THE NATURE OF DAMPED Lyα SYSTEMS AND THEIR HOSTS IN THE STANDARD COLD DARK MATTER UNIVERSE

Using adaptive mesh refinement cosmological hydrodynamic simulations with a physically motivated supernova feedback prescription, we show that the standard cold dark matter model can account for extant observed properties of damped Ly alpha systems (DLAs). With detailed examination of DLAs identified for each redshift snapshot through ray tracing through the simulation volumes containing thousands of galaxies, we find the following: (1) While DLA hosts roughly trace the overall population of galaxies at all redshifts, they are always gas-rich and have tendencies of being slightly smaller and bluer. (2) The history of DLA evolution is cosmological in nature and reflects primarily the evolution of the underlying cosmic density, galaxy size, and galaxy interactions. With higher density and more interactions at high redshift the size of DLAs is a larger fraction of their virial radius. (3) The variety of DLAs at high redshift is richer with a large contribution coming from galactic aqueducts, created through close galaxy interactions. The portion of gaseous disks of galaxies where most stars reside makes a relatively small contribution to DLA incidence at z = 3-4. (4) The majority of DLAs arise in halos of mass M-h = 10(10)-10(12) M-circle dot at z = 1.6-4, as these galaxies dominate the overall population of galaxies then. At z = 3-4, 20%-30% of DLA hosts are Lyman break galaxies (LBGs), 10%-20% are due to galaxies more massive than LBGs, and 50%-70% are from smaller galaxies. (5) Galactic winds play an indispensable role in shaping the kinematic properties of DLAs. Specifically, the high velocity width DLAs are a mixture of those arising in high-mass, high velocity dispersion halos and those arising in smaller mass systems where cold gas clouds are entrained to high velocities by galactic winds. (6) In agreement with observations, we see a weak but noticeable evolution in DLA metallicity. The metallicity distribution centers at [Z/H] = -1.5 to -1 and spans more than three decades at z = 3-4, with the peak moving to [Z/H] = -0.75 at z = 1.6 and [Z/H] = -0.5 by z = 0. (7) The star formation rate of DLA hosts is concentrated in the range 0.3-30 M-circle dot yr(-1) at z = 3-4, gradually shifting lower to peak at similar to 0.5-1 M-circle dot yr(-1) by z = 0. (8) We predict that only 20%-30% of DLAs are within 100 kpc of L* galaxies at z = 3-4, increasing to 30%-50% at z = 1.6.