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

Aims. We wish to study the extent and sub-parsec spatial structure of intervening quasar absorbers, mainly those involving cold neutral and molecular gas. Methods. We have selected quasar absorption systems with high spectral resolution and a good signal-to-noise ratio data, with some of their lines falling on quasar emission features. By investigating the consistency of absorption profiles seen for lines formed either against the quasar continuum source or on the much more extended (Ly α -N v, C iv or Ly β -O vi) emission line region (ELR), we can probe the extent and structure of the foreground absorber over the extent of the ELR (~ 0.3–1 pc). The spatial covering analysis provides constraints on the transverse size of the absorber and thus is complementary to variability or photoionisation modelling studies, which yield information on the absorber size along the line of sight. The methods we used to identify spatial covering or structure effects involve line profile fitting and curve-of-growth analysis. Results. We have detected three absorbers with unambiguous non-uniformity effects in neutral gas. For the extreme case of the Fe i absorber at z abs = 0.45206 towards HE 0001−2340, we derive a coverage factor of the ELR of at most 0.10 and possibly very close to zero; this implies an overall absorber size no larger than 0.06 pc. For the z abs = 2.41837 C i absorber towards QSO J1439+1117, absorption is significantly stronger towards the ELR than towards the continuum source in several C i and C i ⋆ velocity components, pointing to spatial variations of their column densities of about a factor of two and to structures at the 100 au–0.1 pc scale. The other systems with firm or possible effects can be described in terms of a partial covering of the ELR, with coverage factors in the range 0.7–1. The overall results for cold neutral absorbers imply a transverse extent of about five times the ELR size or smaller, which is consistent with other known constraints. Although not our primary goal, we also checked when possible that singly-ionised absorbers are uniform at the parsec scale, in agreement with previous studies. In Tol 0453−423, we have discovered a very unusual case with a small but clearly significant residual flux for a saturated Fe ii λ 2600 line at z abs = 0.72604 seen on Ly α emission, thus with an absorber size comparable to or larger than that of the ELR.