Abstract
We use cosmological smoothed particle hydrodynamics (SPH) simulations of the A cold dark matter (CDM) model to study the abundance of damped Lyman a absorbers (DLAs) in the redshift range z = 0-4.5. We compute the cumulative DLA abundance by using the relation between the DLA cross-section and the total halo mass inferred from the simulations. Our approach includes standard radiative cooling and heating with a uniform ultraviolet background, star formation, supernova feedback and a phenomenological model for feedback by galactic winds. The latter allows us to examine, in particular, the effect of galactic outflows on the abundance of DLAs. We employ the 'conservative entropy' formulation of SPH developed by Springel & Hernquist, which mitigates against the systematic overcooling that affected earlier simulations. In addition, we utilize a series of simulations of varying box-size and particle number to isolate the impact of numerical resolution on our results. We show that the DLA abundance was overestimated in previous studies for three reasons: (i) the overcooling of gas occurring with non-conservative formulations of SPH, (ii) a lack of numerical resolution and (iii) an inadequate treatment of feedback. Our new results for the total neutral hydrogen mass density, DLA abundance and column density distribution function all agree reasonably well with observational estimates at redshift z = 3, indicating that DLAs arise naturally from radiatively cooled gas in dark matter haloes that form in a ACDM universe. Our simulations suggest a moderate decrease in DLA abundance by roughly a factor of 2 from z = 4.5 to 3, consistent with observations. A significant decline in abundance from z = 3 to 1, followed by weak evolution from z = 1 to 0, is also indicated, but our low-redshift results need to be interpreted with caution because they are based on coarser simulations than those employed at high redshift. Our highest resolution simulation also suggests that the halo massscale below which DLAs do not exist is slightly above 10 8 h -1 M ○. at z = 3-4, somewhat lower than previously estimated.
Highlights
Damped Lyman-α absorbers, historically defined as quasar absorption systems with neutral hydrogen column density NHI > 2 × 1020cm−2 (Wolfe et al 1986), are one of the best probes of structure formation in the early universe
We show that the damped Lyman-α absorbers (DLAs) abundance was overestimated in previous studies for three reasons: (1) the overcooling of gas occurring with non-conservative formulations of smoothed-particle hydrodynamics (SPH), (2) a lack of numerical resolution, and (3) an inadequate treatment of feedback
7 DISCUSSION We have used state-of-the-art hydrodynamic simulations of structure formation to investigate the abundance of DLAs in a ΛCDM universe
Summary
Damped Lyman-α absorbers, historically defined as quasar absorption systems with neutral hydrogen column density NHI > 2 × 1020cm−2 (Wolfe et al 1986), are one of the best probes of structure formation in the early universe. Earlier studies by Katz et al (1996) and Hernquist et al (1996) showed that the observed Hi column density distribution can be reproduced within a factor of a few in hydrodynamic simulations based on a CDM model over a wide range of column densities 1014cm−2 ≤ NHI ≤ 1022cm−2 Their results demonstrated that the Ly-α forest develops naturally in the hierarchical clustering scenario of CDM universes, and that DLAs and Lyman-limit systems (NHI ≥ 1017cm−2) arise in these models from radiatively cooled gas inside dark matter haloes that host forming galaxies at high redshift.
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