Physical conditions in high-­z triply ionized carbon: origin and evolution

Abstract

We present detailed photoionization models of well-aligned optically thin C III absorption components at 2.1 < z < 3.4. Using our models we estimate density (nH ), metallicity ([C/H]), total hydrogen column density and line-of-sight thickness (L) in each C III components. Our inferred nH and overdensity (∆) are much higher and favor the absorption originating from gas associated with circumgalactic medium (CGM) and probably not in hydrostatic equilibrium. We show L vs. [C/H] correlation can be well reproduced if L is governed by the product of gas cooling time and sound speed as expected in the case of cloud fragmentation under thermal instabilities. This allows us to explain other observed correlations by simple photoionization considerations. Studying the optically thin C III absorbers over a large z range and probably correlating their z evolution with global starformation rate density evolution can shed light into the physics of cold clump formation and their evolution in the CGM.