Equivalent width

The equivalent width of a spectral line is a measure of the area of the line on a plot of intensity versus wavelength in relation to underlying continuum level.

It is found by forming a rectangle with a height equal to that of continuum emission, and finding the width such that the area of the rectangle is equal to the area in the spectral line.

represents the width of a hypothetical line which drops to an intensity of zero and has the "same integrated flux deficit from the continuum as the true one.

The equivalent width is used as a quantitative measure of the strength of spectral features.

The equivalent width is a convenient choice because the shapes of spectral features can vary depending upon the configuration of the system which is producing the lines.

For instance, the line may experience Doppler broadening due to motions of the gas emitting the photons.

[3] Thus the equivalent width can in many conditions yield the number of absorbing or emitting atoms, by using the curve of growth.

[1] For example, measurements of the equivalent width of the Balmer alpha transition in T Tauri stars are used in order to classify individual T Tauri stars as being classical or weak-lined.

[2] Also, the equivalent width is used in studying star formation in Lyman alpha galaxies, as the equivalent width of the Lyman alpha line is related to the star formation rate in the galaxy.

[4] The equivalent width is also used in many other situations where a quantitative comparison between line strengths is needed.