On the sizes of neutral hydrogen regions giving rise to damped Lyα absorption systems

Abstract

Using quasi-stellar object (QSO) absorption-line spectra obtained along closely spaced sight-lines, we examine the transverse sizes of regions containing large columns of neutral hydrogen gas at redshifts z ≈ 1.5. The observations are primarily of intervening damped Lyα (DLA) and sub-DLA absorption-line systems in gravitationally lensed QSOs. In particular, Hubble Space Telescope spectroscopy of the four-component Cloverleaf QSO (H1413+1143) reveals three new DLA/sub-DLA systems at z ≈ 1.44, 1.49 and 1.66. A neutral hydrogen column density of N HI ≥ 2 × 10 20 atoms cm ―2 is required for a system to be classified as a DLA, but none of the three systems has an H column density above the DLA threshold in all four components. Over component separations < 1.4 arcsec in the Cloverleaf, corresponding to transverse sizes of ≈5―12 h ―1 70 kpc, the HI column densities typically change by factors of ≈2―40. Similar observations of other QSOs containing absorption systems in the DLA regime are summarized from the literature. In addition to establishing approximate sizes for DLA regions, the results have implications for their volume-averaged HI gas number densities and neutral gas masses. By combining our results on DLA absorber sizes with published results on the sizes of lower column density QSO absorbers, which however arise in very ionized regions, we infer the useful relation that the typical transverse size of an absorber in the redshift interval z≈[1,2] is S abs ≈ 11 h ―1 70 [[N HI /10 20 ] ―1/4 kpc.