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Numerical simulations suggest that the first galaxies are formed in protogalactic halos with virial temperatures >= 10^4 K. It is likely that such halos are polluted with trace amounts of metals produced by the first generation of stars. The presence of dust can significantly change the chemistry and dynamics of early galaxies. In this article, we aim to assess the role of dust on the thermal and dynamical evolution of the first galaxies in the presence of a background UV flux, and its implications for the observability of Lyman alpha emitters and sub-mm sources. We have performed high resolution cosmological simulations using the adaptive mesh refinement code FLASH to accomplish this goal. We have developed a chemical network appropriate for these conditions and coupled it with the FLASH code. The main ingredients of our chemical model include the formation of molecules, a multi-level treatment of atomic hydrogen, line trapping of Lyman alpha photons and, photoionization and photodissociation processes in a UV background. We found that the formation of molecules (H_{2} and HD) is significantly enhanced in the presence of dust grains. The presence of a background UV flux strongly influences the formation of molecules by photodissociating them. We explore the evolution after a major merger, leading to the formation of a binary disk. These disks have gas masses of ~10^{7} M_sun at a redshift of 5.4. Each disk lies in a separate subhalo as a result of the merger event. The disks are supported by turbulent pressure due to the highly supersonic turbulence present in the halo. The presence of dust does not significantly reduce the Lyman alpha emission. The emission of Lyman alpha is extended and originates from the envelope of the halo due to line trapping effects. We also find that dust masses of 10^8 M_sun are required to observe the dust continuum emission from z 5 galaxies with ALMA., 12 pages, 10 figures, Accepted for publication in Astronomy and Astrophysics

Context: The HD and H2 molecules play important roles in the cooling of primordial and very metal-poor gas at high redshift. Aims: Grain surface and gas phase formation of HD and H2 is investigated to assess the importance of trace amounts of dust, 10^{-5}-10^{-3} Zo, in the production of HD and H2. Methods: We consider carbonaceous and silicate grains and include both physisorption and chemisorption, tunneling, and realistic grain surface barriers. We find, for a collapsing gas cloud environment with coupled chemical and thermal balance, that dust abundances as small as 10^{-5} solar lead to a strong boost in the H2 formation rate due to surface reactions. As a result of this enhancement in H2, HD is formed more efficiently in the gas phase through the D+ + H2 reaction. Direct formation of HD on dust grains cannot compete well with this gas phase process for dust temperatures below 150 K. We also derive up-to-date analytic fitting formulae for the grain surface formation of H2 and HD, including the different binding energies of H and D. Results: Grain surface reactions are crucial to the availability of H2 and HD in very metal-poor environments. Above metallicities of 10^{-5} solar, the grain surface route dominates the formation of H2, which in turn, drives the formation of HD in the gas phase. At dust temperatures above 150 K, laboratory experiments and theoretical modelling suggest that H2 formation on grains is suppressed while HD formation on grains is not., typos corrected, accepted for publication in Astronomy and Astrophysics