2025ApJ...985...31R

We present a method to classify intergalactic absorption lines and apply it to 10,772 low-redshift systems. The classification gives the number of components (reflecting the kinematics) in four different ionization phases, using detections and nondetections of H I, high (O VI, N V, O V, Ne V, Ne VI, Ne VIII), medium (C III, Si III, O III, O IV, C IV) and low (C II, N II, O II, Si II, He I) ions. In this manner, we unify the “Lyα forest,” “O VI absorbers,” “C IV absorbers,” “Mg II absorbers,” etc. We identify three main absorber types: systems without metals (type A, 86.8% of the sample), systems with high- and/or medium-ionization ions only (type B, 10.2%), and systems with low-ionization lines (type C, 3.0%). Type A mostly has single-component systems (82.2%), type B has 43.2% single-component systems, while type C absorbers mostly have multiple hydrogen components (20.6% are single). The fraction of systems with a given number of components is basically independent of the signal-to-noise ratio. Our scheme describes the observed data for each system and can be used to define samples of similar absorbers. From absorbers with available galaxy information, we find that type A systems occur both near to and far from galaxies, type C systems trace the denser circumgalactic medium (CGM) and group gas, while type B systems trace the tenuous CGM and intergalactic medium. We present new determinations of dn/dz for the 58 most common intergalactic absorption lines. We show that a rest-frame velocity of 3000 km s^–1 relative to the active galactic nucleus is preferred for separating intergalactic from associated absorbers.