Observations of the Gas Reservoir around a Star Forming Galaxy in the Early Universe

TL;DR

This study presents a detailed spectrum of a bright, strongly-lensed galaxy at z=4.9, revealing high H I optical depth in its surrounding IGM, offering insights into early cosmic structure formation.

Contribution

First measurement of H I flux transmission around a high-redshift galaxy, highlighting its dense environment and potential for understanding early large-scale structure evolution.

Findings

High H I optical depth near the galaxy compared to QSOs at similar redshifts.

Presence of transmission spikes indicating complex IGM structure.

Galaxy resides in a region with more neutral hydrogen than typical at that epoch.

Abstract

We present a high signal-to-noise spectrum of a bright galaxy at z = 4.9 in 14 h of integration on VLT FORS2. This galaxy is extremely bright, i_850 = 23.10 +/- 0.01, and is strongly-lensed by the foreground massive galaxy cluster Abell 1689 (z=0.18). Stellar continuum is seen longward of the Ly-alpha emission line at ~7100 \AA, while intergalactic H I produces strong absorption shortward of Ly-alpha. Two transmission spikes at ~6800 Angstroms (A) and ~7040 A are also visible, along with other structures at shorter wavelengths. Although fainter than a QSO, the absence of a strong central ultraviolet flux source in this star forming galaxy enables a measurement of the H I flux transmission in the intergalactic medium (IGM) in the vicinity of a high redshift object. We find that the effective H I optical depth of the IGM is remarkably high within a large 14 Mpc (physical) region surrounding the galaxy compared to that seen towards QSOs at similar redshifts. Evidently, this high-redshift galaxy is located in a region of space where the amount of H I is much larger than that seen at similar epochs in the diffuse IGM. We argue that observations of high-redshift galaxies like this one provide unique insights on the nascent stages of baryonic large-scale structures that evolve into the filamentary cosmic web of galaxies and clusters of galaxies observed in the present universe.