Absorption line systems in simulated galaxies fed by cold streams
  Hydro cosmological simulations reveal that massive galaxies at high redshift
are fed by long narrow streams of merging galaxies and a smoother component of
cold gas. We post-process seven high-resolution simulated galaxies with
radiative transfer to study the absorption characteristics of the gas in
galaxies and streams, in comparison with the statistics of observed
absorption-line systems. We find that much of the stream gas is ionized by UV
radiation from background and local stellar sources, but still optically thick
(N_HI > 10^17 cm^-2) so that the streams appear as Lyman-limit systems (LLSs).
At z>3, the fraction of neutral gas in streams becomes non-negligible, giving
rise to damped Lyman-alpha (DLA) absorbers as well. The gas in the central and
incoming galaxies remains mostly neutral, responsible for DLAs. Within one
(two) virial radii, the covering factor of optically thick gas is <25% (10%)
for LLSs and <5% (1%) for DLAs, slowly declining with time following the
universal expansion. Nevertheless, galaxies and their cold streams in the
studied mass range, M_vir = 10^10 - 10^12 Msun, account for >30% of the
observed absorbers in the foreground of quasars, the rest possibly arising from
smaller galaxies or the intergalactic medium. The mean metallicity in the
streams is ~1% solar, much lower than in the galaxies. The simulated galaxies
reproduce the Lyalpha-absorption equivalent widths observed around Lyman-break
galaxies, but they severely underpredict the equivalent widths in metal lines,
suggesting that the latter may arise from outflows. We conclude that the
observed metal-poor LLSs are likely detections of the predicted cold streams.
Revised analysis of the observed LLSs kinematics and simulations with more
massive outflows in conjunction with the inflows may enable a clearer
distinction between the signatures of the various gas modes.
