Abstract

The Lyman alpha line is a robust tracer of high redshift galaxies. We present estimates of Lyman alpha emission from a protogalactic halo illuminated by UV background radiation fields with various intensities. For this purpose, we performed cosmological hydrodynamics simulations with the adaptive mesh refinement code FLASH, including a detailed network for primordial chemistry, comprising the formation of primordial molecules, a multi-level model for the hydrogen atom as well as the photo-ionization and photo-dissociation processes in a UV background. We find that the presence of a background radiation field J$_{21}$ excites the emission of Lyman alpha photons, increasing the Lyman {$\alpha$} luminosity up to two orders of magnitude. For a halo of \~{} 10$^{10}$ M$_{⊙}$, we find that a maximum flux of 5 {\times} 10$^{-15}$ erg cm$^{-2}$ s$^{-1}$ is obtained for J$_{21}$ {\times} f$_{esc}$ = 0.1, where f$_{esc}$ is the escape fraction of the ionizing radiation. Depending on the environmental conditions, the flux may vary by three orders of magnitude. For J$_{21}$ {\times} f$_{esc}$ $\gt$ 0.1 the Lyman alpha luminosity decreases as the atomic hydrogen abundance becomes rather small. The fluxes derived here can be probed using Subaru and the upcoming James Webb Space Telescope. The emission of Lyman alpha photons is extended and comes from the envelope of the halo rather than its core. In the center of the halo, line trapping becomes effective above columns of 10$^{22}$ cm$^{-2}$ and suppresses the emission of Lyman alpha. In addition, cooling by primordial molecules may decrease the gas temperature in the central region, which further reduces Lyman {$\alpha$} emission. In the central core, H$_{2}$ is photo-dissociated for a background flux of J$_{21}$ {\ge} 1000. For weaker radiation fields, i.e. J$_{21}$ $\lt$ 0.1, H$_{2}$ and HD cooling are particularly strong in the center of the halo, leading to gas temperatures as low as \~{}100 K. We also performed a parameter study with different escape fractions of ionizing photons and explored the relative role of ionizing and dissociating radiation. We find that Lyman alpha emission depends more on the strength of the ionizing background. For a constant ionizing background, the Lyman {$\alpha$} flux increases at least by an order of magnitude for stronger photodissociation.