Intergalactic Transmission and its Impact on the Ly{\alpha} Line

TL;DR

This study models how the intergalactic medium affects the Lyα emission line profiles of distant galaxies, revealing implications for galaxy observations and cosmic reionization history.

Contribution

It introduces detailed cosmological simulations of IGM transmission effects on Lyα lines, considering density and velocity correlations, and constrains the epoch of reionization.

Findings

IGM can significantly suppress the blue part of Lyα lines.

The model offers an alternative explanation to outflows for line asymmetry.

Constraints on reionization epoch to z <~ 8.5.

Abstract

We study the intergalactic transmission of radiation in the vicinity of the Ly{\alpha} wavelength. Simulating sightlines through the intergalactic medium (IGM) in detailed cosmological hydrosimulations, the impact of the IGM on the shape of the line profile from Ly{\alpha} emitting galaxies at redshifts 2.5 to 6.5 is investigated. In particular we show that taking into account the correlation of the density and velocity fields of the IGM with the galaxies, the blue part of the spectrum may be appreciably reduced, even at relatively low redshifts. This may in some cases provide an alternative to the often-invoked outflow scenario, although it is concluded that this model is still a plausible explanation of the many asymmetric Ly{\alpha} profiles observed. Applying the calculated wavelength dependent transmission to simulated spectra from Ly{\alpha} emitting galaxies, we derive the fraction of photons that are lost in the IGM, in addition to what is absorbed internally in the galaxies due to dust. Moreover, by comparing the calculated transmission of radiation blueward of the Ly{\alpha} line, the total optical depth to Thomson scattering of cosmic microwave background, with corresponding observations, we are able to constrain the epoch when the Universe was reionized to z <~ 8.5.