The neutral gas extent of galaxies as derived from weak intervening Ca ii absorbers

We present a systematic study of weak intervening Ca ii absorbers at low redshift (z < 0.5), based on the analysis of archival high-resolution (R ≥ 45   000) optical spectra of 304 quasars and active galactic nuclei observed with VLT/UVES. Along a total redshift path of Δz ≈ 100 we detected 23 intervening Ca ii absorbers in both the Ca ii H & K lines, with rest frame equivalent widths Wr,3934 = 15−799 mÅ and column densities log N(Ca ii) = 11.25−13.04 (obtained by fitting Voigt-profile components). We obtain a bias-corrected number density of weak intervening Ca ii absorbers of at ⟨zabs⟩ = 0.35 for absorbers with log N(Ca ii) ≥ 11.65 (Wr,3934 ≥ 32 mÅ). This is ~2.6 times the value obtained for damped Lyman α absorbers (DLAs) at low redshift. All Ca ii absorbers in our sample show associated absorption by other low ions such as Mg ii and Fe ii; 45 percent of them have associated Na i absorption. From ionization modelling we conclude that intervening Ca ii absorption with log N(Ca ii) ≥ 11.5 arises in DLAs, sub-DLAs and Lyman-limit systems (LLS) at H i column densities of log  N(H i) ≥ 17.4. Using supplementary H i information for nine of the absorbers we find that the Ca ii/H i ratio decreases strongly with increasing H i column density, indicating a column-density-dependent dust depletion of Ca. The observed column density distribution function of Ca ii absorption components follows a relatively steep power law, f(N) ∝ N − β, with a slope of − β = −1.68, which again points towards an enhanced dust depletion in high column density systems. The relatively large cross section of these absorbers together with the frequent detection of Ca ii absorption in high-velocity clouds (HVCs) in the halo of the Milky Way suggests that a considerable fraction of the intervening Ca ii systems trace (partly) neutral gas structures in the halos and circumgalactic environment of galaxies (i.e., they are HVC analogs). Based on the recently measured detection rate of Ca ii absorption in the Milky Way HVCs we estimate that the mean (projected) Ca ii covering fraction of galaxies and their gaseous halos is ⟨fc,CaII⟩ = 0.33. Using this value and considering all galaxies with luminosities L ≥ 0.05 L ⋆ we calculate that the characteristic radial extent of (partly) neutral gas clouds with log N(H i) ≥ 17.4 around low-redshift galaxies is RHVC ≈ 55 kpc.