7.3 Radio and millimeter-wave astronomy

Radio and millimeter-wave astronomy opens a window into the universe's hidden secrets. By studying long wavelengths, scientists can peek through cosmic dust and gas, revealing the chemical makeup of stars, galaxies, and space itself.

This field is crucial for astrochemistry, allowing us to detect molecules in space we can't see with our eyes. From simple hydrogen to complex organic compounds, radio telescopes help us understand the universe's chemical evolution and maybe even the origins of life.

Principles of radio and millimeter-wave astronomy

Wavelengths, frequencies, and emission mechanisms

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• Radio and millimeter-wave astronomy studies celestial objects at wavelengths ranging from about 1 millimeter to 10 meters, corresponding to frequencies between 300 GHz and 30 MHz
• Radio and millimeter-wave emission from astronomical sources is typically produced by various mechanisms:
  • Thermal emission from dust
  • Free-free emission from ionized gas
  • Synchrotron radiation from relativistic electrons
  • Spectral line emission from atoms and molecules

Brightness temperature and spatial resolution

• The intensity of radio and millimeter-wave emission is often expressed in terms of the brightness temperature, which is the temperature a black body would need to have to produce the observed intensity at a given wavelength
• The spatial resolution of a radio or millimeter-wave telescope is determined by the ratio of the wavelength to the diameter of the telescope
  • Larger telescopes provide higher resolution
• Radio and millimeter-wave telescopes often use interferometry, combining signals from multiple telescopes to achieve higher angular resolution than possible with a single telescope

Key molecules and spectral lines

Carbon monoxide and other important molecules

• Carbon monoxide (CO) is one of the most important molecules in radio and millimeter-wave astronomy
  • Its rotational transitions at 115 GHz (J=1-0), 230 GHz (J=2-1), and higher frequencies serve as tracers of molecular gas in the interstellar medium
• Other key molecules observed in radio and millimeter-wave astronomy include:
  • Water (H2O)
  • Ammonia (NH3)
  • Hydrogen cyanide (HCN)
  • Formaldehyde (H2CO)
  • Complex organic molecules such as methanol (CH3OH) and methyl cyanide (CH3CN)

Atomic and ionic spectral lines

• The 21 cm (1420 MHz) spectral line of atomic hydrogen (HI) is a fundamental tool for studying the distribution and kinematics of neutral atomic gas in the Milky Way and other galaxies
• Maser emission, which is the microwave analogue of laser emission, is observed from molecules such as water (H2O), hydroxyl (OH), and methanol (CH3OH) in regions of active star formation and evolved stars
• The spectral lines of ions such as carbon (CII) and nitrogen (NII) are used to study the properties of ionized gas in HII regions and planetary nebulae

Radio and millimeter-wave telescopes

Major interferometric arrays

• The Atacama Large Millimeter/submillimeter Array (ALMA) is a revolutionary interferometer located in the Atacama Desert of Chile
  • Consists of 66 antennas operating at wavelengths from 0.3 to 9.6 millimeters
• The Very Large Array (VLA) is a 27-antenna interferometer located in New Mexico
  • Operates at wavelengths from 0.7 to 400 centimeters
• The NOrthern Extended Millimeter Array (NOEMA) interferometer in France is operated by the Institut de Radioastronomie Millimétrique (IRAM)
• The Submillimeter Array (SMA) is an 8-antenna interferometer located in Hawaii
  • Operates at wavelengths from 0.3 to 1.7 millimeters

Single-dish telescopes

• The Green Bank Telescope (GBT) is a 100-meter single-dish telescope located in West Virginia
  • Operates at wavelengths from 0.3 to 100 centimeters
• The Institut de Radioastronomie Millimétrique (IRAM) operates the 30-meter single-dish telescope in Spain

Astrochemistry research using radio observations

Probing the interstellar medium

• Radio and millimeter-wave observations provide a wealth of information about the chemical composition and physical conditions of the interstellar medium
  • From diffuse clouds to dense star-forming regions
• Spectral line observations of molecules can be used to determine:
  • Temperature of the gas
  • Density of the gas
  • Kinematics of the gas
  • Abundances of different chemical species
• Radio and millimeter-wave observations have led to the detection of over 200 different molecules in the interstellar medium
  • Includes complex organic molecules that may be relevant to the origins of life (amino acids, sugars)

High-resolution studies of star formation and protoplanetary disks

• Interferometric observations with high angular resolution can reveal the detailed structure and dynamics of star-forming regions and protoplanetary disks
  • Areas where astrochemical processes are active
• Radio and millimeter-wave observations are often combined with data from other wavelengths (infrared, optical) to provide a comprehensive understanding of the chemistry and physics of astronomical sources