The Doppler parameter, or Doppler broadening parameter, usually denoted as , is a parameter commonly used in astrophysics to characterize the width of observed spectral lines of astronomical objects. It is defined as , where is the one-dimensional velocity dispersion . Given this parameter, the velocity distribution of the line-emitting/absorbing atoms and ions proximated by a Gaussian can be rewritten as , where is the probability of the velocity along the line of sight being in the interval .
The line width is also often specified in terms of the FWHM (full width at half maximum), which is .
Distribution The Doppler parameters of Lyman-alpha forest absorption lines are in the range 10–100 km s−1, with a median value around that decrease with redshift . Analyses of the HST/COS dataset of low-redshift quasars gives a median parameter of around (, ).
See also Doppler broadening Doppler spectroscopy
References