Effect of radiative transfer on damped Lyman-alpha and Lyman limit
  systems in cosmological SPH simulations
  We study the effect of local stellar radiation and UVB on the physical
properties of DLAs and LLSs at z=3 using cosmological SPH simulations. We
post-process our simulations with the ART code for radiative transfer of local
stellar radiation and UVB. We find that the DLA and LLS cross sections are
significantly reduced by the UVB, whereas the local stellar radiation does not
affect them very much except in the low-mass halos. This is because clumpy
high-density clouds near young star clusters effectively absorb most of the
ionizing photons from young stars. We also find that the UVB model with a
simple density threshold for self-shielding effect can reproduce the observed
column density distribution function of DLAs and LLSs very well, and we
validate this model by direct radiative transfer calculations of stellar
radiation and UVB with high angular resolution. We show that, with a
self-shielding treatment, the DLAs have an extended distribution around
star-forming regions typically on ~ 10-30 kpc scales, and LLSs are surrounding
DLAs on ~ 30-60 kpc scales. Our simulations suggest that the median properties
of DLA host haloes are: Mh = 2.4*10^10 Msun, SFR = 0.3 Msun/yr, M* = 2.4*10^8
Msun, and Z/Zsun = 0.1. About 30 per cent of DLAs are hosted by haloes having
SFR = 1 - 20 Msun/yr, which is the typical SFR range for LBGs. More than half
of DLAs are hosted by the LBGs that are fainter than the current observational
limit. Our results suggest that fractional contribution to LLSs from lower mass
haloes is greater than for DLAs. Therefore the median values of LLS host haloes
are somewhat lower with Mh = 9.6*10^9 Msun, SFR = 0.06 Msun/yr, M* = 6.5*10^7
Msun and Z/Zsun = 0.08. About 80 per cent of total LLS cross section are hosted
by haloes with SFR < 1 Msun/yr, hence most LLSs are associated with low-mass
halos with faint LBGs below the current detection limit.
