Astrophysics (index)about

spectral energy distribution

(function or plot of brightness at each wavelength in the spectrum)

Electromagnetic radiation's spectral energy distribution (SED) is a function, i.e., plot of brightness or flux density versus the frequency or wavelength (the choice of which produce different results: see below). When the SED of an astronomical object is measured from Earth, the effects of Earth atmosphere are relevant. Capturing a wide range of wavelengths requires multiple instruments, which causes data-calibration challenges. But some limited bandwidths can provide signatures for specific types sources.

The alternate term, spectral power distribution (SPD) is used in some other (non-astronomy) fields.

Plotting by wavelength versus plotting by frequency has consequences: the peak wavelength differs in the two cases if each is plotted linearly, a consequence of plotting density functions of values that have a reciprocal relationship. Taking black-body spectrum as an example, plotting wavelengths by millimeter (0mm, 1mm, 2mm, 3mm, ...) crowds all the infrared, visible light, ultraviolet, etc., between 0 and 1 and spreads radio over a wide range. Plotting frequency by 100GHz (0GHz, 100GHz, 200GHz, 300GHz, ..) crowds the radio into 0-300GHz and spreads the others over the rest. The CMB SED's peak is cited at various wavelengths and frequencies, but all are valid for the spectrum representing a temperature of 2.72548 K, depending upon how it is plotted:

plotted by peak frequency peak wavelength peak photon energy
linear wavelength 282 GHz 1.063 mm 1.168 × 10-3 eV
linear frequency 160.23 GHz 1.871 mm 6.26 × 10-4 eV
log of either 222.6 GHz 1.347 mm 9.2 × 10-4 eV

Plots against the log of either produce an equivalent plot: 1mm≡300GHz, 10mm≡30GHz, 100mm≡3GHz, etc.

SED fitting (aka spectral energy distribution fitting) is a method of classifying distant galaxies, to help determine their age and degree of star formation. A challenge is that a young galaxy with dust has a SED across ultraviolet, visible light, and near infrared as an old galaxy. A fitting method is Markov chain Monte Carlo (MCMC), using random walks around a parameter-space to determine the degree of unique fit between an SED and a known type of object. GalMC is an example of software that takes this approach.


Referenced by:
angular power spectrum
Compton reflection
galaxy SED
Herschel Orion Protostar Survey (HOPS)
K correction
linearly variable filter (LVF)
Rayleigh-Jeans law
radio galaxy (RG)
Spitzer Extended Deep Survey (SEDS)
sky subtraction
spectral line energy distribution (SLED)
spectral power distribution (SPD)
spectral correlator
spectral signature
stellar model atmosphere
Sunyaev-Zel'dovich effect (SZ effect)
Thomson optical depth
vegetation red edge (VRE)
Wien's displacement law