A joint model for the emission and absorption properties of damped Lyα absorption systems

The recently discovered population of ultra-faint extended line emitters, with fluxes of a few times 10(-18) erg s(-1) cm(-2) at z similar to 3, can account for the majority of the incidence rate of damped Ly alpha systems (DLAs) at this redshift if the line emission is interpreted as Ly alpha. We show here that a model similar to that proposed by Haehnelt, Steinmetz & Rauch (2000), which reproduces the incidence rate and kinematics of DLAs in the context of Lambda cold dark matter models for structure formation, also reproduces the size distribution of the new population of faint Ly alpha emitters for plausible parameters. This lends further support to the interpretation of the emission as Ly alpha, as well as the identification of the emitters with the hitherto elusive population of DLA host galaxies. The observed incidence rate of DLAs together with the observed space density and size distribution of the emitters suggest a duty cycle of similar to 0.2-0.4 for the Ly alpha emission from DLA host galaxies. We further show that Ly alpha cooling is expected to contribute little to the Ly alpha emission for the majority of emitters. This leaves centrally concentrated star formation at a rate of a few tenths M-circle dot yr(-1), surrounded by extended Ly alpha haloes with radii up to 30-50 kpc, as the most plausible explanation for the origin of the emission. Both the luminosity function of Ly alpha emission and the velocity width distribution of low ionization absorption require that galaxies inside dark matter (DM) haloes with virial velocities less than or similar to 50-70 km s(-1) contribute little to the incidence rate of DLAs at z similar to 3, suggesting that energy and momentum input due to star formation efficiently removes gas from these haloes. Galaxies with DM haloes with virial velocities of 100-150 km s(-1) appear to account for the majority of DLA host galaxies. DLA host galaxies at z similar to 3 should thus become the building blocks of typical present-day galaxies like our Milky Way.