  Cold gas streaming along the dark-matter filaments of the cosmic web is
predicted to be the major provider of resources for disc buildup, violent disk
instability and star formation in massive galaxies in the early universe. We
study to what extent these cold streams are traceable in the extended
circum-galactic environment of galaxies via Ly alpha absorption and selected
low ionisation metal absorption lines. We model the expected absorption
signatures using high resolution zoom-in AMR cosmological simulations. In the
postprocessing, we distinguish between self-shielded gas and unshielded gas. In
the self-shielded gas, which is optically thick to Lyman continuum radiation,
we assume pure collisional ionisation for species with an ionisation potential
greater than 13.6 eV. In the optically thin, unshielded gas these species are
also photoionised by the meta-galactic radiation. In addition to absorption of
radiation from background quasars, we compute the absorption line profiles of
radiation emitted by the galaxy at the centre of the same halo. We predict the
strength of the absorption signal for individual galaxies without stacking. We
find that the Ly alpha absorption profiles produced by the streams are
consistent with observations of absorption and emission Ly alpha profiles in
high redshift galaxies. Due to the low metallicities in the streams, and their
low covering factors, the metal absorption features are weak and difficult to
detect.
