On the relation between high-redshift starburst galaxies and damped Lyα systems

Essentially all high-redshift galaxies show evidence for strong large-scale outflows that should have a profound effect on the structure and kinematics of both the galaxies themselves and their environment. The interstellar absorption spectra of both Lyman break galaxies (LBGs) and local starburst galaxies are remarkably similar to those of damped Ly alpha (DLA) systems in QSO spectra. Their rest-frame UV spectra typically show broad, blueshifted Ly alpha absorption accompanied by low-ionization metallic absorption lines with complex profiles. Hydrodynamical simulations of galactic winds suggest that the absorption arises in a collection of dense clouds entrained in the hot wind and/or in shells swept up by the outflow. It seems likely that outflows would also give rise to DLA absorption when seen in absorption against a background QSO. It is emphasized that some differences are expected between the properties of DLA systems in the emission-weighted galaxy spectra, which always probe the centers of star formation, and those in random sight lines through the outflow. The observed LBGs alone can account for all DLA absorption at z similar to 3 if the cross section for DLA absorption is pir(2) with r = 19 h(-1) kpc. If them cross section is smaller than this, then the fraction of DLA systems arising in outflows could still be significant if there are many wind-driving galaxies below the current detection limits. This is certainly possible since the z = 3 luminosity function is still rising down to the detection limit of 0.1L(*) and observations and simulations of local dwarf galaxies indicate that many of these fainter galaxies drive winds as well.