Abstract
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.
Highlights
The observational frontier of high-redshift galaxies is shifting rapidly (Bouwens et al 2010; Bunker et al 2010; Bouwens et al 2011; Vanzella et al 2011)
We found that the formation of molecules (H2 and HD) is significantly enhanced in the presence of dust grains as compared to only gas phase reactions by up to two orders of magnitude
We have performed 9 cosmological simulations to study the dynamics of disk formation in protogalactic halos under the influence of different UV radiation field strengths and dust-togas ratios
Summary
The observational frontier of high-redshift galaxies is shifting rapidly (Bouwens et al 2010; Bunker et al 2010; Bouwens et al 2011; Vanzella et al 2011). In the very central region, there is a central massive clump with no spiral arm structure, and with very little rotational support. This is because the turbulent velocities are much larger than the rotational velocity, leading to a spherical rather than a disk-type structure. At larger scales >1019.6 cm, the rotational velocity is increased and we see a disk with spiral arms. It is found that turbulence is highly supersonic and turbulent pressure is more important than the thermal pressure This is due to the high virial temperature, which is reflected in the kinetic energy of the gas. To investigate the stability of each disk, we computed the Toomre instability parameter Q, given by
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