Detectability of cold streams into high-redshift galaxies by absorption lines

TL;DR

This study investigates the detectability of cold gas streams feeding high-redshift galaxies through Ly alpha and metal absorption lines using cosmological simulations, finding Ly alpha signals are consistent with observations but metal lines are weak.

Contribution

It models absorption signatures of cold streams in high-redshift galaxies, incorporating self-shielding and ionisation processes, providing predictions for observational detection.

Findings

Ly alpha absorption profiles match observed high-redshift galaxy profiles.

Metal absorption features are weak due to low metallicity and covering factors.

Cold streams produce detectable Ly alpha signals but weak metal lines.

Abstract

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.