the cold, dense regions of the interstellar medium
• However, radio observations generally have lower angular resolution than submillimeter observations
  • The number of observable molecular transitions decreases at longer wavelengths

Comparison with High-Energy Astronomy

• X-ray and gamma-ray astronomy probe the hot, energetic processes in the Universe, such as stellar coronae, supernova remnants, and active galactic nuclei
  • While these wavelengths are not directly sensitive to molecular transitions, they can provide information about the high-energy processes that drive chemical reactions and ionization in astrophysical environments
• Ultraviolet astronomy is sensitive to the electronic transitions of atoms and molecules, and it is particularly useful for studying the chemistry of diffuse interstellar clouds and the ionized regions around hot stars
  • However, ultraviolet observations are strongly affected by interstellar extinction and require space-based observatories

Unique Niche of Infrared and Submillimeter Astronomy

• Infrared and submillimeter astronomy occupy a unique niche in the electromagnetic spectrum, bridging the gap between the cold, dense regions probed by radio observations and the hot, ionized regions studied by ultraviolet, X-ray, and gamma-ray astronomy
• The combination of observations across multiple wavelength ranges provides a comprehensive view of the chemical processes and physical conditions in astrophysical environments
  • Allows the study of the full range of chemical phenomena, from the formation of simple molecules to the synthesis of complex organic compounds
• Infrared and submillimeter observations are crucial for understanding the chemical evolution of the Universe and the origins of life
  • Provides a unique window into the hidden regions of star and planet formation, where the building blocks of life are assembled