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 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 tocloudy 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 (thickness ) and produces the observed C ii and Si iii absorption, while the other has a density of the order of (thickness ) and gives rise to the observed weak C iv absorption. Cloud temperatures are and for photoionized models. Although collisionally ionized clouds with 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 when the ionizing background radiation was more intense. We find that at the denser component resembles a C iv absorber. The second C iii cloud in this absorber may be analogous to a subset of the more diffuse O vi absorbers at higher redshift. [ABSTRACT FROM AUTHOR]