Equivalent width
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The equivalent width of a spectral absorption line is the width of the rectangle which has the same area as that between the intensity profile of the absorption line as a function of wavelength, and the continuum level, with the height of the rectangle being the magnitude of the continuum emission. It is a measure of the strength of spectral features that is primarily used in astronomy.[1] The intensity profile of the absorption line depends upon column density, . Column density is defined as the number of atoms per unit area along the line of sight. The curve of growth describes the dependence of the equivalent width, which is an effective measure of the strength of a feature in an emission or absorption spectrum, on the column density. The shape of this profile is initially Gaussian but moves towards a Lorentzian profile as the column density increases.
Formally, the equivalent width is given by the equation[2]
Here, represents the underlying continuum intensity, while represents the intensity of the actual spectrum (the line and continuum). Then 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."[2] This equation can be applied to either emission or absorption, but when applied to emission, the value of is negative, and so the absolute value is used.
In other words, if the continuum level is constant and the area under/above the emission/absorption line (compared to the continuum) is (the integral above), then (further highlighting the continuum-level dependence). Therefore, for a fixed line strength (), the equivalent width will be smaller for a brighter continuum.
