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We wish to study the extent and subparsec scale spatial structure ofintervening quasar absorbers, mainly those involving neutral and molecular gas.We have selected quasar absorption systems with high spectral resolution andgood S/N data, with some of their lines falling on quasar emission features. Byinvestigating the consistency of absorption profiles seen for lines formedeither against the quasar continuum source or on the much more extendedemission line region (ELR), we can probe the extent and structure of theforeground absorber over the extent of the ELR (0.3-1 pc). The spatial coveringanalysis provides constraints on the transverse size of the absorber and thusis complementary to variability or photoionisation modelling studies. Themethods we used to identify spatial covering or structure effects involve lineprofile fitting and curve of growth analysis.We have detected three absorberswith unambiguous non uniformity effects in neutral gas. For one extreme case,the FeI absorber at z_abs=0.45206 towards HE 0001-2340, we derive a coveragefactor of the ELR of at most 0.10 and possibly very close to zero; this impliesan absorber overall size no larger than 0.06 pc. For the z_abs=2.41837 CIabsorber towards QSO J1439+1117, absorption is significantly stronger towardsthe ELR than towards the continuum source in several CI and CI* velocitycomponents pointing to factors of about two spatial variations of their columndensities and the presence of structures at the 100 au - 0.1 pc scale. Theother systems with firm or possible effects can be described in terms ofpartial covering of the ELR, with coverage factors in the range 0.7 - 1. Theoverall results for cold, neutral absorbers imply a transverse extent of aboutfive times or less the ELR size, which is consistent with other knownconstraints.

Extent and structure of intervening absorbers from absorption lines redshifted on quasar emission lines