Aims. Doubly ionized silicon (Si iii) is a powerful tracer of diffuse ionized gas inside and outside of galaxies. It can be observed in the local Universe in ultraviolet (UV) absorption against bright extragalactic background sources. We here present an extensive study of intervening Si iii-selected absorbers and study the properties of the warm circumgalactic medium (CGM) around low-redshift (z ≤ 0.1) galaxies. Methods. We analyzed the UV absorption spectra of 303 extragalactic background sources, as obtained with the Cosmic Origins Spectrograph (COS) on-board the Hubble Space Telescope (HST). We developed a geometrical model for the absorption-cross section of the CGM around the local galaxy population and compared the observed Si iii absorption statistics with predictions provided by the model. We also compared redshifts and positions of the absorbers with those of ∼64 000 galaxies using archival galaxy-survey data to investigate the relation between intervening Si iii absorbers and the CGM. Results. Along a total redshift path of ∆z ≈ 24, we identify 69 intervening Si iii systems that all show associated absorption from other low and high ions (e.g., H i, Si ii, Si iv, C ii, C iv). We derive a bias-corrected number density of dN/dz(Si iii) = 2.5 ± 0.4 for absorbers with column densities log N(Si iii) > 12.2, which is ∼3 times the number density of strong Mg ii systems at z = 0. This number density matches the expected cross section of a Si iii absorbing CGM around the local galaxy population with a mean covering fraction of f c = 0.69. For the majority (∼60 percent) of the absorbers, we identify possible host galaxies within 300 km s −1 of the absorbers and derive impact parameters ρ < 200 kpc, demonstrating that the spatial distributions of Si iii absorbers and galaxies are highly correlated.Conclusions. Our study indicates that the majority of Si iii-selected absorbers in our sample trace the CGM of nearby galaxies within their virial radii at a typical covering fraction of ∼70 percent. We estimate that diffuse gas in the CGM around galaxies, as traced by Si iii, contains substantially more (more than twice as much) baryonic mass than their neutral interstellar medium.

We use archival UV absorption-line data from HST/STIS to statistically analyse the absorption characteristics of the high-velocity clouds (HVCs) in the Galactic halo towards more than 40 extragalactic background sources. We determine absorption covering fractions of low-and intermediate ions (O i, C ii, Si ii, Mg ii, Fe ii, Si iii, C iv, and Si iv) in the range f c = 0.20−0.70. For detailed analysis we concentrate on Si ii absorption components in HVCs, for which we investigate the distribution of column densities, b-values, and radial velocities. Combining information for Si ii and Mg ii, and using a geometrical HVC model we investigate the contribution of HVCs to the absorption cross section of strong Mg ii absorbers in the local Universe. We estimate that the Galactic HVCs would contribute on average ∼52 percent to the total strong Mg ii cross section of the Milky Way, if our Galaxy were to be observed from an exterior vantage point. We further estimate that the mean projected covering fraction of strong Mg ii absorption in the Milky Way halo and disc from an exterior vantage point is f c,sMgII = 0.31 for a halo radius of R = 61 kpc. These numbers, together with the observed number density of strong Mg ii absorbers at low redshift, indicate that the contribution of infalling gas clouds (i.e., HVC analogues) in the halos of Milky Way-type galaxies to the cross section of strong Mg ii absorbers is < 34 percent.These findings are in line with the idea that outflowing gas (e.g., produced by galactic winds) in the halos of more actively star-forming galaxies dominate the absorption-cross section of strong Mg ii absorbers in the local Universe.