The z= 0.0777 C iii absorber towards PHL 1811 as a case study of a low-redshift weak metal line absorber

We consider the physical conditions and origin of the z = 0.0777 absorption system observed in C iii, C ii, Si iii, C iv, O vi and H i absorption along the line of sight towards the quasar PHL 1811. We analysed the Hubble Space Telescope/Space Telescope Imaging Spectrograph and Far Ultraviolet Spectroscopic Explorer spectra of this quasar and compared the results to cloudy photoionization and collisional ionization models in order to derive densities, temperatures and metallicities of the absorbing gas. The absorption can be explained by two C iii clouds, offset by 35 km s^-1 in velocity, with metallicities of approximately one-tenth the solar value. One cloud has a density of the order of n_H = 1.2^+0.9_-0.5 × 10^-3 cm^-3 (thickness 0.4^+0.3_-0.2 kpc) and produces the observed C ii and Si iii absorption, while the other has a density of the order of n_H = 1.2^+0.9_-0.5 × 10^-5 cm^-3 (thickness 80⁺⁷⁰_-40 kpc) and gives rise to the observed weak C iv absorption. Cloud temperatures are ∼14 000⁺³⁰⁰⁰_-2000 K and ∼34 000⁺²⁰⁰⁰_-4000 K for photoionized models. Although collisionally ionized clouds with T∼ 70 000 K are possible, they are less likely because of the short cooling time-scales involved. Previous studies revealed no luminous galaxy at the absorber's redshift, so it is probably related to tidal debris, ejected material, a dwarf galaxy or other halo material in a galaxy group. Our models also indicate that one of the two clouds would produce detectable weak Mg ii absorption if spectral coverage of that transition existed. We predict what the system would look like at z ∼ 1 when the ionizing background radiation was more intense. We find that at z ∼ 1 the denser component resembles a C iv absorber. The second C iii cloud in this z = 0.0777 absorber may be analogous to a subset of the more diffuse O vi absorbers at higher redshift.