= Lyman limit system
{c}
{title2=$N_{\rm HI}\gtrsim10^{17.2}\,\mathrm{cm}^{-2}$}
A Lyman limit system has approximately unit or greater optical depth at the neutral-hydrogen <Lyman limit> near $91.2\,\mathrm{nm}$. The threshold follows from $\tau_{912}=N_{\rm HI}\sigma_{912}$ with $\sigma_{912}\simeq6.3\times10^{-18}\,\mathrm{cm}^2$. Its strong continuum absorption distinguishes it from ordinary weak <Lyman-alpha forest> lines. Classification conventions often separate <damped Lyman-alpha systems> as the highest-column category.
Back to article page
