At --, the diffuse radio continuum is dominated by Galactic synchrotron emission from relativistic cosmic-ray electrons spiralling in the Galactic magnetic field. It therefore traces an energetic-particle population and the magnetic field rather than gas density alone. Thermal free--free radiation from ionized gas becomes more important at higher radio frequencies and traces the emission measure .
The neutral-hydrogen 21-centimeter line is the ground-state hyperfine transition of atomic hydrogen. In the optically thin regime its velocity-integrated brightness is proportional to the H I column density, while the Doppler-resolved line maps line-of-sight velocity. Optical-depth measurements against a continuum source additionally constrain the spin temperature, so the line primarily traces atomic-gas density and kinematics.
Low rotational transitions of carbon monoxide are excited by collisions in cold molecular clouds. CO survives in shielded gas and is readily observable, so the carbon-monoxide tracer of molecular hydrogen uses its integrated intensity with to infer the otherwise difficult-to-observe column. Line ratios also constrain excitation temperature and density, but the dominant large-scale use is to map molecular-gas mass and velocity.
Interstellar dust absorbs ultraviolet and optical starlight and thermally reradiates it. Far-infrared emission comes mainly from large cool grains near thermal equilibrium and probes dust column density times a temperature-dependent emissivity. Mid-infrared emission emphasizes warmer grains, stochastically heated small grains, and aromatic features near star-forming regions. These bands therefore trace both dust mass and the intensity of the radiation field that heats it.
Near-infrared light is dominated over much of the Milky Way by old, cool, low-mass stars, with extra emission from young stars and hot dust in active regions. Because interstellar dust attenuates it much less strongly than optical light, near-infrared surface brightness and star counts are useful tracers of stellar mass and the obscured Galactic bulge and bar.
Optical continuum light maps photospheric emission from stars and therefore depends on stellar density, luminosity, temperature, age, and composition. Recombination and forbidden lines additionally trace warm ionized gas. Strong wavelength-dependent absorption and scattering by interstellar dust obscure the inner disk, so an optical map is also a map of the foreground extinction.
Galactic X rays include thermal bremsstrahlung and line emission from gas at roughly --, together with nonthermal emission from compact binaries, pulsars, and supernova remnants. They consequently trace hot plasma, shocks, accretion, and other high-energy populations; soft X rays are strongly absorbed by intervening gas.
Diffuse Galactic gamma-ray emission is produced by neutral-pion decay after collisions of cosmic-ray nuclei with gas, electron bremsstrahlung in gas, and inverse-Compton scattering of relativistic electrons from starlight and microwave photons. Compact objects and supernova remnants add resolved sources. Gamma rays therefore probe cosmic-ray populations, their target gas or radiation fields, and particle acceleration.
The differing maps follow from radiative transfer and from the different source functions of the bands. Low-frequency radio synchrotron emission passes through dust and follows cosmic-ray electrons and magnetic fields over a thick halo. Infrared emission follows dust mixed with dense gas and reprocessed starlight, whereas optical light follows unobscured stars and is strongly reshaped by extinction. The same matter distribution can therefore appear smooth at radio wavelengths, thin and clumpy in the infrared, and broken by dark dust lanes in the optical.
The Galactic scale height of a material tracer is set by vertical gravity balanced by its random, thermal, turbulent, magnetic, or cosmic-ray support. It also depends on source lifetime and transport. Cold molecular gas and dust have small velocity dispersions and settle into a thin layer. Relativistic electrons diffuse or advect far from their sources and radiate in a vertically extended magnetic field, so Galactic synchrotron emission is broad. Cosmic rays similarly fill a halo, and inverse-Compton gamma rays can be produced wherever they meet an extended radiation field; gamma-ray absorption within the Galaxy is also weak. Molecular and infrared emission remain concentrated around the thin cold-gas and young-star disk.
The spatial correlation between infrared and CO emission follows because molecular clouds contain dust and are the sites of massive-star formation. Young stars heat nearby dust, while CO maps their molecular fuel. Converting integrated CO intensity to column density gives a molecular-gas surface-density profile; converting extinction-corrected infrared luminosity to a star formation rate gives the corresponding star-formation profile. Kinematic distances from velocity-resolved CO, with care near the Galactic center and for the near--far distance ambiguity, then estimate the radial relation between gas and star formation.
The three-dimensional Galactic bar is reconstructed by combining complementary distance and velocity information. Extinction-corrected near-infrared star counts reveal its projected stellar density, and standard-candle populations such as red-clump stars show that its near end is brighter and closer than its far end. CO and H I longitude--velocity diagrams trace noncircular gas streams and shocks in a barred potential. Maser parallaxes, proper motions, radial velocities, and dynamical forward models then constrain the bar angle, length, pattern speed, and vertical box or peanut shape.
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