Lyα Emitting Galaxies as a Probe of Reionisation

Abstract

The Epoch of Reionization (EoR) represents a milestone in the evolution of our Universe. Star-forming galaxies that existed during the EoR likely emitted a significant fraction (∼ 5 − 40%) of their bolometric luminosity as Lyα line emission. However, neutral intergalactic gas that existed during the EoR was opaque to Lyα emission that escaped from galaxies during this epoch, which makes it difficult to observe. The neutral intergalactic medium (IGM) may thus reveal itself by suppressing the Lyα flux from background galaxies. Interestingly, a 'sudden' reduction in the observed Lyα flux has now been observed in galaxies at z > 6. This review contains a detailed summary of Lyα radiative processes: I describe (i) the main Lyα emission processes, including collisional-excitation & recombination (and derive the origin of the famous factor '0.68'), and (ii) basic radiative transfer concepts, including e.g. partially coherent scattering, frequency diffusion, resonant versus wing scattering, optically thick versus 'extremely' optically thick (static/outflowing/collapsing) media, and multiphase media. Following this review, I derive expressions for the Gunn-Peterson optical depth of the IGM during (inhomogeneous) reionisation and post-reionisation. I then describe why current observations appear to require a very rapid evolution of volume-averaged neutral fraction of hydrogen in the context of realistic inhomogeneous reionisation models, and discuss uncertainties in this interpretation. Finally, I describe how existing & futures surveys and instruments can help reduce these uncertainties, and allow us to fully exploit Lyα emitting galaxies as a probe of the EoR.

Illuminating gas inflows/outflows in the MUSE deepest fields: Lyα nebulae around forming galaxies atz≃ 3.3

“…For such a medium, the emerging Lyα spectrum is double peaked, with the peak separation set by the total HI column density of the cloud (e.g., Harrington 1973;Neufeld 1990;Dijkstra 2014):…”

“…In the case of cooling radiation, the Hα flux that is associated with Lyα should be ∼ 100 times weaker (Dijkstra 2014), and would likely be undetectable. However, for the 'star formation' and 'fluorescence' models, the Hα flux should be significantly stronger.…”

Cosmology with intensity mapping techniques using atomic and molecular lines

“…On the other hand case B recombinations occur in a UV opaque medium, i.e, optically thick to Lyman series and ionizing photons, where direct recombinations to the ground state are not aloud. For a review please refer to Dijkstra (2014). Collisional excitations give an extra contribution which is an order of magnitude lower, R Lyα exc = 4.0 × 10 41 erg/s, at a gas temperature of 10 4 K. For now we will ignore this contribution since it was not considered in the estimation of an observationally determined f Lyα esc .…”

Giant Lyα Nebulae in the Illustris Simulation

“…The Lyα radiative transfer calculations are carried out using the Monte-Carlo radiative transfer (MCRT) code tlac (Gronke & Dijkstra 2014). General descriptions of MCRT are given in Dijkstra (2014) or Laursen (2010). The specific settings of tlac we employed are identical to Gronke & Dijkstra (2016) and we merely summarize them here.…”

“…Extended faint Lyα emission originating from these regions directly probes this gas, uniquely so at higher redshifts where the observation of other emission lines is challenging (for reviews see, e.g., Barnes et al 2014;Dijkstra 2014;Hayes 2015). These "Lyα nebulae" are often dubbed "Lyα blobs" (LABs), or when fainter and surrounding a galaxy also called "Lyα halos" (LAHs).…”