= Solution
<Hydrogen> is the most abundant element, and its <Lyman-alpha line> is a strong resonance from the ground state. Very small neutral fractions in an otherwise ionized <intergalactic medium> therefore produce detectable <Lyman-alpha absorption>. A long <quasar> sightline samples a great many modest density enhancements in the cosmic gas, with a much larger covering area than the compact high-column regions that produce <damped Lyman-alpha systems>.
Heavy-element absorption additionally requires metal enrichment and enough ions in the relevant ionization state. Many weak <Lyman-alpha forest> absorbers contain too few such ions to produce detectable metal lines at the same sensitivity, while high-column <Lyman limit systems> and <damped Lyman-alpha systems> are intrinsically rarer. Thus \b[abundant hydrogen, a sensitive resonance and the large interception area of diffuse gas together explain the forest's numerical dominance]. The comparison is between absorber classes at specified observational sensitivity; it does not mean that a metal-rich system has only one spectral transition.
Back to article page
