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(Abridged) We present a systematic study of weak intervening CaII absorbersat 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 observedwith VLT/UVES. Along a total redshift path of Dz~100 we detected 23 interveningCaII absorbers in both the CaII H & K lines, with rest frame equivalent widthsW_r,3934=15-799 mA and column densities log N(CaII)=11.25-13.04. We obtain abias corrected number density of weak intervening CaII absorbers ofdN/dz=0.117+-0.044 at z=0.35 for absorbers with log N(CaII)>11.65. This is ~2.6times the value obtained for damped Lyman alpha absorbers (DLAs) at lowredshift. From ionization modeling we conclude that intervening CaII absorptionwith log N(CaII)>11.5 arises in optically thick neutral gas in DLAs, sub-DLAsand Lyman limit systems (LLS) at HI column densities of log N(HI)>17.4. Therelatively large cross section of these absorbers together with the frequentdetection of CaII absorption in high velocity clouds (HVCs) in the halo of theMilky Way suggests that a considerable fraction of the intervening CaII systemstrace dusty neutral gas structures in the halos and circumgalactic environmentof galaxies (i.e., they are HVC analogs). Considering all galaxies withluminosities L>0.05L* we calculate that the characteristic radial extent of(partly) neutral gas clouds with log N(HI)>17.4 around low-redshift galaxies isR_HVC ~ 55 kpc.

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